Text-Terminal-HOWTO
David S. Lawyer <mailto:dave@lafn.org>
v1.13, June 2000
This document explains what text terminals are, how they work, how to
install and configure them, and provides some info on how to repair
them. If you don't have a terminal manual, it may be of help. While
it's written for real terminals on a Linux system, some of it is also
applicable to terminal emulation and may be helpful for non-Linux sys�
tems.
______________________________________________________________________
Table of Contents
1. Introduction
1.1 Copyright, Trademarks, Disclaimer, & Credits
1.1.1 Copyright
1.1.2 Disclaimer
1.1.3 Trademarks.
1.1.4 Credits
1.2 Future Plans: You Can Help
1.3 New Versions of this HOWTO
1.4 Related HOWTO's
1.5 Terminology Used in this Document
1.6 What is a Terminal ?
2. Types of Terminals
2.1 Dumb Terminals
2.2 Text Terminals
2.3 Graphics Terminals
2.3.1 Serial Line Graphics Terminals
2.3.2 Fast Graphics Terminals (often known by other names)
2.4 Network Computers (NCs)
2.5 Emulation on a PC
3. Quick Install
4. Why Use a Terminal ?
4.1 Intro to Why Use a Terminal
4.2 Lower Hardware Costs ?
4.3 Control of Software
4.4 Hardware Upgrades
4.5 Other Advantages of Terminals
4.6 Major Disadvantages of Terminals
4.7 Are Text Terminals Obsolete ?
5. Overview of How Terminals Work (in Linux)
5.1 Device Names
5.2 Login/Logout
5.3 Half/Full Duplex
5.4 Terminal Memory
5.5 Commands for the Terminal
5.6 Lack of Standardization Solved by Terminfo
5.7 The Interface
5.8 Emulation
5.9 The Console
6. Terminal Special Files such as /dev/tty
6.1 Serial Port Terminals
6.2 Pseudo Terminals
6.3 The Controlling Terminal /dev/tty
6.4 /dev/ttyIN "Terminals"
6.5 The Console: /dev/ttyN
6.6 Creating a Device with "mknod"
7. Some Details on How Terminals Work
7.1 Terminal Memory Details
7.2 Early Terminals
7.3 Escape Sequences and Control Codes (intro)
7.3.1 Control Codes
7.3.2 Escape Sequences
7.4 Display Attributes & Magic Cookies
8. Special Features of Some Terminals
8.1 Color
8.2 Multiple Sessions
8.3 Printer/Auxiliary Port
8.4 Pages
8.5 Character-Sets
8.6 Fonts
8.7 Keyboards & Special Keys
9. Terminal Emulation; the Console
9.1 Intro to Terminal Emulation
9.2 Don't Use TERM For Emulation
9.3 Communication (Dialing) programs
9.3.1 Emulation under X-Windows
9.3.2 Real Terminals Better
9.4 Testing Terminal Emulation
9.5 The Linux Console
9.6 Emulation Software
9.6.1 Make a Linux PC a terminal
9.6.2 Make a non-Linux PC a terminal
10. Flow Control (Handshaking)
10.1 Why Is Flow Control Needed ?
10.2 Padding
10.3 Overrunning a Serial Port
10.4 Stop Sending
10.5 Keyboard Lock
10.6 Resume Sending
10.7 Hardware Flow Control (RTS/CTS etc.)
10.7.1 RTS/CTS, DTR, and DTR/DSR Flow Control
10.7.2 Connecting Up DTR or DTR/DSR Flow Control
10.7.3 Old RTS/CTS Handshaking Is Different
10.7.4 Reverse Channel
10.8 Is Hardware Flow Control Done by Hardware ?
10.9 Obsolete ?? ETX/ACK or ENQ/ACK Flow Control
11. Physical Connection
11.1 Multiport I/O Cards (Adapters)
11.2 Direct Cable Connection.
11.2.1 Null Modem Cable Pin-out (3, 4, or 5 conductor)
11.2.2 Standard Null Modem Cable Pin-out (7 conductor)
11.2.3 Length Limitations
11.2.4 Hardware Flow Control Cables
11.2.5 Cable Tips
11.2.6 A Kludge using Twisted-Pair Cable
11.2.7 Cable Grounding
11.3 Modem Connection
11.3.1 Dialing Out From a Terminal
11.3.2 Terminal Gets Dialed Into
11.4 Terminal Server Connection
11.5 Connector and Adapter Types
11.5.1 Sex of Connector/Adapters
11.5.2 Types of Adapters
11.5.3 DB Connectors
11.5.4 RJ Modular Connectors
11.6 Making or Modifying a Cable
11.6.1 Buy or Make ?
11.6.2 Pin Numbers
11.6.3 Installing DB Connectors on Cable Ends
11.6.4 Installing RJ Connectors
12. Set-Up (Configure) in General
12.1 Intro to Set-Up
12.2 Terminal Set-Up (Configure) Overview
12.3 Computer Set-Up (Configure) Overview
12.4 Many Options
12.5 Communication Interface Options
12.5.1 Speed
12.5.2 Parity & Should You Use It ?
12.5.3 Bits/Character
12.5.4 Which Flow Control (Handshaking) ?
12.5.5 Port Select
12.6 Quick Attempt
13. Terminal Set-Up (Configure) Details
13.1 Send Escape Sequences to the Terminal
13.2 Older Terminals Set-Up
13.3 Getting Into Set-Up (Configuration) Mode
13.4 Communication Options
13.5 Saving the Set-up
13.6 Set-Up Options/Parameters
13.7 Emulation {Personality} {{Terminal Modes}}
13.8 Display Options
13.8.1 Character Cell Size {Char Cell}
13.8.2 Columns/Lines
13.8.3 Cursor
13.8.4 Display Attributes (Magic Cookies)
13.8.5 Display Control Characters {Monitor}
13.8.6 Double Width/Height
13.8.7 Reverse Video {Display} (Background Light/Dark)
13.8.8 Status Line
13.8.9 Upon 80/132 Change: Clear or Preserve?
13.9 Page Related Options
13.9.1 Page Size
13.9.2 Coupling (of cursor & display)
13.10 Reporting and Answerback
13.10.1 Answerback Message (String)
13.10.2 Auto Answerback
13.10.3 Answerback Concealed
13.10.4 Terminal ID {ANSI ID}
13.11 Keyboard Options
13.11.1 Keyclick
13.11.2 Caps Lock {Keylock}
13.11.3 Auto Repeat {Repeat}
13.11.4 Margin Bell
13.11.5 Remapping the Keys
13.11.6 Corner Key (for Wyse only)
13.11.7 Numeric Keypad or Arrow Keys Sends
13.11.8 What does shifted-del and shifted-bs send?
13.11.9 PC Scan Codes
13.11.10 Alternate Characters
13.12 Meaning of Received Control Codes
13.12.1 Auto New Line {Newline}
13.12.2 Auto Line Feed {Rcv CR}
13.12.3 Recognize Del (Wyse Only ??) or Null
13.13 Where New Text Goes
13.13.1 Line Wrap
13.13.2 Scrolling
13.13.3 New Page?
13.14 Function Keys
13.15 Block Mode Options
13.15.1 Forms Display
13.15.2 Send Entire Block ?
13.15.3 Region to Send
13.15.4 Block/Page terminator
13.16 Locks
13.17 Screen Saver {Scrn Saver}
13.18 Printer
14. Computer Set-Up (Configure) Details
14.1 Getty (in /etc/inittab)
14.1.1 Agetty (may be named getty)
14.1.1.1 Agetty's detection of parity
14.1.1.2 Agetty's parity with 8-bit data bytes
14.1.2 getty (part of getty_ps)
14.1.3 mgetty
14.2 Stty & Setserial
14.3 Setserial
14.3.1 Introduction
14.3.2 Probing
14.3.3 Boot-time Configuration
14.3.4 Configuration Scripts/Files
14.3.5 Edit a script (after version 2.15: perhaps not)
14.3.6 New configuration method using /etc/serial.conf
14.3.7 IRQs
14.4 Stty
14.4.1 Introduction
14.4.2 Using stty for a "foreign" terminal
14.4.2.1 Old redirection method
14.4.3 Two interfaces at a terminal
14.4.4 Where to put the stty command ?
14.5 Terminfo & Termcap (brief)
14.6 Setting TERM and TERMINFO
14.6.1 What is the terminfo name of my terminal ?
14.7 Rarely Needed /etc/ttytype File
14.8 Login Restrictions
14.9 Run Command Only If TERM=my_term_type
14.9.1 Example for ls Function
15. Terminfo and Termcap (detailed)
15.1 Intro to Terminfo
15.2 Terminfo Database
15.2.1 Introduction
15.2.2 Where is the database located ?
15.2.2.1 Compiled database locations
15.2.2.2 Source-code database locations
15.2.3 Terminfo Compiler (tic)
15.2.4 Look at Your Terminfo
15.2.5 Deleting Data Not Needed
15.3 Bugs in Existing Terminfo Files (and Hardware)
15.4 Modifying Terminfo Files
15.5 Init String
15.6 TERM Variable
15.7 Terminfo/Termcap Documents
16. Using the Terminal
16.1 Intro to Using Terminal
16.2 Starting Up the Terminal
16.3 Terminal (Serial) Device Driver
16.4 Problems with Editors
16.4.1 emacs and ^S, ^Q
16.4.2 vi and Cursor-Keys
16.5 Color ls Corruption
16.6 Display Freezes (hung terminal)
16.7 Corrupted Terminal Interface
16.7.1 Symptoms and Some Fixes
16.7.2 Sent Terminal Binary Characters
16.7.3 Abnormally Exited a Program
16.8 Special (Control) Characters
16.8.1 Command-Line Editing
16.8.2 Interrupting (& Quit, Suspend, EOF, Flush)
16.8.3 Stop/Start Scrolling
16.8.4 Take Next Character Literally
16.9 Viewing Latin-1 Files on a 7-bit Terminal
16.10 Inspecting the Interface
16.11 Changing the Terminal Settings
16.11.1 setterm
16.11.2 tput
16.11.3 echo
16.11.4 Saving Changes
16.12 Make a Terminal the Console
16.12.1 For Kernels 2.2 or higher
16.12.2 For Kernels before 2.2
16.12.3 Can I Run Linux without a Monitor (PC Console) ?
16.13 Multiple Sessions
16.14 Logging Out
16.15 Chatting between Terminals, Spying
16.16 Sharing the Serial Port
17. Trouble-Shooting (software)
17.1 Terminal Was Working OK
17.2 Terminal Newly Installed
17.3 Is the Terminal OK ?
17.4 Missing Text
17.5 Getty Respawning Too Rapidly
17.5.1 Serial Module Not Loaded
17.5.2 No Modem Control Voltage
17.5.3 Key Shorted
17.6 Fails Just After Login
17.7 Can't Login
17.8 Garbled Login Prompt
17.9 No Sign of any Login Prompt
17.9.1 Diagnose Problem from the Console
17.9.2 Measure Voltages
17.10 Slow: pauses of several seconds between bursts of characters
17.11 Terminal doesn't scroll
17.12 Serial Monitoring/Diagnostics
17.13 Local Mode
17.14 Serial Electrical Test Equipment
17.14.1 Breakout Gadgets, etc.
17.14.2 Measuring Voltages
17.14.3 Taste Voltage
18. Repair & Diagnose
18.1 Repair Books & Websites
18.1.1 Books
18.1.2 Websites
18.2 Safety
18.3 Appearance of Display
18.4 Diagnose
18.4.1 Terminal Made a Noise
18.4.2 Terminal Made No Noise
18.5 Error Messages on the Screen
18.5.1 Keyboard Error
18.5.2 Checksum Error in NVR
18.6 Capacitors
18.7 Keyboards
18.7.1 Interchangeability
18.7.2 How They Work
18.7.3 One Press Types 2 Different Characters
18.7.4 Modern vs Old Keyboards
18.7.5 Keyboard Doesn't Work At All
18.7.6 Typing b Displays bb, etc. (doubled)
18.7.7 The Keyboard Types By Itself
18.7.8 Liquid Spilled on the Keyboard
18.7.9 Cleaning Keyboard Contacts
18.7.9.1 Keyboards with Membranes
18.7.9.2 Keyboards with Individual Switches
19. Appendix A: General
19.1 List of Linux Terminal Commands
19.1.1 Sending a Command to the Terminal
19.1.2 Configuring the Terminal Device Driver
19.1.3 Terminfo
19.1.4 Other
19.2 The Internet and Books
19.2.1 Terminal Info on the Internet
19.2.2 Books Related to Terminals
19.2.3 Entire Books on Terminals
19.2.4 Books with Chapters on Terminals
19.3 Non-Linux OS's
20. Appendix B: Escape Sequence Commands Terminology
20.1 Esc Sequence List
20.2 8-bit Control Codes
20.3 Printer Esc
20.4 Reports
20.5 Cursor Movements
20.6 Pages (definition)
21. Appendix C: Serial Communications on EIA-232 (RS-232)
21.1 Intro to Serial Communication
21.2 Voltages
21.2.1 Voltage for a Bit
21.2.2 Voltage Sequence for a Byte
21.3 Parity Explained
21.4 Forming a Byte (Framing)
21.5 Limitations of EIA-232
21.5.1 Low Speed & Short Distance
21.5.2 Successors to EIA-232
21.5.3 Line Drivers
21.6 Synchronization & Synchronous
21.6.1 How "Asynchronous" is Synchronized
21.6.2 Defining Asynchronous vs Synchronous
21.6.3 Synchronous Communication
21.7 Block Mode
21.7.1 Intro to Block Mode
21.7.2 Types of Block Modes, Forms
21.7.3 Efficiency
21.8 EIA-232 (RS-232) Books
21.9 Serial Software
22. Appendix D: Notes by Brand Name
22.1 CIT
22.2 IBM Terminals
22.2.1 IBM 3153
22.3 Teletypes
22.4 VT (DEC)
22.5 Wyse
22.5.1 Wyse 60
22.5.2 Wyse 85
22.5.3 Wyse 99-GT
22.5.4 Wyse 150
______________________________________________________________________
1. Introduction
For a quick attempt to install a terminal see ``Quick Install''.
1.1. Copyright, Trademarks, Disclaimer, & Credits
1.1.1. Copyright
Copyright 1998-2000 by David S. Lawyer. <mailto:dave@lafn.org>
Please freely copy and distribute (sell or give away) this document in
any format. Forward any corrections and comments to the document
maintainer. You may create a derivative work and distribute it
provided that you:
1. Send your derivative work (in the most suitable format such as
sgml) to the LDP (Linux Documentation Project) or the like for
posting on the Internet. If not the LDP, then let the LDP know
where it is available. Except for a translation, send a copy to
the previous maintainer's url as shown in the latest version.
2. License the derivative work in the spirit of this license or use
GPL. Include a copyright notice and at least a pointer to the
license used.
3. Give due credit to previous authors and major contributors.
If you're considering making a derived work other than a translation,
it's requested that you discuss your plans with the current
maintainer.
1.1.2. Disclaimer
While I haven't intentionally tried to mislead you, there are likely a
number of errors in this document. Please let me know about them.
Since this is free documentation, it should be obvious that I cannot
be held legally responsible for any errors.
1.1.3. Trademarks.
Any brand names (starts with a capital letter) should be assumed to be
a trademark). Such trademarks belong to their respective owners.
1.1.4. Credits
Much of the section "Physical Connection" is from Serial-HOWTO v.
1.11 by Greg Hankins (with his permission). His "How Do I Set Up A
Terminal Connected To My PC?" was incorporated into v1.00 at various
places. v1.09 has about 25 changes (and error corrections) suggested
by Alessandro Rubini who reviewed this HOWTO.
1.2. Future Plans: You Can Help
Please let me know of any errors in facts, opinions, logic, spelling,
grammar, clarity, links, etc. But first, if the date is over a few
months old, check to see that you have the latest version. Please
send me any info that you think belongs in this document.
Starting with version 1.00, a first attempt was made to help people
set up terminals without recourse to a terminal manual. Much more is
needed in this respect. One way to solve this problem would be if
terminal manufacturers put their manuals on the Internet. I suggest
that you encourage them to do so. The task of providing information
on how to configure most terminals in this HOWTO is daunting. There
are so many different terminals, but there are far fewer models than
there used to be in the 1980,s so the task is not totally infeasible.
Please send me any surplus terminal manuals which you may have,
especially on terminals made within the past 10 years (but I'll accept
older ones also). Also, you might want to write up something on a
certain terminal to put in the Appendix D: Notes by Brand Name.
1.3. New Versions of this HOWTO
New versions of the Text-Terminal-HOWTO will be available to browse
and/or download at LDP mirror sites. For a list of mirror sites see:
<http://linuxdoc.org/mirrors.html>. Various formats are available.
If you only want to quickly check the date of the latest version look
at <http://linuxdoc.org/HOWTO/Text-Terminal-HOWTO.html>. The version
your are currently reading is: v1.13, June 2000 . New in this version
is: killing getty permanently, reset bug+, man lilo. .
1.4. Related HOWTO's
Go to the websites shown above to get these.
� Serial-HOWTO has info on Multiport Serial Cards used for both
terminals and banks of modems. It has general technical info on
the serial port including troubleshooting it.
� Modem-HOWTO
� Serial-Programming-HOWTO
� NCD-X-Terminal mini-HOWTO
� Xterminal-HOWTO (unmaintained). It's at
<http://sunsite.unc.edu/pub/Linux/docs/HOWTO/unmaintained/mini/Xterminal>
1.5. Terminology Used in this Document
Configuration means the same as set-up. While Linux commands take
options (using - symbols), options in a broader sense include various
other types of choices. Install in the broad sense includes setting
up (configuring) software and hardware. A statement that I suspect is
true (but may not be) ends with 2 question marks: ?? If you know for
sure, let me know.
1.6. What is a Terminal ?
A terminal consists of a screen and keyboard that one uses to
communicate remotely with a (host) computer. One uses it just like it
was a personal computer but the terminal is remote from its host
computer (on the other side of the room or even on the other side of
the world). Programs execute on the host computer but the results
display on the terminal screen. Its computational ability is
relatively low (otherwise it would be a computer and not a terminal).
This computational ability is generally limited to the ability to
display what is sent to it (possibly including full-screen graphics)
and the ability to send to the host what is typed at the keyboard.
In the days of mainframes from the mid 1970's to the mid 1980's, most
people used terminals to communicate with computers. They typed in
programs, ran programs, wrote documents, issued printing commands,
etc. A cable connected the terminal to the computer (often
indirectly). It was called a terminal since it was located at the
terminal end of this cable.
If you've been using Linux (except for X-Window use) with a computer
monitor and keyboard you already know what a terminal is because you
have been using one (or more precisely a "virtual terminal"). The
monitor (along with the keyboard) is emulating a terminal. In X-
Windows the programs xterm, rxvt, and zterm emulate terminals.
A real terminal is different from a monitor because it's a different
electronic setup. A text terminal is often connected to a serial port
of the computer via a long cable. Thus, in contrast to a monitor
which is usually located right next to the computer, a terminal may be
quite a distance away from its host computer. The video card inside a
computer stores the video image seen on the monitor screen. For a
terminal, the equivalent of this video card is built right into the
terminal but since text terminals are often monochrome without much
graphics, the capabilities of its "video card" are rather weak. Also,
most text terminals do not have mice.
In network client-server terminology, one might think that the
terminal is the client and that the host computer is the server. The
terminal has been called a "thin client" by some. But it is not
actually a "client" nor is the host a "server". The only "service"
the host provides is to receive every letter typed at the keyboard and
react to this just like a computer would. The terminal is like a
window into the computer just like a monitor (and keyboard) are. You
may have already used virtual terminals in Linux (by pressing Left
Alt-F2, etc.). A real terminal is just like running such a virtual
terminal but you run it on its own terminal screen instead of having
to share the monitor screen. In contrast to using a virtual terminal
at the console (monitor), this allows another person to sit at the
real terminal and use the computer simultaneously with others.
2. Types of Terminals
2.1. Dumb Terminals
There are various conflicting definitions of "dumb terminal" but as
time goes by, more and more terminals are called dumb. This document
mainly covers text terminals which display only text on the screen.
It might be titled "Dumb-Terminal-HOWTO" but in some magazines
articles any terminal, no matter how smart, including ones which
present a full graphical user interface (GUI), are called dumb. If
all terminals are "dumb" then there is no point of prefixing the word
"dumb" to terminal (except as a sales pitch to sell computers or the
like in place of "smart" terminals). Due to the ambiguous meaning of
"dumb terminal" it is not classified here as a type of terminal.
2.2. Text Terminals
For a text terminal, a 2-way flow of information between the computer
and the terminal takes place over the cable that connects them
together. This flow is in ASCII bytes where each byte usually
represents a character. Bytes typed at the keyboard go to the
computer and most bytes from the computer are displayed on the
terminal screen. Special bytes (or sequences of bytes) from the
computer tell the terminal where to move the cursor to, what to erase,
where to begin and end underlining and/or blinking and/or bold, etc.
There are often hundreds of such special commands and many terminals
can even change fonts.
The communication uses characters (letters) encoded using a code chart
for the character set being used. Usually, the first 128 bytes out of
256 possible bytes use ASCII codes. Terminals for Unix-like systems,
normally connect to computers via a cable running between the
asynchronous serial ports (RS-232-C = EIA-232-D) of the host computer
and terminal. Sometimes the connection is via modem or terminal
server, etc.
Other names for text terminals are "serial terminal", "character-cell
terminal", "ASCII/ANSI terminal", "asynchronous terminal", "data
terminal", "video terminal" and "video display terminal" (VDT). In
olden days "video display unit" (VDU) was used for terminals but
strictly speaking, it excludes the keyboard.
"Block mode" was used exclusively by old IBM mainframe terminals but
many modern terminals also have this capability (which is not used
much). The characters you type are temporarily retained in the
terminal memory (and may possibly be edited by a built-in editor at
the terminal). Then when the send key (or the like) is pressed, a
block of characters (sometimes just a line of characters) is sent to
the computer all at once. Block mode (as of late 1998) is not
supported by Linux. See section ``Block Mode''.
2.3. Graphics Terminals
To a limited degree some ASCII symbols can provide graphics on text
terminals. One may form arrows <--- and draw boxes with _ and |.
With special graphic character sets, even more is possible. None of
these are really graphics terminals. However, the term "graphics
terminal" is sometimes applied to all text-only terminals since text
is a limited form of graphics.
There are two basic types of graphics displays: raster and vector
(rarely used). Raster graphics (bit-mapped) puts dots on the screen
by horizontal scan lines drawn by an electron beam (or by activating
pixels or dots on a flat screen). Vector graphic displays are usually
for monochrome screens that don't have any dots. They use smart
electronics to draw lines and curves with an electron beam that can
move in any direction (at any angle and location). Vector graphics
draws high quality lines without zig-zags but is both rare and
expensive. Raster graphics is almost universally used today. For
PC's, images encoded in vector graphic format are sometimes used but
they are translated to raster graphics format for display (with a drop
in image quality).
2.3.1. Serial Line Graphics Terminals
Most of this document also applies to these. Most of these can also
function as text terminals. The protocols for such graphics include:
Tektronix Vector Graphics, ReGIS (DEC), Sixel (DEC), and NAPLPS (North
American Presentation Level Protocol Syntax).
2.3.2. Fast Graphics Terminals (often known by other names)
None of these covered in this document. A terminal that deserves to
be called smart is a graphics terminal which can rapidly display full-
screen graphics just like a PC monitor. It will also have a mouse.
Bytes sent to it often represent bit-maps for pictures (and other
graphics). It will often use a high-speed connection to its host
computer using twisted pair or coax cable. X-Window terminals are
such devices. See the link to Xterminal-HOWTO at ``Related HOWTO's''
For displaying a MS-Windows GUI there are at various types of
interfaces and terminals: Winterm using WinFrame software from Citrix
is one. Another (based in part on Citrix's code) is Hydra (code name)
by Microsoft, also known as "Windows Terminal Server" which works with
versions 4 or higher of MS Windows NT. Citrix uses its ICA protocol
and has created an add-on to Hydra known as pICAsso so that WinFrame
(ICA) based terminals can use the Hydra system. Hydra is also multi-
user. There is also the "MultiConsole Personal Terminal" by Unbounded
Technologies and Tektronix had its own multi-user interface but will
now support Hydra. A magazine article in 1997 called Winterm a "dumb
terminal" but it's really pretty smart. Such terminals are often
called "thin clients", but some thin clients are more that just
terminals as they can execute Java code sent to them, etc.
2.4. Network Computers (NCs)
These are neither true computers nor terminals but are something in-
between. One type of network computer (NC's) is a computer with a CPU
but no hard Disk. They are full-graphics and connect to a server
computer. They are different from terminals since the programs they
run execute on their own CPU chips. Java code may be sent to them for
execution. IBM calls this a "Network Station". They should work on
IP networks and might work under a server running Linux. Wintel
established a "NetPC" which, unlike the above, is almost a PC
computer. However, it has no removable disks so users can't install
their own software or obtain copies of anything. For using the Sun
JavaStation NC under Linux see the JavaStation-HOWTO released in Apr.
24, 2000.
Although the promoters of NCs and related Window-Terminals projected
that they would replace millions of PCs, it hasn't yet happened. A
major reason is that PCs have come down in price in recent years so
that they are often even cheaper than NCs, etc. Thus for terminals,
the Text-Terminal still predominates.
2.5. Emulation on a PC
Since a PC has a screen and keyboard (as does a terminal) but also has
much more computing power, it's easy to use some of this computing
power to make the PC computer behave like a text terminal. This is
called "terminal emulation". They usually emulate text-terminals.
See ``Terminal Emulation''
3. Quick Install
This is a quick procedure to install a terminal without going through
a ``Setup'' procedure for both the terminal and the host computer. It
probably will not work right if the terminal happens to have been set
up incompatible with the computer. If you don't understand some of it
you'll need to consult other parts of this document for more info.
To install a terminal, first look in /etc/termcap or terminfo.src to
find an entry for it (see ``Terminfo and Termcap (detailed)'').
Figure out what serial port you'll connect it to and what the tty
designation is for that port (e.g. ttyS1, see ``Device Names''). As
the root user, edit /etc/inittab and add a getty command next to the
other getty commands. The format of the getty command depends on
which getty program you use. agetty (called just getty in the Debian
distribution) is the easiest (no configuration file). See the "info"
or "man re getty. For getty parameters use the terminfo (or termcap)
name (such as vt100) for your terminal. Type in a baud-rate that the
terminal supports. But if you set the baud too high you may need to
use (See``Flow Control'').
Then physically connect the main serial port of the terminal to the
chosen serial port of the computer with a null-modem cable and turn on
the terminal. Don't expect most ready-made cables to be wired
correctly for hardware flow control. Make sure the baud-rate of the
terminal is set the same as you gave to getty and that its "data bits"
is 8. Then at the computer console type "init q" to apply the changes
you made to the inittab file. You should now see a login prompt at
the terminal. If you don't, tap the terminal's return key. If this
doesn't work read more of this document and/or see ``Trouble-
Shooting''.
4. Why Use a Terminal ?
4.1. Intro to Why Use a Terminal
PC's are so powerful today that just one PC can often support several
persons using it at once, especially if they are doing low-load tasks
such as text editing, data entry, etc. One way to do this is to
connect a number of terminals to a single PC (or other host computer)
by modems or direct cable connection. To do this, it's usually best
to have a multi-user operating system such as Linux so that each user
at a terminal can use the computer independently. This has been
called "time sharing" but it's not good terminology today since
"distributed" computing over a network is also a type of time sharing.
It might be better described as "centralized" computing. But the
central computer may be connected to the rest of the world via a
network so that terminal users may send email, browse the Internet
with the "lynx" browser, etc. So it's not exactly "centralized"
either.
Terminals have seldom been used with PC's because the popular
operating systems used for them (Windows, DOS, and Mac) were not
multiuser until 1998 (available for MS Windows NT) and previously
could not support terminals very well. Now that Linux, a multiuser
operating system, is freely available for PC's, the use of terminals
with PC's becomes more feasible. The drawback is that text terminals
are not smart enough to support the type of graphical user interface
(GUI) that many computer users today normally expect.
4.2. Lower Hardware Costs ?
When Computers (including PCs) were quite expensive, lower hardware
costs was a significant advantage of using terminals. Today with
cheap PCs, the cost savings is problematical. Here's what I wrote
years ago when PCs were more expensive. It's still true today but of
less significance.
If several people use the same computer as the same time, there is a
reduction in the amount of hardware needed for the same level of
service. One type of savings is due to code sharing. The application
files on hard disks are shared as well as shared libraries in memory
(even when people are running different programs provided they use
some of the same functions in their code). Another type of savings is
due to reduction of peak load. The hardware of a single PC may be
idle most of the time as people slowly type in information, think,
talk, or are away from their desks. Having several people on the same
computer at once makes good use of much of this idle time which would
otherwise be wasted.
These savings are substantial. One may roughly estimate (using
statistical theory) that for 9 persons (8 terminals & 1 console) the
shared PC only needs only about 3 times as much capacity (in memory,
disk storage, CPU power, etc.) as a single PC in order to provide the
same level of service per person. Thus the computational hardware for
such a shared system should only cost about 1/3 as much per user.
However, the cost of the display hardware (CRT's, keyboards, video
electronics, etc.) is about the same for both cases. The terminals
have the added cost of requiring additional serial ports at the host
computer.
For a fair comparison with PC's, the terminals should have the same
capabilities as the PC monitors. Unfortunately, color graphic
terminals for Linux (X-windows) with high speed communication is a
niche market with high prices so in this case there is not likely to
be any savings in hardware costs. But for text terminals there will
be some savings, especially if the terminals are obtained used at low
cost.
4.3. Control of Software
For centralized computing, software (and the updates to software) only
need be installed and configured on one host computer instead of
several. The person in charge of this computer may control and
configure the software which is installed on it. This is advantageous
if the person controlling the host computer does an excellent job and
knows about the needs and preferences of the other users. Users can
be restricted in playing games or surfing the Internet by not
installing the software (or by otherwise restricting access to it).
Whether or not centralized control is desirable depends on the
situation.
4.4. Hardware Upgrades
With terminals, the computer hardware upgrades take place on only one
computer instead of many. This saves installation labor effort.
While the cost of the hardware for the host computer upgrade will be
more than that for a single PC (since the host needs more computing
power than a PC), the cost will be significantly less than upgrading
the hardware of a number of PC's being used instead of terminals.
4.5. Other Advantages of Terminals
� The elimination of noise from fans and disk drives provided the
terminals are not close to the computer.
� The users of the terminals can share data and files and send e-mail
to each other. It's similar to a local network.
4.6. Major Disadvantages of Terminals
� Text terminals have no high-speed graphic display (or high
resolution graphics) although they can often use graphic character
sets to draw boxes, etc. This lack limits the software that may be
used on it.
� If the host computer goes down, then no one can use the terminals
either (unless there is a "standby" host computer to connect to).
4.7. Are Text Terminals Obsolete ?
Text terminals are technologically obsolete because for a slightly
higher cost of hardware, one could build a smart terminal (with the
same quality of display). This wasn't always the case since around
1980 memory cost thousands of dollars per megabyte. Today with low
costs for memory and processors, one could make a text terminal smart
for only about a 10% or 20% increase in hardware cost.
The reasons that text terminals are not yet obsolete are:
� There is no satisfactory standard interface for smart graphics
terminals. The MS Hydra system is for MS Windows NT, while X-
Windows is not as efficient as it should be (and X-Windows
terminals are too costly).
� Many people don't need full screen graphics.
� Text terminals are low in cost and allegedly take longer to become
obsolete, yet can give access to a much newer (and powerful)
computer.
� Since running a text-terminal (in contrast to a full-graphics
terminal) doesn't consume much of a modern PC's resources, a large
number of terminals may be efficiently run from one PC.
5. Overview of How Terminals Work (in Linux)
See also section ``Some Details on How Terminals Work''
5.1. Device Names
Each terminal is connected to a serial port on the host computer
(often just a PC). The ports have names: ttyS0, ttyS1, ttyS2 etc.
These are represented by special files found in the /dev (device)
directory. /dev/ttyS0 corresponds to COM1 in DOS or Windows. ttyS1
is COM2, etc. See ``Terminal Special Files'' for details on these and
related "devices" such as cua.
5.2. Login/Logout
When the host computer starts up it runs the program getty. The getty
program runs the "login" program to log people in. See ``Getty (in
/etc/inittab)''. A "login:" prompt appears on the screen. People at
the terminals log in (after giving their passwords) and then have
access to the computer. When it's time to shut the terminal down, one
normally logs out and turns the terminal off. See ``Login
Restrictions'' regarding restricting logins (including allowing the
root user to log in at terminal).
5.3. Half/Full Duplex
If one watches someone typing at a terminal, the letters one types
simultaneously appear on the screen. A naive person might think that
what one types is being sent directly from the keyboard to the screen
with a copy going to the computer (half-duplex like, see next
paragraph). What is usually going on is that what is typed at the
keyboard is directly sent only to the host computer which in turn
echoes back to the terminal each character it receives (called full-
duplex). In some cases (such as passwords or terse editor commands)
the typed letters are not echoed back.
Full-duplex means that there are two (dual) one-way communication
links. Full-duplex is the norm for terminals. Half-duplex is half of
a duplex, meaning that there is only a single one-way communication
link. This link must be shared by communications going in both
directions and only one direction may be used at a time. In this case
the computer would not be able to echo the characters you type (and
send to it) so the terminal would need to also send each character you
type directly to the terminal screen. Some terminals have a half-
duplex mode of operation which is seldom used.
5.4. Terminal Memory
The image on a CRT tube will fade away almost instantly unless it is
frequently redrawn on the screen by a beam of electrons shot onto the
face of the tube. Since text sent to a terminal needs to stay on the
screen, the image on the screen must be stored in the memory chips of
the terminal and the electron beam must repeatedly scan the screen
(say 60 times per second) to maintain the image. See ``Terminal
Memory Details'' for more details.
5.5. Commands for the Terminal
The terminal is under the control of the computer. The computer not
only sends the terminal text to display on the screen but also sends
the terminal commands which are acted on. These are ``Control Codes''
(bytes) and ``escape sequences''. For example, the CR (carriage
return) control code moves the cursor the the left hand edge of the
screen. A certain escape sequence (several bytes where the first byte
is the "escape" control code) can move the cursor to the location on
the screen specified by parameters placed inside the escape sequence.
The ``first terminals'' had only a few such commands but modern
terminals have hundreds of them. The appearance of the display may be
changed for certain regions: such as bright, dim, underline, blink,
and reverse video. A speaker in a terminal can "click" when any key
is pressed or beep if a mistake has occurred. Function keys may be
programmed for special meanings. Various fonts may exist. The
display may be scrolled up or down. Specified parts of the screen may
be erased. Various types of flow control may be used to stop the flow
of data when bytes are being sent to the terminal faster than the
terminal can handle them. There are many more as you will see from
looking over an advanced terminal manual or from the Internet links
``Esc Sequence List''
5.6. Lack of Standardization Solved by Terminfo
While terminals made for the US all used the same ASCII code for the
alphabet (except for IBM terminals which used EBCDIC), they
unfortunately did not all use the same escape sequences. This
happened even after various ANSI (and ISO) standards were established
since these standards were never quite advanced enough. Furthermore,
older terminals often lacked the capabilities of newer terminals.
This might cause problems. For example, the computer might send a
terminal an escape sequence telling it to split the screen up into two
windows of specified size, not realizing that the terminal was
incapable of doing this.
To overcome these problems a database called "termcap" (meaning
"terminal capabilities") was established. Termcap was superceded by
"terminfo". This database resides in certain files on the computer
and has a section of it (sometimes an entire file) for each model of
terminal. For each model (such as VT100) a list of capabilities is
provided including a list of certain escape sequences available. For
example blink=\E5m means that to make the cursor start blinking the
terminal must be sent: Escape 5 m. See Section ``Termcap and Terminfo
(detailed)'' for more details. Application programs may utilize this
database by calling certain C-Library functions. One large set of
such programs (over 200) is named "ncurses" and are listed in the
manual page for "ncurses".
5.7. The Interface
The environment variable TERM is the type of terminal Linux thinks you
are using. Some application programs use this to look up the
capabilities in the terminfo database so TERM needs to be set
correctly. But there is more to a correct interface than the
computer knowing about the capabilities of the terminal.
For bytes to flow from the computer to the terminal the terminal must
be set to receive the bytes at the same baud rate (bits per second) as
they are sent out from the terminal. If the terminal is set to
receive at 19,200 baud and the computer sends out characters at 9600
baud, only garbage (or perhaps nothing) will be seen on the screen.
One selects the baud rate for a terminal (as well as many other
features) from the terminals "set-up" menus at the terminal. Most
terminals have a large number of options in their "set-up" menus (see
``Terminal Set-Up (Configure) Details''). The computer serial port
has options also and these options must be set up in a compatible way
(see ``Computer Set-Up (Configure) Details''.
5.8. Emulation
Most terminals today have more than one emulation (personality or
"terminal mode"). The terminal model numbers of terminals formerly
made by DEC (Digital Equipment Corporation now Compaq) start with VT
(e.g. VT100). Many other terminals which are not VT100 may be set up
to emulate a VT100. Wyse is a major terminal manufacturer and most of
their terminals can emulate various DEC terminals such at VT100 and
VT220. Thus if you want to, say, use a VT320 terminal you may either
use a real VT320 in "native" personality or possibly use some other
terminal capable of emulating a VT320. The "native" personalities
usually have more capabilities so, other things being equal, "native"
is usually the best to use.
The most common type of emulation is to use a PC like it was a vt100
terminal (or the like). Programs loaded into the PC's memory permits
this. In Linux (unless you're in X-windows) the PC monitor (called
the console) emulates a terminal of type "Linux" (close to vt100).
Even certain windows within X-windows emulate terminals. See
``Terminal Emulation''.
5.9. The Console
On a PC, the monitor is normally the console. It emulates a terminal
of type "Linux". One logs on to it as a virtual terminal. See ``The
Console: /dev/tty?''. It receives messages from the kernel regarding
booting and shutdown progress. One may have the messages that
normally go to the console, go to the terminal. To get this you must
manually patch the kernel, except that for kernel 2.2 (or higher) it
is a "make config" option. See ``Make a Terminal the Console''.
6. Terminal Special Files such as /dev/tty
"tty" is an abbreviation for "Teletype". The first terminals were
Teletypes (like remotely controlled typewriters). See subsection
``Teletypes''. A list of Linux devices (the stuff in the /dev
directory) may be found in "Linux Allocated Devices" which should be
included with kernel sources. It "describes" what each device used
for in only a word or two but doesn't tell you how to use them.
6.1. Serial Port Terminals
The computer considers each serial port to be a "device". It's
sometimes called a terminal device since at one time terminals were
the common use for the serial port. For each such serial port there
is a special file in the /dev (device) directory. /dev/ttyS0 is the
special file for the serial port known as COM1 in the DOS/Windows
world. To send text to a terminal you may redirect standard output of
some command-line command to the appropriate special file. For
example typing "echo test > /dev/ttyS1" at the command prompt should
send the word "test" to the terminal on ttyS1 (COM2) provided you have
write permission on /dev/ttyS1. Similarly, typing "cat my_file >
/dev/ttyS0" will send the contents of the file my_file to COM1
(ttyS0).
In addition to ttyS0 (/dev/ttyS0), ttyS1, ttyS2, etc. (the "S" stands
for Serial port) there is also a "cua" series: cua0, cua1, cua2, etc.
cua0 is the same port as ttyS0, etc. The "cu" of cua stands for
CalloUt. The ttyS series are Posix compliant while using cua may
permit the opening of a port that the modem control lines say is not
ready. Starting with kernel version 2.2 cua is obsolete and a warning
message is issued when you attempt to use it (although it still
works). For the past few years it has only been included with Linux
for backwards compatibility. A programmer can arrange things so that
ttyS can behave just like cua, so cua is not really needed.
6.2. Pseudo Terminals
Pseudo terminals have no physical connector on the computer. They are
used to emulate a serial port. They have no IO address nor IRQ. For
example, if someone connects via telnet to your computer over a
network, they may wind up connected to the device /dev/ptyp2 (a pseudo
terminal port). In X-Windows, the terminal emulator program, xterm
(or rxvt), uses pseudo terminals. Ham radio programs under Linux also
use them. Using certain application software it is possible to have 2
or more pseudo terminals attached to the same physical serial port.
Pseudo terminals come in pairs such as ttyp3 and ptyp3. The pty... is
the master or controlling terminal and the tty... is the slave. ttyq5
is also a pseudo terminal as is ttysc (c is a hexadecimal digit).
More precisely, pseudo master terminals are /dev/pty[p-s]N and the
corresponding slaves are /dev/tty[p-s]N where N is a hexadecimal digit
0-f. The slave /dev/ttyp looks just like a serial port to an
application program that reads and writes to it. However the
application program is really communicating with whatever program is
writing and reading the corresponding master /dev/pty. Thus the
master and slave are really the same "port" but one is used by the
application program and the other is used by a network program (or the
like) which supplies (and gets) data to/from the "port".
Unix98 doesn't use the above but instead uses a "pty master" which is
/dev/ptmx. This can supply a pty on demand. Details on using pseudo
terminals with other programs are in a few manual pages but a manual
page devoted to only to pseudo terminals is needed for Linux. Some
other unix-like systems have such a page named "pty - pseudo terminal
driver".
6.3. The Controlling Terminal /dev/tty
/dev/tty stands for the controlling terminal (if any) for the current
process. To find out which tty's are attached to which processes use
the "ps -a" command at the shell prompt (command line). Look at the
"tty" column. For the shell process you're in, /dev/tty is the
terminal you are now using. Type "tty" at the shell prompt to see
what it is (see manual pg. tty(1)). /dev/tty is something like a link
to the actually terminal device name with some additional features for
C-programmers: see the manual page tty(4).
6.4. /dev/ttyIN "Terminals"
N stands for an integer. One use of these in Linux is with the ISDN
driver package: isdn4linux. The ttyIN is something like ttySN. There
is also a cuiN which is something like cuaN. The ttyI and cui
emulate modems and may be given modem commands.
6.5. The Console: /dev/ttyN
In Linux the PC monitor is usually called the console and has several
device special files associated with it: tty0, tty1, tty2, etc. When
you log in you are on tty1. To go to tty2 (on the same screen) For
tty3 use Left Alt-F3, etc. These (tty1, tty2, tty3, etc.) are called
"virtual terminals". tty0 is just an alias for the current virtual
terminal and it's where messages from the system are sent. Thus
messages from the system will be seen on the console (monitor)
regardless of which virtual terminal it is displaying.
You may log in to different virtual terminals and thus have a few
different sessions with the computer going on at the same time. Only
the system or the root user may write to /dev/tty0 to which
/dev/console is sometimes linked. For more info on the console see
``The Linux Console''.
6.6. Creating a Device with "mknod"
The /dev directory comes supplied with many device special files. If
you need something that's not there you may try to create it with the
"mknod" command. See the manual page ttys(4) for how to do this for
serial ports. To use mknod you must know the major and minor device
numbers. You might be able to infer the numbers you need by using the
"ls -l" command in the /dev directory. It will display the major and
minor numbers of existing special files.
7. Some Details on How Terminals Work
If you know almost nothing about terminals, it's suggested that you
first read ``Introduction'' and also read ``Overview of How Terminals
Work''.
7.1. Terminal Memory Details
The terminal screen refreshes itself at perhaps 60 times per second
from an image stored in the memory of the terminal. For a PC the
monitor's image is stored on the video card inside the computer but
for a terminal, the equivalent of the video card is inside the
terminal. For a text terminal the storage of the image uses little
memory. Instead of putting every dot (pixel) on the screen into
memory and requiring the storage of about a quarter-million dots, a
much more efficient method of storage is used.
A screen-full of text may be represented inside the terminal memory by
ASCII bytes, one for each character on the screen. An entire screen
only takes about 2K ASCII bytes. To display these characters, the
terminal must also know the bit-map (the shape) of each of the almost
100 printable ASCII characters. With a bit-map of a character using
say 15 bytes, only about 1.5K of memory is needed for the bit-maps of
all the ASCII characters (the font). This ASCII text and font memory
is scanned so that the resulting image is put on the screen about 60
times each second. This is a form of shared memory where a single
bit-map of a letter such as the letter e, is shared by all of the many
letter e's which appear on a screen-full of text. Low memory
requirements meant low costs to produce monitors in the early 1980's
when the cost of memory was several thousand times higher than it is
today (costing then several dollars per kilobyte).
7.2. Early Terminals
The first terminals were something like remotely controlled
typewriters which could only "display" (print on paper) the character
stream sent to them from the computer. The earliest models were
called ``Teletypes''. The name "tty" is just an abbreviation for
"Teletype". Early terminals could do a line feed and a carriage
return just like a typewriter and ring a bell when a bell character
was received. Due to the lack of significant capabilities this was
the first type of terminal to be labeled "dumb". This type of
terminal interface (using a terminal type called "dumb") is sometimes
used today when the computer can't figure out what kind of a terminal
it is communicating with.
7.3. Escape Sequences and Control Codes (intro)
Terminals have many capabilities some of which are always present and
some of which require commands from the computer to change or
activate. To exercise all these capabilities under the control of the
computer requires that special codes be established so that the
computer can tell the terminal what to do. There are two major type
of such codes: escape sequences and control codes (control
characters). There are many times more escape sequences than control
codes.
7.3.1. Control Codes
The control codes (or control characters) consist of the first 32
bytes of the ASCII alphabet. They include the following: carriage-
return (cursor to far left), line-feed (cursor down one line),
backspace, escape-character, tab, and bell. They do not normally show
on the screen. There is usually a command which you may give to your
terminal which will result in them being displayed when they are
received by the terminal. It's called something like "Display
Controls" or "Monitor". If you do this then the display may look a
mess since escape sequences, which all start with the ESC (escape)
control character, are no longer executed. Words which should appear
at the top or bottom of the screen will show up in other locations.
The escape sequences to reposition the cursor display on the screen
but the cursor doesn't move to where the escape sequence says.
7.3.2. Escape Sequences
Since there are not nearly enough control codes to do everything (and
for some reason, not all of them are utilized) many escape sequences
are used. They consist of the "escape" (ESC) control character
followed by a sequence of ordinary characters. Upon receiving an
escape character, the terminal examines the characters following it so
that it may interpret the sequence and carry out the intended command
from the computer. Once it recognizes the end of a valid sequence,
further characters received just display on the screen (unless they
are control codes or more escape sequences). Some escape sequences
may take parameters (or arguments) such as the coordinates on the
screen to move the cursor to. The parameters become a part of the
escape sequence. An ``Esc Sequence List'' is on the web for some
terminals, but it's terse.
A list of the escape sequences for your terminal should be in the
"programmers manual" for the terminal. Except for very old terminals,
there may be two or three hundred such sequences. If you don't have a
such manual it's not easy to find them. Some of the sequences are
available on the Internet. One link is ``Esc Sequence List''. By
searching the Internet for one sequence (such as ESC[5m) you may come
across a long list of them.
Another way to determine some of them is to find the terminfo entry
(termcap) for the terminal and mentally decode it. See ``Terminfo and
Termcap (detailed)'' in this document and/or the ``Termcap Manual'' on
the Internet. Unfortunately, the terminfo (termcap) for a terminal
often does not list all of the escape sequences which the terminal has
available for use, but fortunately, the most important ones are
usually there.
7.4. Display Attributes & Magic Cookies
Terminals have various methods of generating character attributes such
as bold, reverse-video, underlining, etc. There should be no need for
the user to worry about how how this is done, except that it creates
problems for some old terminals and there is sometimes an option for
this in the set-up menu of newer terminals.
The magic cookie method is obsolete. It's the simplest (and worst)
method of defining attributes: Use a certain byte for the start of an
attribute and another to end that attribute. For example, a "start
underlining" magic cookie byte is placed just before the first word to
be underlined. These extra bytes are put into the memory of the
screen page, just like character bytes that display as characters.
But this might foul up the count of the number of characters per line
since non-printable magic cookie characters are intermingled with
other printable characters. This sometimes causes problems.
A better method which uses more memory is to assign an attribute byte
(or half=byte, etc.) to each displayed character. This method is used
by PC video cards (for text) for the common PC monitor.
8. Special Features of Some Terminals
8.1. Color
While the common monochrome terminal is not a color terminal it may
have a fixed "color" display other than white such as green or amber.
All terminals have black (electron beam turned off = zero brightness).
A real color terminal can change the color of the text and background
to many different colors while a monochrome terminal can only change
the brightness of a fixed color.
However, changing the brightness, etc. gives a lot of possibilities.
For example, a black and white (monochrome) terminal can have white,
grey, and black by varying the brightness. Some words can be black on
a light grey background while other are highlighted by black on white.
In addition there is white on black, underlining, and blinking.
Color works like the color on a computer monitor or TV screen. The
CRT has three colors of dots on it with each color controlled by its
own electron beam (3 beams). Monochrome has inherently better
resolution since it doesn't depend on dots permanently fixed on the
screen. For text terminals the only use of color is to differentiate
text and this advantage is not always worth the cost of worse
resolution. Thus monochrome may be better since it also costs less.
8.2. Multiple Sessions
For dual sessions the terminal has two serial ports of equal status.
Each port is connected to a serial port on a different computer. Thus
one may log in to two different computers with each session displaying
in a split-screen window. Alternatively, each session may run full-
screen with a "hot" key (or the like) to switch between sessions. One
could also connect to two different serial ports on the same computer
and log in twice (similar to "virtual terminals" at the console). The
program "screen" will make any ordinary terminal (single session)
connected to a single computer run two or more "sessions".
8.3. Printer/Auxiliary Port
Many terminals have a connector on the rear for such a port. It may
be labeled as "Aux" or "Printer", etc. Some printer ports are for
parallel printers while others are for serial printers. If a printer
is connected to the printer or auxiliary port, then pressing certain
keys will print the screen. One may also have everything that
displays on the screen go also to the printer. If the port is an
auxiliary port, one may connect this to another computer and almost
have dual sessions as above. However, the video memory inside the
terminal may not retain both sessions so you may need to refresh the
screen when switching to the other session. There will likely not be
a hot key either but possibly a programmable function key may be
programmed to do this. There exists various key combinations and
escape sequences for controlling such a port. See ``Printer Esc''.
There is a program called vtprint which is designed to send a print
job (text only) to your terminal to be printed on a printer attached
to the terminal. It's homepage is . It's also included (as of 1998)
in the Debian distribution of Linux. xprt (also in Debian) seems to
do something similar, but only for X-Window terminals ??
8.4. Pages
Many terminals permit the storage of more than one page in their video
memory. Sometimes the page size is the same as the screen, but
sometimes it is larger so that scrolling will reveal unseen parts of a
page. So when one looks at a screen, there may be hidden text on the
same page above or below the display. In addition, if there is more
than just one page, there may be hidden text on these other pages.
One use for pages is on terminals that support dual sessions. Each
session may have its own page and one may switch back and forth
between them.
Even if you only have a one-page-terminal with the page sized equal to
what is displayed on the screen, you will still see other pages of a
file (etc.) as the host sends more data to the terminal. One
advantage to having additional pages stored in the terminal memory is
so that you can jump to them instantly without waiting a second or so
for them to be transmitted from the host.
Multiple pages is supported by ncurses. There is also a commercial
program called "Multiscreen" which supports multiple pages but
probably not for Linux ?? Multiscreen is reported to be part of SCO
and is something like the virtual terminals on a Linux PC console.
The Linux program "screen" makes it look like you have multiple pages
but they are stored in the computer and but you can have only one
page-like window for each running program.
8.5. Character-Sets
A character-set is normally represented by a list (or table or chart)
of characters along with the byte code assigned to each character.
The codes for a byte range from 0 to 255 (00 to FF in hexadecimal).
In MS-DOS, character-set tables are called "code-pages". You should
examine such a table if you're not familiar with them. They are
sometimes included in printer and terminal manuals but also may be
found on the Internet.
ASCII is one of the most common character-sets used on text terminals.
It is a 7-bit code but can be made into 8-bit if the first bit (high
order bit) is always set to 0. Other character-sets are usually
available (except on very old terminals where the only choice is
ASCII). The first half of most character-sets are the conventional
ASCII characters and the second half (the characters with the high-
order bit set to 1) belong to a wide variety of character-sets.
Character sets are often ISO standards. To get specialized character
sets on a terminal, you may need to download a soft-font for that
character-set into the memory of the terminal.
Besides ASCII, there are some other common character-sets, all 8-bit.
CP stands for Code Page character sets invented by IBM: CP-437 (DOS
ECS), CP-850 (Multilingual Latin 1 --not the same as ISO Latin-1),
ISO-8859-1 (Latin-1), ANSI (derived from Latin-1). MS Windows uses
ANSI while the Internet often uses Latin-1. There are several
ISO-8859 character sets in addition to Latin-1. These include Greek
(-7), Arabic (-6), Eastern European (-2), and a replacement for
Latin-1 (-15) called Latin-9. There are many others. For example,
KOI8-R is more commonly used for Russian than IS0-8859-5. Unicode is
a very large character-set where each character is represented by 2
bytes instead on just one byte.
More info re character-sets are:
� Manual pages: ASCII and latin1
� HOWTO's for various languages (likely written in that language).
See "Cyrillic" for Russian.
� A tutorial on character code issues
<http://www.hut.fi/~jkorpela/chars.html> Clearly written.
� Languages, Countries and Character Sets
<http://www.w3.org/International/O-charset-lang.html>
� Languages of the World by Computers ...
<http://www.threeweb.ad.jp/logos>
� Links re Internationalization
<http://linux.monnet.ru/books/locale/locale_i.html> A long list of
links (in Russian but most words in English).
Once you've found the character set name (or alpha-numeric
designation) you are interested in, you may search for more info about
it on the Internet.
8.6. Fonts
Most terminals made after the mid 1980's can accept downloaded soft-
font. This means that they can display almost any character set
provided that you can find the soft-font for it. If you can't find
the needed soft-font, you can always create your own. A free font
editor for this is called BitFontEdit (written by the author of this
document) and (in 1998) was at
Europe: <http://www.funet.fi/pub/culture/russian/comp/cyril-term/>
N. America:
<ftp://cs.utk.edu/pub/shuford/terminal/BitFontEdit.tar.gz>
8.7. Keyboards & Special Keys
Terminal keyboards often have a number of keys that one doesn't find
on a PC keyboard. Few (if any) actual terminals will have all of
these keys and most will have additional keys not listed here. Some
have a large number of special purpose keys such as terminals made for
use with cash registers. There are often many more key meanings than
shown here since these keys often have extended meanings when used in
conjunction with other keys (such as shift and control).
� BREAK sends a very long 0 bit (space = +12 V) of duration 300 to
700 milliseconds to the host. The host may interpret this as an
interrupt if stty has set brkint or ignore it if ignbrk is set.
� NO SCROLL stops the screen from scrolling like ^S does. Depressing
it again resumes scrolling. Uses flow control signals to do this.
� REPEAT if held down with an other key, forces repeated output of
that other key even if the auto-repeat option is set to off.
� LINE FEED sends the line feed character ^J to the host. Seldom
used.
� SET-UP allows the manual configuration of the terminal via menus.
Sometimes purposely disabled by putting a block under it so it
can't be pressed down. Sometimes another key such as shift or
control must be pressed at the same time. See ``Getting Into Set-Up
(Configuration) Mode''.
� LOCAL disconnects the terminal from the host. In local, what one
types goes directly to the screen. Useful for testing.
� RETURN is the same as the "enter" key on a PC. It usually sends a
carriage return to the host which normally get translated to a new-
line character by the host's device driver. On some terminals it
may be set up to send something else.
� F1, F2, ... or PF1, PF2, ... are function keys which usually may be
programmed to send out a sequence of bytes (characters). See
``Function Keys''
9. Terminal Emulation; the Console
9.1. Intro to Terminal Emulation
Since a PC has a screen and keyboard (as does a terminal) but also has
much more computing power, it's easy to use some of this computing
power to make the PC computer behave like a text terminal. This is
one type of terminal emulation. Another type of terminal emulation is
where you set up a real terminal to emulate another brand/model of
terminal. To do this you select the emulation you want (called
"personality" in Wyse jargon) from the terminal's set-up menu. This
section is about the first type of emulation: emulating a terminal on
a PC.
Emulation software is available for MS Windows and comes built-in with
recent versions of MS Windows. Most Linux free software can only
emulate a VT100, VT102, or VT100/ANSI. If you find out about any
others, let me know. Since most PC's have color monitors but VT100
and VT102 were designed for a monochrome monitor, the emulation
usually adds color capabilities (and a choice of colors). Sometimes
the emulation is not 100% perfect but this usually causes few
problems. For using a Mac computer to emulate a terminal see the
mini-howto: Mac-Terminal.
9.2. Don't Use TERM For Emulation
Some have erroneously thought that they could create an emulator at a
Linux console (monitor) by setting the environment variable TERM to
the type of terminal they would like to emulate. This does not work.
The value of TERM only tells an application program what terminal you
are using. This way it doesn't need to interactively ask you this
question. If you're at the PC monitor it's a terminal of type "Linux"
and your can't change this. So you must set TERM to "Linux".
If you set it to something else you are fibbing to application
programs. As a result they will incorrectly interpret certain escape
sequences from the console resulting in a corrupted interface. Since
the Linux console behaves almost like a vt100 terminal, it could still
work almost OK if you falsely claimed it was a vt100 (or some other
terminal which is something like a vt100). It may seeming work OK
most of the time but once in a while will make a mistake when editing
or the like.
9.3. Communication (Dialing) programs
Dialing programs for making a PPP connection to the Internet don't
normally include any terminal emulation. But some other modem dialing
programs (such as minicom or seyon) do. Using them one may (for
example) dial up public libraries to use their catalogs and indexes,
(or even read magazine articles). They are also useful for testing
modems. Seyon is only for use with X-windows and can emulate
Tektronix 4014 terminals.
The communication program kermit doesn't do terminal emulation as it
is merely a semi-transparent pipe between whatever terminal you are on
and the remote site you are connected to. Thus if you use kermit on a
Linux PC the terminal type will be "Linux". If you have a Wyse60
connected to your PC and run kermit on that, you will appear as a
Wyse60 to the remote computer (which may not be able to handle Wyse60
terminals). Minicom emulates a VT102 and if you use it on Wyse60
terminal the Wyse escape sequences will get translated to VT102 escape
sequences before the data goes out to the modem. Kermit can't do
this.
Emulators exist under DOS such as telix and procomm work just as well.
The terminal emulated is often the old VT100, VT102, or ANSI (like
VT100).
9.3.1. Emulation under X-Windows
Xterm (obsolete ??) may be run under X-Windows which can emulate a
VT102, VT220, or Tektronix 4014. There is also an xterm emulation
(although there is no physical terminal named "xterm"). If you don't
need the Tektronix 4014 emulation (a vector graphics terminal; see
``Graphics Terminals'') you may use eterm. Predecessors to eterm are
rxvt and xvt. eterm supports pixmaps.
For non-Latin alphabets, kterm is for Kanji terminal emulation (or for
other non-Latin alphabets) while xcin is for Chinese. There is also
9term emulation. This seems to be more than just an emulator as it
has a built-in editor and scroll-bars. It was designed for Plan 9, a
Unix-like operating system from AT&T.
9.3.2. Real Terminals Better
Unless you are using X-Windows with a large display, a real terminal
is often nicer to use than emulating one. It usually costs less, has
better resolution for text, and has no disk drives to make annoying
noises.
9.4. Testing Terminal Emulation
For the VT series terminals there is a test program: vttest to help
determine if a terminal behaves correctly like a vt53, vt100, vt102,
vt220, vt320, vt420 etc. There is no documentation but it has menus
and is easy to use. To compile it run the configure script and then
type "make". It may be downloaded from:
<ftp://ftp.clark/net:/pub/dickey/vttest/>. An alternate download site
is: <http://sunsite.unc.edu/pub/Linux/utils/console/>
9.5. The Linux Console
The console for a PC Linux system is normally the computer monitor.
It emulates a terminal of type "Linux". There is no way (unless you
want to spend weeks rewriting the kernel code) to get it to emulate
anything else. Setting the TERM environment variable to type of
terminal other than "Linux" will not result in emulating that other
terminal. It will only result in a corrupted interface since you have
falsely declared (via the TERM variable) that your "terminal" is of a
type different from what it is. See ``Don't Use TERM For Emulation''
The "Linux" emulation is flexible and has features which go well
beyond those of the vt102 terminal which it was intended to emulate.
These include the ability to use custom fonts and easily re-map the
keyboard (without patching the source code and recompiling the kernel
as is required for the case of a real terminal). These extra features
reside in the console driver software and not in the emulation
software but the results are like it was part of the emulation.
Many commands exist (see Keyboard-and-Console-HOWTO) to utilize these
added features. Real terminals, which use neither scan codes nor VGA
cards, unfortunately can't use most of these features. One may
recompile Linux to make a terminal receive the messages which normally
go to the console. See ``Make a Terminal the Console''.
9.6. Emulation Software
Emulators often don't work quite right so before purchasing software
you should try to throughly check out what you will get.
9.6.1. Make a Linux PC a terminal
Unless you want to emulate the standard vt100 (or close to it). There
doesn't seem to be much free terminal emulation software available for
Linux. The free programs minicom and seyon (only for X-windows) can
emulate a vt100 (or close to it). Seyon can also emulate a Tektronix
4014 terminal.
TERM (non-free from Century Software)
<http://www.ecc400.com/censoft/termunix.html> can emulate Wyse60, 50;
VT 220, 102, 100, 52: TV950, 925, 912; PCTERM; ANSI; IBM3101; ADM-1l;
WANG 2110. Block mode is available for IBM and Wyse. It runs on a
Linux PC.
9.6.2. Make a non-Linux PC a terminal
Emulators exist which run on non-Linux PCs. They permit you to use a
non-Linux-PC as a terminal connected to a Linux-PC. Under DOS there
is telix and procomm. Windows comes with "HyperTerminal" (formerly
simply called "Terminal" in Windows 3.x and DOS). Competing with this
is "HyperTerminal Private Edition"
<http://www/hilgraeve.com/htpe/index.html> which is non-free to
business. It can emulate vt-220. Turbosoft's (Australia) TTWin
<http://www.turbosoft.com.au/> can emulate over 80 different terminals
under Windows.
For the Mac Computer there is emulation by Carnation Software
<http://www.webcom.com/carn/carnation/panel-default.html>
One place to check terminal emulation products is Shuford's site, but
it seems to lists old products (which may still work OK). The fact
that most only run under DOS (and not Windows) indicates that this
info is dated. See
<http://www.cs.utk.edu/~shuford/terminal/term_emulator_products.txt>.
10. Flow Control (Handshaking)
Flow control (= handshaking = pacing) is to prevent too fast of a
flow of bytes from overrunning a terminal, computer, modem or other
device. Overrunning is when a device can't process what it is
receiving quickly enough and thus loses bytes and/or makes other
serious errors. What flow control does is to halt the flow of bytes
until the terminal (for example) is ready for some more bytes. Flow
control sends its signal to halt the flow in a direction opposite to
the flow of bytes it wants to stop. Flow control must both be set at
the terminal and at the computer.
There are 2 types of flow control: hardware and software (Xon/Xoff or
DC1/DC3). Hardware flow control uses dedicated signal wires such as
RTS/CTS or DTR/DSR while software flow control signals by sending DC1
or DC3 control bytes in the normal data wires. For hardware flow
control, the cable must be correctly wired.
The flow of data bytes in the cable between 2 serial ports is bi-
directional so there are 2 different flows (and wires) to consider:
1. Byte flow from the computer to the terminal
2. Byte flow from the terminal keyboard to the computer.
10.1. Why Is Flow Control Needed ?
You might ask: "Why not send at a speed slow enough so that the device
will not be overrun and then flow control is not needed?" This is
possible but it's usually significantly slower than sending faster and
using flow control. One reason for this is that one can't just set
the serial port baud rate at any desired speed such as 14,500, since
only a discrete number of choices are available. The best choice is
to select a rate that is a little higher than the device can keep up
with but then use flow control to make things work right.
If one decides to not use flow control, then the speed must be set low
enough to cope with the worst case situation. For a terminal, this is
when one sends escape sequences to it to do complex tasks that take
more time than normal. In the case of a modem (with data compression
but no flow control) the speed from the computer to the modem must be
slow enough so that this same speed is usable on the phone line, since
in the worst case the data is random and can't be compressed. If one
failed to use flow control, the speed (with data compression turned
on) would be no faster than without using any compression at all.
Buffers are of some help in handling worst case situations of short
duration. The buffer stores bytes that come in too fast to be
processed at once, and saves them for processing later.
10.2. Padding
Another way to handle a "worst case" situation (without using flow
control or buffers) is to add a bunch of nulls (bytes of value zero)
to escape sequences. Sometimes DEL's are used instead provided they
have no other function. See ``Recognize Del''.
The escape sequence starts the terminal doing something, and while the
terminal is busy doing it, it receives a bunch of nulls which it
ignores. When it gets the last null, it has completed its task and is
ready for the next command. This is called null padding. These nulls
formerly were called "fill characters". These nulls are added just to
"waste" time, but it's not all wasted since the terminal is usually
kept busy doing something else while the nulls are being received. It
was much used in the past before flow control became popular. To be
efficient, just the right amount of nulls should be added and figuring
out this is tedious. It was often done by trial and error since
terminal manuals are of little or no help. If flow control doesn't
work right or is not implemented, padding is one solution. Some of
the options to the stty command involve padding.
10.3. Overrunning a Serial Port
One might wonder how overrunning is possible at a serial port since
both the sending and receiving serial ports involved in a transmission
of data bytes are set for the same speed (in bits/sec) such as 19,200.
The reason is that although the receiving serial port electronics can
handle the incoming flow rate, the hardware/software that fetches and
processes the bytes from the serial port sometimes can't cope with the
high flow rate.
One cause of this is that the serial port's hardware buffer is quite
small. Older serial ports had a hardware buffer size of only one byte
(inside the UART chip). If that one received byte of data in the
buffer is not removed (fetched) by CPU instructions before the next
byte arrives, that byte is lost (the buffer is overrun). Newer
UART's, namely most 16550's, have 16-byte buffers (but may be set to
emulate a one-byte buffer) and are less likely to overrun. It may be
set to issue an interrupt when the number of bytes in its buffer
reaches 1, 4, 8, or 14 bytes. It's the job of another computer chip
(usually the main CPU chip for a computer) to take these incoming
bytes out of this small hardware buffer and process them (as well as
perform other tasks).
When contents of this small hardware receive buffer reaches the
specified limit (one byte for old UART'S) an interrupt is issued.
Then the computer interrupts what it was doing and software checks to
find out what happened. It finally determines that it needs to fetch
a byte (or more) from the serial port's buffer. It takes these
byte(s) and puts them into a larger buffer (also a serial port buffer)
that the kernel maintains in main memory. For the transmit buffer,
the serial hardware issues an interrupt when the buffer is empty (or
nearly so) to tell the CPU to put some more bytes into it to send out.
Terminals also have serial ports and buffers similar to the computer.
Since the flow rate of bytes to the terminal is usually much greater
than the flow in the reverse direction from the keyboard to the host
computer, it's the terminal that is most likely to suffer overrunning.
Of course, if you're using a computer as a terminal (by emulation),
then it is likewise subject to overrunning.
Risky situations where overrunning is more likely are: 1. When
another process has disabled interrupts (for a computer). 2. When the
serial port buffer in main (or terminal) memory is about to overflow.
10.4. Stop Sending
When its appears that the receiver is about to be overwhelmed by
incoming bytes, it sends a signal to the sender to stop sending. That
is flow control and the flow control signals are always sent in a
direction opposite to the flow of data which they control (although
not in the same channel or wire). This signal may either be a control
character (^S = DC3 = Xoff) sent as an ordinary data byte on the data
wire (in-band signalling), or a voltage transition from positive to
negative in the dtr-to-cts (or other) signal wire (out-of-band
signalling). Using Xoff is called "software flow control" and using
the voltage transition in a dedicated signal wire (inside the cable)
is called hardware flow control.
10.5. Keyboard Lock
With terminals, the most common case of "stop sending" is where the
terminal can't keep up with the characters being sent to it and it
issues a "stop" to the PC. Another case of this is where someone
presses control-S. Much less common is the opposite case where the PC
can't keep up with your typing speed and tells the terminal to stop
sending. The terminal "locks" its keyboard and a message or light
should inform you of this. Anything you type at a locked keyboard is
ignored.
The term "locked" is also sometimes used for the common case of where
the computer is told to stop sending to a terminal. The keyboard is
not locked so that whatever you type goes to the computer. Since the
computer can't send anything back to you, characters you type don't
display on the screen and it may seem like the keyboard is locked.
Scrolling is locked (scroll lock) but the keyboard is not locked.
10.6. Resume Sending
When the receiver has caught up with its processing and is ready to
receive more data bytes it signals the sender. For software flow
control this signal is the control character ^Q = DC1 = Xon which is
sent on the regular data line. For hardware flow control the voltage
in a signal line goes from negative (negated) to positive (asserted).
If a terminal is told to resume sending the keyboard is then unlocked
and ready to use.
10.7. Hardware Flow Control (RTS/CTS etc.)
Some older terminals have no hardware flow control while others used a
wide assortment of different pins on the serial port for this. For a
list of various pins and their names see ``Standard Null Modem Cable
Pin-out''. The most popular pin to use seems to be the DTR pin (or
both the DTR pin and the DSR pin).
10.7.1. RTS/CTS, DTR, and DTR/DSR Flow Control
Linux PC's use RTS/CTS flow control, but DTR/DSR flow control (used by
some terminals) behaves similarly. DTR flow control (in one direction
only and also used by some terminals) is only the DTR part of DTR/DSR
flow control.
RTS/CTS uses the pins RTS and CTS on the serial (EIA-232) connector.
RTS means "Request To Send". When this pin stays asserted (positive
voltage) at the receiver it means: keep sending data to me. If RTS is
negated (voltage goes negative) it negates "Request To Send" which
means: request not to send to me (stop sending). When the receiver is
ready for more input, it asserts RTS requesting the other side to
resume sending. For computers and terminals (both DTE type equipment)
the RTS pin sends the flow control signal to the CTS pin (Clear To
Send) on the other end of the cable. That is, the RTS pin on one end
of the cable is connected to the CTS pin at the other end.
For a modem (DCE equipment) it's a different scheme since the modem's
RTS pin receives the signal and its CTS pin sends. While this may
seem confusing, there are valid historical reasons for this which are
too involved to discuss here.
Terminals usually have either DTR or DTR/DSR flow control. DTR flow
control is the same as DTR/DSR flow control but it's only one-way and
the DSR pin is not used. For DTR/DSR flow control at a terminal, the
DTR signal is like the signal sent from the RTS pin and the DSR pin is
just like the CTS pin.
10.7.2. Connecting Up DTR or DTR/DSR Flow Control
Some terminals use only DTR flow control. This is only one-way flow
control to keep the terminal from being overrun. It doesn't protect
the computer from someone typing too fast for the computer to handle
it. In a standard null modem cable the DTR pin at the terminal is
connected to the DSR pin at the computer. But Linux doesn't support
DTR/DSR flow control (although drivers for some multiport boards may
support DTR/DSR flow control.) A way around this problem is to simply
wire the DTR pin at the terminal to connect to the CTS pin at the
computer and set RTS/CTS flow control (stty crtscts). The fact that
it's only one way will not affect anything so long as the host doesn't
get overwhelmed by your typing speed and drop RTS in a vain attempt to
lock your keyboard. See ``Keyboard Lock''. For DTR/DSR flow control
(if your terminal supports this two-way flow control) you do the
above. But you also connect the DSR pin at the terminal to the RTS
pin at the computer. Then you are protected if you type too fast.
10.7.3. Old RTS/CTS Handshaking Is Different
What is confusing is that there is the original use of RTS where it
means about the opposite of the previous explanation above. This
original meaning is: I Request To Send to you. This request was
intended to be sent from a terminal (or computer) to a modem which, if
it decided to grant the request, would send back an asserted CTS from
its CTS pin to the CTS pin of the computer: You are Cleared To Send to
me. Note that in contrast to the modern RTS/CTS bi-directional flow
control, this only protects the flow in one direction: from the
computer (or terminal) to the modem.
For older terminals, RTS may have this meaning and goes high when the
terminal has data to send out. The above use is a form of flow
control since if the modem wants the computer to stop sending it drops
CTS (connected to CTS at the computer) and the computer stops sending.
10.7.4. Reverse Channel
Old hard-copy terminals may have a reverse channel pin (such as pin
19) which behaves like the RTS pin in RTS/CTS flow control. This pin
but will also be negated if paper or ribbon runs out. It's often
feasible to connect this pin to the CTS pin of the host computer.
There may be a dip switch to set the polarity of this signal.
10.8. Is Hardware Flow Control Done by Hardware ?
Some think that hardware flow control is done by hardware but (unless
you are using an intelligent serial card with several serial ports)
it's actually done by your operating system software. UART chips and
associated hardware usually know nothing at all about hardware flow
control. When a hardware flow control signal is received, the signal
wire flips polarity and the hardware gives an electrical interrupt
signal to the CPU. However, the hardware has no idea what this
interrupt means. The CPU stops what it was doing and jumps to a table
in main memory that tells the CPU where to go to find a program which
will find out what happened and what to do about it.
There's another way this could have been implemented since when the
polarity flips the hardware could have been configured to send an
electrical interrupt signal to the CPU. Then the CPU would stop what
it was doing, jump to a service routine of the serial driver, check
registers in the serial hardware to find out what happened, and make a
note not to resume the flow after the service routine is exited. This
might be a little more efficient, but it seems that Linux doesn't do
it this way. I once thought it did.
Note that with either method the flow of bytes stops nearly instantly.
However, any bytes (up to 16) which were already in the serial port's
hardware transmit buffer will still get transmitted. Using software
flow control requires that each incoming byte be checked to see if
it's an "off" byte. These bytes are delayed by passing thru the
16-byte receive buffer. If the "off" byte was the first byte into
this buffer, there could be a wait while 15 more bytes were received.
Then the 16 bytes would get read and the "off" byte found. This extra
delay doesn't happen with hardware flow control.
10.9. Obsolete ?? ETX/ACK or ENQ/ACK Flow Control
This is also software flow control and requires a device driver that
knows about it. Bytes are sent in packets (via the async serial port)
with each packet terminated by an ETX (End of Text) control character.
When the terminal gets an ETX it waits till it is ready to receive the
next packet and then returns an ACK (Acknowledge). When the computer
gets the ACK, it then send the next packet. And so on. This is not
supported by Linux ?? Some HP terminals use the same scheme but use
ENQ instead of ETX.
11. Physical Connection
Multiport boards allow many terminals (or modems) to be connected to
one PC computer. A terminal may be connected to its host computer
either by a direct cable connection, via a modem, or via a terminal
server.
11.1. Multiport I/O Cards (Adapters)
Additional serial cards may be purchased which have many serial ports
on them called "multiport boards". These boards are not covered in
this HOWTO but there is quite a lot of coverage in the Serial-HOWTO
One company which had (in 1998) below average prices is ByteRunner
<http://www.byterunner.com/cgi-bin/goto.cgi?FILE=iocards.html>.
11.2. Direct Cable Connection.
The simplest way to connect a terminal to a host computer is via a
direct connection to a serial port on the computer. You may also use
some the info in this section for connecting one computer to another
(via the serial port). Most PC's come with a couple of serial ports,
but one is usually used by a mouse. For the EIA-232 port, you need a
null modem cable that crosses over the transmit and receive wires. If
you want hardware flow control, you will probably use the DTR pin (or
both the DTR and DSR pins).
Make sure you have the right kind of cable. A null modem cable bought
at a computer store may do it (if it's long enough), but it probably
will not work for hardware flow control. Such a cable may be labeled
as a serial printer cable. See this to help determine if you should
``Buy or Make'' your own cable. Make sure you are connecting to your
PC's serial port at the male DB25 or the DB9, and not your parallel
port (female DB25).
11.2.1. Null Modem Cable Pin-out (3, 4, or 5 conductor)
These 3 diagrams are for real text-terminals. But you could use them
to connect up 2 PCs if you substitute RTS for DTR and CTS for DSR.
(Don't use 4-conductors for PC-to-PC). For terminals, if you only
have DTR flow control (one-way) you may eliminate the RTS-to-DSR wire.
If you have no hardware flow control, then you may also eliminate the
CTS-to-DTR wire. Then if you have 2@ twisted pairs, you may then use
2 wires for signal ground per ``A Kludge using Twisted-Pair Cable''.
For a DB25 connector on your PC, you need:
PC male DB25 Terminal DB25
TxD Transmit Data 2 --> 3 RxD Receive Data
RxD Receive Data 3 <-- 2 TxD Transmit Data
SG Signal Ground 7 --- 7 SG Signal Ground
CTS Clear To Send 5 <--20 DTR Data Terminal Ready
RTS Request To Send 4 --> 6 DSR Data Set Ready
If you have a DB9 connector on your PC, try the following:
PC DB9 Terminal DB25
RxD Receive Data 2 <-- 2 TxD Transmit Data
TxD Transmit Data 3 --> 3 RxD Receive Data
SG Signal Ground 5 --- 7 SG Signal Ground
CTS Clear To Send 8 <--20 DTR Data Terminal Ready
RTS Request To Send 7 --> 6 DSR Data Set Ready **
If you have a DB9 connector on both your serial port and terminal:
PC DB9 Terminal DB9
RxD Receive Data 2 <-- 3 TxD Transmit Data
TxD Transmit Data 3 --> 2 RxD Receive Data
SG Signal Ground 5 --- 5 SG Signal Ground
CTS Clear To Send 8 <-- 4 DTR Data Terminal Ready
RTS Request To Send 7 --> 6 DSR Data Set Ready **
The above don't have modem control lines so be sure to give a "local"
option to getty (which is equivalent to "stty clocal"). Also if you
need hardware flow control it must be enabled at your computer (use a
-h flag with agetty) ( equivalent to "stty crtscts" ).
11.2.2. Standard Null Modem Cable Pin-out (7 conductor)
The following 3 diagrams show full "standard" null modem cables. One
that you purchase is apt to be wired this way. They will work fine
for computer-to-computer connections. They also work for real
terminals using software (Xon/Xoff) flow control (or no flow control).
However, they don't work for terminal hardware flow control since most
real terminals support DTR or DTR/DSR flow control (handshaking) but
Linux doesn't.
PC male DB25 Terminal DB25
TxD Transmit Data 2 --> 3 RxD Receive Data
RxD Receive Data 3 <-- 2 TxD Transmit Data
RTS Request To Send 4 --> 5 CTS Clear To Send
CTS Clear To Send 5 <-- 4 RTS Request To Send
DSR Data Set Ready 6
|
DCD Carrier Detect 8 <-- 20 DTR Data Terminal Ready
SG Signal Ground 7 --- 7 SG Signal Ground
6 DSR Data Set Ready
|
DTR Data Terminal Ready 20 --> 8 DCD Carrier Detect
Alternatively, a full DB9-DB25 null modem cable (will not work with
terminal hardware handshaking; see above):
PC DB9 Terminal DB25
RxD Receive Data 2 <-- 2 TxD Transmit Data
TxD Transmit Data 3 --> 3 RxD Receive Data
6 DSR Data Set Ready
|
DTR Data Terminal Ready 4 --> 8 DCD Carrier Detect
GND Signal Ground 5 --- 7 GND Signal Ground
DCD Carrier Detect 1
|
DSR Data Set Ready 6 <-- 20 DTR Data Terminal Ready
RTS Request To Send 7 --> 5 CTS Clear To Send
CTS Clear To Send 8 <-- 4 RTS Request To Send
(RI Ring Indicator 9 not needed)
(Yes, the pins 2 and 3 really do have the opposite meanings in DB9
connectors than in DB25 connectors!)
Here's how to null-modem connect two DB9's together (but DTR flow
control will not work):
PC DB9 DB9
RxD Receive Data 2 <-- 3 TxD Transmit Data
TxD Transmit Data 3 --> 2 RxD Receive Data
6 DSR Data Set Ready
|
DTR Data Terminal Ready 4 --> 1 DCD Carrier Detect
GND Signal Ground 5 --- 5 GND Signal Ground
DCD Carrier Detect 1
|
DSR Data Set Ready 6 <-- 4 DTR Data Terminal Ready
RTS Request To Send 7 --> 8 CTS Clear To Send
CTS Clear To Send 8 <-- 7 RTS Request To Send
RI Ring Indicator 9 (not used)
Using the above 2 connections provide full modem control signals and
seemingly allow one to set "stty -clocal". Then one must turn on the
terminal first (asserts DTR) before the port may be opened in a normal
manner by getty, etc. But there is likely to be trouble if you fail
to turn on the terminal first (see ``Getty Respawning Too Rapidly'').
For this reason one should use "stty clocal" which is the default
(ignores modem control lines) and the additional wires in these cables
then serve no useful purpose.
In olden days when it may not have been this easy to ignore modem
control signals etc, the following "trick" was done for cables that
lacked conductors for modem control: on your computer side of the
connector, connect RTS and CTS together, and also connect DSR, DCD and
DTR together. This way, when the computer needs a certain handshaking
signal to proceed, it will get it (falsely) from itself.
11.2.3. Length Limitations
A cable longer than a 50 feet or so may not work properly at high
speed. Much longer lengths sometimes work OK, especially if the speed
is low and/or the cable is a special low-capacitance type and/or the
electronics of the receiving end are extra sensitive. It is claimed
that under ideal conditions at 9600 baud, 1000 feet works OK. One way
to cover long distances is to install 2@ line drivers near each serial
port so as to convert unbalanced to balanced (and conversely) and then
use twisted pair cabling. But line drivers are expensive.
11.2.4. Hardware Flow Control Cables
If you expect to use hardware flow control (handshaking) you will
likely need to make up your own cable (or order one made). Of course,
if the connecters on the ends of a used cable remove, you might rewire
it. See ``Installing DB Connectors''. You will need to determine
whether or not the terminal uses the DTR pin for this, and if not,
what pin (or pins) it uses. The set-up menus may give you a clue on
this since there may be an option for enabling "DTR handshaking" (or
flow control) which of course implies that it uses the DTR pin. It
may also use the DSR pin. See ``Hardware Flow Control'' for a
detailed explanation of it. Older terminals may have no provision for
hardware flow control.
11.2.5. Cable Tips
The normal "straight thru" cable will not work unless you are using it
as an extension cable in conjunction with either a null modem cable or
a null modem adapter. Make sure that the connectors on the cable ends
will mate with the connectors on the hardware. One may use telephone
cable which is at least 4-conductor (and possibly twisted pair).
Shielded, special low-capacitance cable computer cable is best.
11.2.6. A Kludge using Twisted-Pair Cable
Although none of the EIA-232 signals are balanced for twisted pair one
may attempt to use twisted-pair cable with it. Use one pair for
transmit and another for receive. To do this connect signal ground to
one wire in each of these 2 pair. Only part of the signal ground
current flows in the desired wire but it may help. Due to the lower
inductance of the twisted pair circuit (as compared to ground return
current by some other path) more return (ground) current will confine
itself to the desired twisted pair than one would expect from only
resistance calculations. This is especially true at higher
frequencies since inductive impedance increases with frequency. The
rectangular wave of the serial port contains high frequency harmonics.
11.2.7. Cable Grounding
Pin 1 (of a DB25) should be chassis ground (also earth ground) but on
cheap serial ports it may not even be connected to anything. A 9-pin
connector doesn't even have a chassis ground. The signal ground is
pin 7 and is usually grounded to chassis ground. This means that part
of the signal current will flow thru the ground wires of the building
wiring (undesirable). Cable shields are supposed to be only grounded
at one end of the cable, but it may be better to ground both ends
since it's better to have current in the shield than in the building
wiring ??
11.3. Modem Connection
Using a terminal-modem combination (without a computer) one may
connect to BBS's. Some BBS's (such a free-nets) permit Internet
access via the text browser lynx which will work on text terminals.
Thus with an old terminal and external modem, one may connect to the
Internet. If one connects to a host computer on which one has an
account, then one may sometimes store ones work (or downloads) there.
11.3.1. Dialing Out From a Terminal
Instead of connecting a terminal (or computer emulating a terminal)
directly to a host computer using a cable it may be connected to the
host via a telephone line (or dedicated private line) with a modem at
each end of the line. The terminal (or computer) will usually dial
out on a phone line to a host computer.
Most people use a PC and modem for dialing out. The PC could have a
terminal connected to a serial port and the person at the terminal may
dial out using the PC. Connecting a real terminal directly to an
external modem is more difficult since the real terminal isn't very
intelligent and doesn't give as much feedback to the user. For
dialing out, many terminals can store one or more telephone numbers as
messages which may be "set-up" into them and are sent out to the modem
by pressing certain function keys. Many modems can also store phone
numbers. The modem initiation sequence must precede the telephone
number. When the outgoing call is answered by another modem at the
other end of the phone line, the the host computer on this modem may
run a getty program to enable you to log in.
11.3.2. Terminal Gets Dialed Into
It's common for a computer running Linux to get dialed into. The
caller gets a login prompt and logs in. At first glance, it may seem
strange how a dumb terminal (not connected to any computer) could
accept an incoming call, but it can. One possible reason for doing
this is to save on phone bills where rates are not symmetric. Your
terminal needs to be set up for dial-in: Set the modem at your
terminal for automatic answer (Register S0 set to 2 will answer on the
2nd ring). You turn on the terminal and modem before you expect a
call and when the call comes in you get a login prompt and log in.
The host computer that dials out to your terminal needs to do
something quite unusual. As soon as your modem answers, it needs to
run login (getty). A host may do this by running the Linux program
"callback" sometimes named "cb". Callback is for having computer A
call computer B, and then B hangs up and calls A back. This is what
you want if you are using computer A to emulate a terminal. For the
case of a real terminal this may be too complex a task so the host may
utilize only the "back" part of the callback program. The setup file
for callback must be properly configured at the host. Callback makes
the call to the terminal and then has mgetty run a login on that port.
Mgetty by itself (as of early 1998) is only for dial-in calls but
there is work being done to incorporate callback features into it and
thus make it able to dial-out. As of early 1999 it didn't seem to
have been done.
11.4. Terminal Server Connection
One use for terminal servers is to connect many terminals (or modems)
to a high speed network which connects to host computers. Of course
the terminal server must have the computing power and software to run
network protocols so it is in some ways like a computer. The terminal
server may interact with the user and ask what computer to connect to,
etc. or it may connect without asking. One may sometimes send jobs to
a printer thru a terminal server.
A PC today has enough computing power to act like a terminal server
for text terminals except that each serial port should have its own
hardware interrupt. PC's only have a few spare interrupts for this
purpose and since they are hard-wired you can't create more by
software. A solution is to use an advanced multiport serial card
which has its own system of interrupts (or on lower cost models,
shares one of the PC's interrupts between a number of ports). See
Serial-HOWTO for more info about such cards. If such a PC runs Linux
with getty running on many serial ports it might be thought of as a
terminal server. It is in effect a terminal server if it is linked to
other PC's over a network and if its job is mainly to pass thru data
and handle the serial port interrupts every 14 (or so) bytes.
Software called "radius" is sometimes used.
Terminal servers evolved to serve more than just terminals. They also
serve PC's which emulate terminals, and were sometimes connected to a
bank of modems connected to phone lines. With the advent of 56k
digital modems that require a digital connection to service an
incoming phone call, a digital interface to the telephone company was
needed. This (and more) is provided today by "remote access servers"
which have replaced the terminal server. Instead of many individual
telephone line cables connected to a terminal server, one now finds
just a few cables with many digitized telephone calls on a each cable
(multiplexed). The multitude of connectors needed for large numbers
of terminals or modems is no longer present on a remote access server
and thus the successor to the terminal server can't readily serve
text-terminals anymore.
11.5. Connector and Adapter Types
A connector is more-or-less permanently attached to the end of a cable
or to a hardware unit. There are two basic types of connectors used
in serial communications: 1. DBxx with pins (such as DB25) and 2.
modular telephone-style connectors.
An adapter looks about like a connector but it has two ends. It is
just like a cable that is so short that there is no cable part left at
all --just different connectors on each end is all that remains. The
adapter just plugs in on each side. It allows two incompatible
connectors to mate with each other by going in between them.
Sometimes the purpose of the adapter is to interchange wires.
Obviously, one may use a special cable (perhaps homemade) as a
substitute for an adapter.
11.5.1. Sex of Connector/Adapters
Connectors (or one side of adapters) are either male or female. The
connectors that have pins are male and the ones that have sockets
(sometimes also called pins) are female. For modular connectors, the
ones with exposed contacts are plugs while the ones with internal
contacts (not easy to see) are jacks. Plugs are male; jacks are
female.
11.5.2. Types of Adapters
There are three basic types of adapters: null modem, gender changers
and port adapters. Some adapters perform more than one of these three
functions.
� null modem adapter: Reroutes wires. Like a null modem cable.
� gender changer: Changes the sex of a cable end. Two connectors of
the same sex can now connect (mate) with each other.
� port adapter: Goes from one type of connector to another (DB9 to DB
25, etc.)
11.5.3. DB Connectors
(For how to install a DB connector on the ends of a cable see
``Installing DB Connectors''.) These come in 9 or 25 pins. The
EIA-232 specs. call for 25 pins but since most of these pins are not
used on ordinary serial ports, 9 pins is sufficient. See ``DB9-DB25''
for the pin-out. The pins are usually numbered if you look closely
enough or use a magnifying glass.
11.5.4. RJ Modular Connectors
These look like modern telephone connectors but are sometimes not
compatible with telephone connectors. See also ``Installing RJ
Connectors''. They may be 6, 8, or 10 conductor. RJ11/14 is a 4-6
conductor telephone plug. A look-alike is a MMJ connector
(6-conductor) used on later model VT (and other) terminals. MMJ has
an offset tab and is not compatible with RJ11/14. However, some
connectors have been made that are compatible with both MMJ or
RJ11/14. The MMJ pin-out is: 1-DTR, 2-TXD, 3-TXD GND, 4-RXD GND,
5-RXD, 6-DSR.
A null-modem cable with MMJ (or RJ11/14) connectors will connect: 1-6,
2-5, and 3-4. Note that such a cable support DTR/DSR flow control
which is not supported (yet) by Linux. Making up your own 6-conductor
null-modem cable is very simple if you understand that the ordinary
4-conductor telephone cable from the wall to your telephone, used in
hundreds of millions of homes, is also a null-modem cable. Find one
and wire your cable the same way.
If you lay such a cable (or your terminal null-modem cable) flat on
the floor (with no twists) you will note that both plugs on the ends
have their gold contacts facing up (or both facing down). Although
it's symmetrical, it is also null- modem if you think about it a bit.
One may put a few such cables together with inline couplers and
everything works OK because each inline coupler is also a null-modem
adapter. Two null-modem devices in series result in a straight-thru
connection.
RJ45 and RJ48 are usually 8-conductor modular telephone plugs.
However some are 10-conductor and are allegedly wider and will not
mate with 8-conductor ones. They are used for both flat telephone
cable and round twisted pair cable. The cable end of the connector
may be different for round and flat cable and both RJ45 and RJ48 may
be 8 or 10 conductor so make sure you get the right one. RJ48 has an
extra tab so that a RJ48 plug will not push into a RJ45 jack (but a
RJ45 plug will mate with a RJ48 jack). They're used on some multiport
serial cards and networks. Heres the pin numbers for an 8-conductor:
Plug Jack
(Looking at the end (Looking at the cavity
end of a cable) in a wall)
.__________. .__________.
| 87654321 | | 12345678 |
|__. .__| |__. .__|
|____| |____|
11.6. Making or Modifying a Cable
11.6.1. Buy or Make ?
You may try to buy a short, null modem cable. They are often labeled
as serial printer cables (but serial printers are not very popular
today and neither are the cables). Unfortunately, they will probably
not work for hardware flow control. Make sure the connectors on the
cable ends will fit the connectors on your computer and terminal.
But if you need longer cables to connect up terminals or need hardware
flow control, how do you get the right cables? The right ready-made
cables may be difficult to find (you might find them by searching the
Internet), especially if you want to use a minimum (say 4) of
conductors. One option is to get them custom made, which is likely to
be fairly expensive although you might find someone to make them at
prices not too much higher than ready-made cable (I did). Another
alternative is to make your own. This may require special tools. If
you can find it, used cable can be a real bargain but you will
probably need to rewire the connectors. Most connectors that come
with short cables are permanently molded to the cable and can't be
rewired but most custom-made and homemade cables have connectors that
can be rewired. One advantage of making your own cable is that the
skills you learn will come in handy if a cable breaks (or goes bad) or
if you need to make up another cable in a hurry.
11.6.2. Pin Numbers
The numbers of the pins should be engraved in the plastic of the
connector. Each pin should have a number next to it. You may need a
magnifying glass to read them.
11.6.3. Installing DB Connectors on Cable Ends
See ``DB Connectors'' for a brief description of them. Unfortunately,
most cables one purchases today have molded connectors on each end and
can't be modified. Others have connectors which unscrew and can be
rewired. If you are making up cable or modifying an existing one then
you need to know about pins. There are two types: soldered and
crimped.
The crimped pins require a special crimping tool and also need an
"insertion/extraction" tool. But once you have these tools, making up
and modifying cable may be faster than soldering. If you are
connecting two wires to one pin (also needed if you want to jumper one
connected pin to another pin) then soldering is faster (for these
pins). This is because the crimped pins can only take one wire each
while the soldered ones can accept more than one wire per pin.
To insert crimped pins just push them in by hand or with the tool.
Removing a pin with the tool is a little tricky. These directions can
be best understood if you have both the tool and wires in front of
you. You must put the tool tip around a wire at the the back of a pin
and insert it as far as it will go into the hole (about 1 1/2 cm.).
Some tools have a mark (such as a tiny hole) on them to indicate how
far to insert it. The tool tip should have a tapered gap so that you
may get the tip around the wire by starting it in where the gap is
wider than the wire. The tool may have 2 tips. The one that is the
most difficult to get around the wire is also the one that removes the
wire the easiest since it almost completely envelops the wire.
With the tip properly inserted pull on both the tool and the wire with
a gentle pull. If it doesn't come out, the tool was likely not
inserted correctly so either push it in more or twist it to a
different position (or both). Perhaps you should have used the other
tip that goes more around the pin. Using this tool, one may readily
convert a straight-thru cable to a null-modem cable, etc.
There can be problems using the "insertion/extraction" tool. If the
tools will not insert on the back of the pin, it could be that the pin
was not neatly crimped to the wire and is sort of square where it
should be round, etc. If a pin starts to come out but will not pull
out all the way, the pin may be bent. Look at it under a magnifying
glass. Straightening a pin with needle-nose pliers may damage the
gold plating. Sometimes a stuck pin may be pushed out with a thick
screwdriver blade tip (or the like) but if you push too hard you may
gouge the plastic hole or bend the pin:.
Don't try soldering unless you know what you're doing or have read
about how to do it.
11.6.4. Installing RJ Connectors
These are telephone modular connecters one type of which is used for
most ordinary telephones. But there are many different types (see
``RJ Modular Connectors'').
These are not easy to reuse. You might be able to pull the wires out,
push in something wedged that would lift up the gold-colored contacts
and reuse the connector. There are special crimping tools used to
install them; a different tool for each type.
If you don't have a crimping tool, installation is still possible (but
difficult) using a small screwdriver (and possibly a hammer). Push in
the cable wires and then push each gold-colored contact down hard with
a small screwdriver that will just fit between the insulating ridges
between the contacts. You may damage it if you fail to use a
screwdriver with a head almost the same thickness as the contacts or
if the screwdriver slips off the contact as you are pushing it down.
You may also use a small hammer to pound on the screwdriver (push
first by hand).
Be sure to not hurt the "remove lever" on the connecter when you push
in the contacts. Don't just set it down on a table and push in the
contacts. Instead, put a shim (about 1 mm thick) that fits snugly in
the crevice between the lever and the body. For such a shim you may
use thick cardboard, several calling cards, or wood. Since the bottom
of the connector (that you will put on the table) isn't level (due to
the "remove lever), make sure that the table top has something a
little soft on it (like a sheet of cardboard) to help support the non-
level connector. Even better would be to put another 1mm shim under
the first 6mm of the connector, supporting it just under where you see
the contacts. A soft tabletop wouldn't hurt either. Another method
(I've never done this) is to hold the connector in a vice but be
careful not to break the connector.
As compared to using a crimping tool, installing it per above takes a
lot longer and is much more prone to errors and failure but it's
sometimes more expedient and a lot cheaper than buying a special tool
if you only have one or two connectors to install.
12. Set-Up (Configure) in General
12.1. Intro to Set-Up
Configuring (Set-Up) involves both storing a configuration in the non-
volatile memory of the terminal, and putting commands in start-up
files (on you hard disk) that will run each time the computer is
powered on (or possibly only when the run-level changes). This
section gives an overview of configuring and covers the configuring of
the essential communication options for both the terminal and the
computer. The next two major sections cover in detail the
configuration of the terminal (see ``Terminal Set-Up'' and the
computer (see ``Computer Set-Up (Configure) Details''.
12.2. Terminal Set-Up (Configure) Overview
When a terminal is installed it's necessary to configure the physical
terminal by saving (in its non-volatile memory which is not lost when
the terminal is powered off) the characteristics it will have when it
is powered on. You might be lucky and have a terminal that has
already been set-up correctly for your installation so that little or
no terminal configuration is required.
There are two basic ways of configuring a terminal. One is to sit at
the terminal and go thru a series of set-up menus. Another is to send
escape sequences to it from the host computer. Before you can send
anything to the terminal (such as the above escape sequences), its
``Communication Interface'') options such as the baud rate must be set
up to match those of the computer. This can only be done by sitting
at the terminal since the communications must be set up right before
the computer and the terminal can "talk" to each other. See
``Terminal Set-Up''.
12.3. Computer Set-Up (Configure) Overview
Besides possibly sending escape sequences from the computer to
configure the terminal, there is the configuring of the computer
itself to handle the terminal. If your lucky, all you need to do is
to put a "getty" command in the /etc/inittab file so that a "login:"
prompt will be sent to the terminal when the computer starts up. See
``Getty (in /etc/inittab)'' for details. this for the computer.
The computer communicates with the terminal using the device driver
software (part of the kernel). The serial device driver has a default
configuration and is also partly (sometimes fully) configured by the
getty program before running "login" at each terminal. However,
additional configuration is sometimes needed using programs named
"stty" and "setserial". These programs (if needed) must be run each
time the computer starts up since this configuration is lost each time
the computer powers down. See ``Computer Set-Up (Configure)
Details''.
12.4. Many Options
There are a great many configuration options for you to choose from.
The communication options must be set right or the terminal will not
work at all. Other options may be set wrong, but will cause no
problem since the features they set may not be used. For example, if
you don't have a printer connected to the terminal it makes no
difference how the printer configuration parameters are set inside the
terminal. This last statement is not 100% correct. Suppose that you
have no printer but the compute (by mistake) sends the terminal a
command to redirect all characters (data) from the computer to the
printer only. Then nothing will display on the screen and your
terminal will be dead. Some terminals have a configuration option to
inform the terminal that no printer is attached. In this case the
terminal will ignore any command to redirect output to the "printer"
and the above problem will never happen. However, this doesn't help
much since there are many other erroneous commands that can be sent to
your terminal that will really foul things up. This is likely to
happen if you send the terminal a binary file by accident.
In some cases a wrong setting will not cause any problem until you
happen to run a rare application program that expects the terminal to
be set a certain way. Other options govern only the appearance of the
display and the terminal will work fine if they are set wrong but may
not be as pleasant to look at.
Some options concern only the terminal and do not need to be set at
the computer. For example: Do you want black letters on a light
background? This is easier on the eyes than a black background.
Should a key repeat when held down? Should the screen wrap when a
line runs off the right end of the screen? Should keys click?
12.5. Communication Interface Options
Some of these communication settings (options) are for both the
terminal and the computer and they must be set exactly the same for
both: speed, parity, bits/character, and flow control. Other
communication options are only set at the terminal (and only a couple
of these are essential to establish communications). Still others
such as the address and interrupt (IRQ) of the physical port ttyS2 are
set only at the computer using the "setserial" command. Until all of
the above essential options are compatibly set up there can be no
satisfactory serial communication (and likely no communication at all)
between the terminal and the computer. For the terminal, one must set
these options manually by menus at each terminal (or by using some
sort of special cartridge at each terminal). The host computer is
configured by running commands each time the computer is powered up
(or when people log in). Sometimes the getty program (found in the
/etc/inittab file) which starts the login process will take care of
this for the computer. See ``Getty (in /etc/inittab)''
The settings for both the computer and the terminal are:
� ``Speed (bits/second) ''
� ``Parity''
� ``Bits per Character ''
� ``Flow Control ''
Some essential settings for the terminal alone are:
� ``Port Select''
� Set communication to full duplex (=FDX on Wyse terminals)
If the ``Getty (in /etc/inittab)'' program can't set up the computer
side the way you want, then you may need to use one (or both) of the
``Stty & Setserial'' commands.
12.5.1. Speed
These must be set the same on both the terminal and the computer. The
speed is the bits/sec (bps or baud rate). Use the highest speed that
works without errors. Enabling flow control may make higher speeds
possible. There may be two speeds to set at the terminal: Transmit
and Receive, sometimes abbreviated T and R. Usually they are both set
the same since stty in Linux doesn't seem to have the option yet of
setting them differently. (There is an option to do this with the
"stty" command but it seems to actually set them both the same.)
Common speeds are 300, 600, 1200, 2400, 4800, 9600, 19200, 38400, ...
The slower speeds (like 600) are for printers and hard-copy terminals.
12.5.2. Parity & Should You Use It ?
For a definition see ``Parity Explained''. Parity-disabled is often
the default. To enable parity, you must both enable it and then
select either even or odd parity. It probably makes no difference if
it's odd or even. For terminals there are sometimes settings for both
transmit and receive parity. You should set both of these the same
since stty at the computer doesn't permit setting them differently.
The PC serial port usually can't support different parities either.
Some terminal are unable to set receive parity and will simply always
ignore received parity bits. On some older terminals if you use
8-data-bits per byte then parity will not work since there is no room
in the hardware for the extra parity bit.
Should you use parity at all? Parity, while not really necessary, is
nice to have. If you don't have parity, then you may get an incorrect
letter here and there and wind up trying to correct spelling errors
that don't really exist. However parity comes at a cost. First, it's
more complicated to set up since the default is usually no parity.
Secondly, parity will slow down the speed with which bytes travel over
the serial cable since there will be one more bit per byte. This may
or may not slow down the effective speed.
For example, a hard-copy terminal is usually limited by the mechanics
of the printing process. Increasing the bytes/sec when the computer
(its UART chip) is transmitting only results in more flow-control
"halt" signals to allow the mechanical printing to catch up. Due to
more flow-control waits the effective speed is no better without
parity than with it. The situation is similar for some terminals:
After you implement parity there may be fewer flow-control waits per
unit time resulting in more bits/sec (average). However, due to the
added parity bits the bytes/sec (average) stays the same.
One option is to install terminals with no parity. Then if parity
errors are noticed, it can be implemented later. To spot possible
errors with no parity, look for any spelling errors you don't think
you made. If you spot such an error, refresh the screen (retransmit
from the computer). If the error goes away, then it's likely a parity
error. If too many such errors happen (such as more than one every
few hundred screens) then corrective action is needed such as: Enable
parity and/or reduce speed, and/or use a shorter/better cable.
Enabling parity will not reduce the number of errors but it will tell
you when an error has happened.
Just the opposite policy is to initially enable parity. Then if no
parity errors (error symbols on the CRT) are ever seen (over a
reasonable period of time, say a month or two) it may be safely
disabled.
12.5.3. Bits/Character
This is the character size (the number of data bits per character
excluding any parity bit). For ASCII it's 7, but it's 8 for ISO
character sets. If you are only going to use ASCII characters, then
select 7-bits since it's faster to transmit 7 bits than 8. Some older
terminals will only display 7-bit characters.
12.5.4. Which Flow Control (Handshaking) ?
The choice is between "hardware" (for example dtr/cts) or "software"
(Xon/Xoff) flow control. (The Adds terminal menu incorrectly use
"Xon/Xoff" to mean any kind of flow control.) While hardware flow
control may be faster (if the one or two extra wires for it are
available in the cable and if the terminal supports it) in most cases
Xon/Xoff should work OK. Some people report that they solved
disturbing problems (see below) by converting to hardware flow control
but software flow control has worked fine at other installations (and
for me personally).
If you use software (Xon/Xoff) flow control and have users who don't
know about it, then they may accidentally send an Xoff to the host and
lock up their terminal. While it's locked, they may type frantically
in a vain attempt to unlock it. Then when Xon is finally sent to
restore communication, all that was typed in haste gets executed,
perhaps with unexpected results. They can't do this with hardware
flow control. See ``Flow Control'' for an explanation of flow
control.
12.5.5. Port Select
Since most terminals have two or more connectors on the back, it is
usually possible to assign one of these connecters to connect to the
host computer and assign another connector to be the printer port.
The connector may have a name next to it (inspect it) and this name
(such as Aux, Serial 2, or Modem) may be assigned to either be the
main host connection or the printer connection (or the like).
12.6. Quick Attempt
While all the above may seem overly complex, to get a terminal working
is often fairly simple. The ``Quick Install'' section describes a
simple way to try to do this. But if that doesn't work or if you want
to make the display look better and perform better, more reading will
be needed.
13. Terminal Set-Up (Configure) Details
Except for the next subsection on sending escape sequences to the
terminal, this section mainly presents the details of setting up the
terminal manually by sitting at the terminal and going thru menus. If
you haven't already done so, you should read ``Terminal Set-Up
(Configure) Overview''. It's best if you have a terminal manual, but
even it you don't there is information here on many of the options
which you might possibly need to set.
The communication parameters such as its baud rate must always be set
up at the terminal since if this is not done there can be no
communication with the terminal. Once communication is established
you have two choices for doing the rest the terminal configuration.
You may continue to configure manually at the terminal and save the
results in the terminal's non-volatile memory or you may do this by
sending escape sequences to the terminal from the computer each time
the terminal is powered on (or the like).
If you know how to set up and save a good configuration inside the
terminal it may be the best way. If you don't, you might want to just
send the init string from terminfo to your terminal each time you use
the terminal. Perhaps doing nothing will still give you a usable
terminal. You (or an application program) can always change things by
sending certain escape sequences to the terminal.
13.1. Send Escape Sequences to the Terminal
Once the communication interface is established, the rest of the
configuration of the terminals may sometimes be done by sending escape
sequences to the terminals from the computer. If you have a large
number of terminals, it may be worthwhile to write (or locate) a shell
script to automatically do this. There may (or may not) be a command
you can send to a terminal to tell it to save its current set-up in
its non-volatile memory so that it will be present the next time the
terminal is powered on.
There is an simple way to send these escape sequences and a complex
way. Using the simple way, you never look up escape sequences but
issue commands that automatically find an appropriate escape sequence
in the terminfo database and send that. Unfortunately, not all the
escape sequences which you might want to send are always in the
terminfo database. Thus the more complex (but possibly better) way is
to directly send escape sequences.
For this complex method you'll need an advanced manual. Old terminal
manuals once included a detailed list of escape sequences but newer
ones usually don't. To find them you may need to purchase another
manual called the "programmers manual" (or the like) which is not
supplied with the terminal. A ``Esc Sequence List'' for some
terminals is on the Internet but it's terse and likely incomplete.
Even without a manual or the like, you may still send commands to
configure the terminal by using the programs "tput" and "setterm".
See ``Changing the Terminal Settings''. You could just send the
terminal an init string from the terminfo entry if the init string
sets up the terminal the way want it. See ``Init String''. Unless
you plan to have these sequences sent from the computer to the
terminal each time the terminal is powered on, you must somehow save
the settings in the non-volatile memory of the terminal.
13.2. Older Terminals Set-Up
On older terminals look at the keyboard for labels just above the top
row of numeric keys. If they exist, these labels may be what these
keys do in set-up mode. Some older terminals may have only one "set-
up" menu. Still older ones have physical switches. In some cases not
all the switches are well labeled but they may be well concealed. Of
course, if you set something with a switch, it's "saved" and there is
no need to save the setting in non-volatile memory.
13.3. Getting Into Set-Up (Configuration) Mode
To select options (configure) at the terminal, you must first enter
"set-up" mode and then select options (i.e. configure) using menus
stored inside the terminal and displayed on the screen. To do this,
the terminal does not even need to be connected to a computer. How to
get into set-up mode is covered in the terminal's manual, but here's
some hints that may help:
If there's a "set-up" key try pressing it. Also try it shifted.
� Wyse: First try the shifted "Select" key; then substitute Ctrl for
shifted in all of the above.
� VT, Dorio: F3 may be the set-up key. On VT420 and later models
this key may have been programmed to do something else so turn off
the power. When you turn on the power again, hit the F3 key as
soon as you get an initial screen message.
� IBM: 3151: Ctrl-ScrollLock. 3153: Ctrl-Minus_on_Keypad (or like
3151)
To move around in the set-up menus, try the arrow keys. Use Return,
Space, or a special key ("toggle" on old terminals) to select. To
exit set-up mode select exit from a menu (or on some older terminals
press the set-up key again).
13.4. Communication Options
For the terminal to work at all, speed, parity, bits/character, and
communication mode must be set correctly. Incorrect flow control may
cause loss and/or corruption of data seen on the screen. The essential
communication options were dealt with (for both the terminal and
computer) in another section: See ``Communication Interface''. The
following list provides some links to that section, as well as some
additional communication options set only at the terminal.
� ``Speed (bits/second) '' (baud rate): 9600, 19200, etc.
� ``Parity'' none, even, odd, mark, space
� ``Bits per Character '' {Data}: 7 or 8
� ``Flow Control:'' or Handshake {Hndshk}: none, Xon-Xoff, or
hardware (DTR, etc).
� Receiver Handshake {Rcv Hndshk} protects data being Received by the
terminal by transmitting flow-control signals to the host.
� Transmitter Handshake {Xmt Hndshk} is protection of data being
Transmitted by the terminal. The terminal receives flow-control
signals (and locks/unlocks the keyboard). Includes "Incoming
Xon/Xoff".
� number of stop bits: 1 or 2. See ``Voltage Sequence for a Byte''
� Flow control level {Rcv Hndshk Level} {{Xoff at ...}}: Flow control
will send "stop" when this number of bytes in the terminal's buffer
is exceeded.
� ``Communication Mode'' {Comm}: ``Full Duplex {FDX}, Half Duplex
{HDX}'' {{Local Echo}}, ``Local Mode'' {{Online/Local}}
� Transmit Rate (Speed) Limit {Xmt Lim}: limits the transmit rate to
the specified cps (chars/sec) even though the baud rate setting may
be at a higher speed.
� Function-Key Rate Limit: as above but for function key messages.
� ``Port Select'': Which physical connecter is for the host {Host
Port} ?
13.5. Saving the Set-up
Your set-up must be saved in the non-volatile memory of the terminal
so that it will be effective the next time you turn on the terminal.
If you fail to save it, then the new settings will be lost when you
turn off the terminal. Before you go to the trouble of setting up a
terminal, make sure that you know how to save the settings. For
modern terminals the save command is done via a menu. In some older
terminals, only the manual tells how to save. For many of these you
press Ctrl-S to save.
13.6. Set-Up Options/Parameters
See the Teemworld's Set-Up <http://www.pericom-
usa.com/twdocs/doc/twusec7.htm> for a description of many of these
parameter as used in terminal emulation. Emulation is often a little
different than an actual terminal.
What follows in this section describes some of the options which are
available in the set-up menus of many terminals. Options are also
called parameters or features. Many options may be called "modes".
Setting options is often called "configuring". Many of these options
may also be set by sending certain escape sequences to the terminal.
Different models and brands of terminals have various options and the
same option may be called by different names (not all of which are
given here) Terse names used by Wyse are enclosed in {...}. Names
used mostly for VT terminals are enclosed in {{...}}.
13.7. Emulation {Personality} {{Terminal Modes}}
Most modern terminals can emulate several other terminals. The
terminal can likely do more if it is set to emulate itself (actually
no emulation) {native personality}. Sometimes there are 2 different
emulations for the same model of terminal. For example VT220-7
emulates a VT220 with 7-bits/byte while VT220-8 emulates a VT220 with
8-bits/byte (256 possible characters).
Older models of terminals usually have fewer features than newer
models. Suppose one wanted to emulate an old terminal but also wanted
some of the advanced capabilities of the later model terminal they are
sitting at. This is sometimes possible (to some degree). This
feature is sometimes called {Enhance} (or Enhanced ??).
13.8. Display Options
13.8.1. Character Cell Size {Char Cell}
This is the size of the cell in which a character fits. It is
measured in pixels (=tiny dots). The more dots, the better the
resolution. 10x16 is 10 dots wide by 16 dots high (16 rows and 10
columns). Note the notation is inverted as compared to the notation
for matrix dimensions which gives rows (height) first.. Also, the
character cell includes rows and columns of pixels allocated for the
space between adjacent characters so the cell size which defines the
boundaries of an actual character may be smaller.
13.8.2. Columns/Lines
Usually 80 columns and 24 or 25 lines. This means that there may be
up to 80 characters in a row (line) on the screen. Many terminals
have a 132 column option but unless you have a large screen, the tiny
characters may be hard to read. {{Set 132 column mode}}. If you set
25 lines, make sure that this is in the terminfo. You should also put
"export LINES=25" into /etc/profile. If you don't it might result in
a scrolling problem (see ``Terminal doesn't scroll''
13.8.3. Cursor
The cursor may be set to appear as a rectangle (= block) {Blk}. Other
options are underline {Line} or blinking. I prefer non-blinking
{Steady} block since it's big enough to find quickly but there is no
distractive blinking. If you set it invisible (an option on some
terminals) it will disappear but new letters will appear on the screen
as you type at the invisible cursor.
13.8.4. Display Attributes (Magic Cookies)
``Display Attributes'' may either be magic cookies or be attribute
bytes assigned to each character. For magic cookies, there is a limit
to their extent: Are they in effect to the end of the line or to the
end of the page? It's best to use attribute bytes (which could
actually be half-bytes = nibbles).
13.8.5. Display Control Characters {Monitor}
May be called various names such as "Display Controls". When off
(normal) it's "Interpret Controls". When set on, you see the escape
sequences from the host (which you normally never see on the screen).
So that these sequences may be viewed in sequence on a line, they are
not acted upon (interpreted) by the terminal. Except that a CR LF
sequence creates a new line. See ``Control Codes''.
13.8.6. Double Width/Height
Some terminals can have their characters double width and/or double
height. This feature is seldom needed. When changing a line to
double width (DW) the right half (RH) is pushed off the screen and
there is the question of whether or not to delete (erase) it.
"Preserve" means to keep the RH of DW lines. When in double height
mode, it may be necessary to send each such line twice (the 2nd time
down one row) in order to get a double-height line on the screen.
13.8.7. Reverse Video {Display} (Background Light/Dark)
Normal video is light (white, green, amber) letters (foreground) on a
dark (black) background. Reverse video {Display Light} is the
opposite: black text on a light background. This is easier on the
eyes (unless the room is dark).
13.8.8. Status Line
A status line is a line at the top or bottom of the screen that
displays info about the application program you are running. It's
often highlighted in some way. With a status line enabled, an
application can send the terminal a special escape sequence which
means that the text that follows is for the status line. However,
many applications don't use this feature but instead only simulate a
real status line by direct cursor positioning. The ordinary user
looking at it doesn't know the difference.
13.8.9. Upon 80/132 Change: Clear or Preserve?
When switching the number of columns from 80 to 132 (or conversely)
should the data displayed in the old format be erased (cleared) or
preserved? {80/132 Clr} {{Screen Width Change}}. It should make no
difference how you set this option since if an application program
uses 132 columns, it should set this option appropriately via a
control sequence.
13.9. Page Related Options
For a Wyse terminal to be able to access multiple pages of display
memory {Multipage} must be set to on.
13.9.1. Page Size
The terminal memory may be divided up into a number of pages. See
``Pages'' and ``Pages (definition)'' for explanations of pages. You
may partition the page memory into a number of pages of selected
length. Linux applications don't seem to use pages at present so it
shouldn't make much difference how you set this up.
13.9.2. Coupling (of cursor & display)
The terminal memory may be divided up into a number of pages. See
``Pages'' and ``Pages'' for explanations of pages. When the cursor is
moved to a location in video memory not currently displayed (such as
another page, or on the same page but to a location not displayed on
the screen) should the display change to let one view the new cursor
location? If so, this is called "Coupling". For cursor movement
within the same page there is "Vertical Coupling" and "Horizontal
Coupling". For movement to another page there is "Page Coupling".
13.10. Reporting and Answerback
The terminal will identify itself and its state, or send out a pre-
recorded message in response to certain escape sequences.
13.10.1. Answerback Message (String)
You may write a short message during set-up which may optionally be
sent to the host at power-up or be sent to the host in response to a
request from the host (perhaps the ENQ (inquire) control character).
13.10.2. Auto Answerback
If set, sends the answerback message to the host at power-on without
the host asking for it. Do any "getty" processes look for this ??
13.10.3. Answerback Concealed
If set, will never let anyone see the answerback message (except of
course the host computer). If it needs to be changed, deselect
"answerback concealed" and the formerly concealed message will be
destroyed so you then may enter a new message (but you don't get to
see the old one).
13.10.4. Terminal ID {ANSI ID}
The terminal send this reply in answer to a request for identity.
13.11. Keyboard Options
13.11.1. Keyclick
When set, pressing any key makes a click (broadcast by a tiny
loudspeaker in the keyboard). These clicks annoy some people and I
think it's best to set keyclick off.
13.11.2. Caps Lock {Keylock}
When the Caps-Lock key is down, should only the alphabetic keys
generate shifted characters? If set to {Caps} or upper-case-only then
hitting a number key with the Caps-Lock on will type the number. To
get the symbol above the number one must manually hold down the shift
key. This is the normal mode. If set to {Shift} then all keys type
the shifted character when Caps-Lock is on (hitting the 5 key should
type % without holding down Shift, etc.).
13.11.3. Auto Repeat {Repeat}
If a key is held down then that key is repeatedly "typed". This is
handy for repeatedly typing the same character to create a line across
the page.
13.11.4. Margin Bell
When the cursor is 8 columns away from the right side of the display,
a bell is rung (like on an old typewriter). Almost all editors will
automatically create a new line if needed (no need to hit the Return
key) so this feature is seldom needed.
13.11.5. Remapping the Keys
The code sent to the host when a key is pressed is normally the ASCII
code for that key (depends also on Shift and Control key). On some
terminals you may make any key send any code you wish. That is, you
may completely remap the keyboard by setting up the terminal that way.
This may be useful for some foreign languages and Dvorak keyboard
layouts, etc. which permit one to type faster.
13.11.6. Corner Key (for Wyse only)
Wyse terminals have a key near the lower left corner which may be set
to do various things. Its may be labelled "Funct", "Compose
Character", "Alt", "Hold" or "Scroll Lock". Early models don't have
all of the following options: When set to {Hold} No-Scroll it stops
the flow of data (using flow control) to the terminal. Hitting the
key again restores normal flow. When set to {Compose} it permits one
to generate a limited number of pre-defined non-Latin characters.
When set to Meta, it makes it a meta shift key which sets the high-
order bit on each byte. When set to {Funct} (and pressed) any
alphanumeric key pressed gets a header (SOH) and trailer (CR) byte
framing the ASCII byte code. When set to {Kpd Compose} (and pressed)
then typing a decimal number on the numeric keys (followed by "enter")
sends out the same number in hexadecimal ??
13.11.7. Numeric Keypad or Arrow Keys Sends
The numeric keypad (the rectangle of mostly numeric keys to the right
of the main part of the keyboard) can be set to send special codes
which will do special things in certain application programs. Ditto
for the arrow keys. There is thus a "normal" mode where they send
what is shown on the keycap (or the normal code sequence for an arrow-
key) and an "application" mode where a special escape sequence is
sent. In some cases there is a "hex" numeric mode which is almost
like normal numeric mode except that 6 non-numeric keys send the
letters A-F. Thus one may type for example "B36F" on the numeric
keypad.
13.11.8. What does shifted-del and shifted-bs send?
Depending on how they're set up shifted-del sometimes sends the
control character CAN and shifted backspace sometimes sends DEL.
13.11.9. PC Scan Codes
Newer terminals can emulate a PC keyboard by sending PC scan codes
(see Keyboard-and-Console-HOWTO) instead of ASCII codes. This would
be used if you were directly connected to a PC running Dos/Windows.
Set {Keycode} to {Scan}. Emulating the Dec "PCTerm" should do this
and more. A serial port under Linux can't cope with such scan codes.
13.11.10. Alternate Characters
Some keys may have alternative letters on them. When keys is set to
"Typewriter" they send what they would normally send on a typewriter.
When keys is set to something else the alternative characters are
sent.
13.12. Meaning of Received Control Codes
13.12.1. Auto New Line {Newline}
In this case "New Line" means a new line starting at the left margin
below the current line. In Linux and C "new line" (NL) may have a
different meaning: the line-feed control character LF also called new-
line or NL. This is because in Linux text files, the LF character
means a "new line starts here" so it's labeled NL. Normally, a LF
(NL) sent to a terminal only results in the cursor jumping down one
line below where is was and does not move the cursor back to the start
of this "new line".
If Auto New Line is set, the above "normal" situation is canceled and
a physical new line is created on the display upon receiving a LF from
the host. This is exactly what one wants in Linux. Except that (when
Auto New Line is set) the Return (or Enter) key sends a CR LF sequence
to the host (for Wyse and VT100, but for VT420 ??). Since Linux uses
LF as a "new line" marker in files, Linux would like only a LF to be
sent (and not a CR LF). Thus the "New Line" option is seldom used.
Instead, the required translations are made by the serial port device
driver by default. It is as if one gave the command "stty onlcr
icrnl". But you don't need to do this since it's the default.
13.12.2. Auto Line Feed {Rcv CR}
This is just another type of "Auto New Line". When a CR (carriage
return) character is received, a LF (line feed) action is added
resulting in a new line being displayed. Since Linux marks the end of
lines with LF, this option is not used.
13.12.3. Recognize Del (Wyse Only ??) or Null
If off, the DEL character received by the terminal is ignored. If on
the DEL performs a destructive backspace. Null characters are usually
ignored in any case. Both DEL and NULL are sometimes used for
padding. See ``Padding''
13.13. Where New Text Goes
13.13.1. Line Wrap
Also called "Auto Wrap(around)". What happens when the right edge of
the screen is reached (col. 80, etc) and no return character (or the
like) has been sent from the host? If Line Wrap is set, then the rest
of the line displays on the line below, etc. Otherwise, the rest of
the line is lost and is not seen on the screen. Any good application
should handle the situation correctly (provided the terminfo knows how
Line Wrap is set). Thus even if Line Wrap is not set, the application
should either wrap the screen for long lines or provide another way
for you to view the cutoff tail of long lines (by use of the arrow
keys, etc). But a raw copy command (and other situations) may not do
this so it's often best to set line wrap.
For an 80 col. screen, most terminals only wrap if the 81st character
from the host is a graphic (printable) character. This allows for the
case where 81st character from the host might be "return" or a
"newline" (non-graphic characters) which means that the application is
handing the wrapping OK and intervention by the terminal is not
needed.
13.13.2. Scrolling
Scrolling {Scrl} is where all the lines on the screen move up or down.
Its also called "panning" which includes movement sideways. In
ordinary scrolling lines roll off the bottom or top of the screen and
disappear, and new lines from the host appear at the opposite edge
(top or bottom). There are 3 types of this: smooth, jump, or burst.
Burst is not really scrolling since its an instant replacement of an
old screenfull by a new one (although some lines on the new screen may
be from the old screen). Jump is where new lines jump into view one
at a time. Smooth {Smth} is where the text moves at a steady speed
upward or downward. If the smooth scroll rate is slow enough, one may
read the newly visible lines when they are still scrolling (in
motion).
Smooth scrolling on slow terminals was once useful since one could
continue reading as the display was scrolling. But with higher baud
rates, jump scroll is so fast that little time is lost as the new
display appears. Since it takes a little longer to read scrolling
text than fixed text, it may actually waste more time if smooth
scrolling is selected.
If (auto)scrolling {Autoscrl} is disabled, then new text from the host
must go somewhere so it is put at the top of the display. If the old
text is not erased, the new text merges (nonsensically) into the old.
If the old text is erased, then the new text is out of context. So
keep (auto)scrolling enabled.
13.13.3. New Page?
See ``Pages'' and ``Pages'' for explanations of pages. When the
current page is full (the last line is finished) should the page
scroll, or should a new page be created (leaving the previous page
stored in the terminal's display memory). If {Autopage} is set, then
a new page is created. Since you are probably not using pages, you
should probably set this to off.
13.14. Function Keys
These are the keys labeled F1, F2, etc. On older terminals they may
be labeled PF1, PF2, etc. where the P stands for Programmable. Some
keyboards have both. One may program (redefine) these keys to send
out a string of user-defined bytes. They may often be easily
"programmed" using a certain set-up menu {FKey}. On some terminals,
one may also specify where this string is sent to when the key is
pressed. In "normal" mode, pressing the key is just like typing the
string at the keyboard. In "local" mode pressing the key sends it to
the terminal (just like if the terminal was in local mode). This may
be used to send escape sequences to the terminal so as to configure it
in a special way. In "remote" mode, the string is always sent out the
serial port to the host computer (even if the terminal is in local
mode).
13.15. Block Mode Options
Some options are only for the case of ``Block Mode''. This option is
powerful since it provides forms and takes load off the host computer
by transmitting in bursts. But it's more complicated to set up and is
thus not used too much.
13.15.1. Forms Display
In block mode some regions of the screen are for the text of forms and
are thus write-protected "Prot" {WPRT}. Options may set the
characters in these regions to appear dim, reverse video {WPRT Rev},
and/or underlined {WPRT Undrln}. {WPRT Intensity} may be set to dim,
normal, or even blank (invisible)
13.15.2. Send Entire Block ?
Should write-protected text (the original text in the form) be sent to
the host upon transmission of a block: {Send All} or is write-
protected text also read-protected: {Send Erasable}
13.15.3. Region to Send
Should the entire screen be sent or just the scrolling region? {Send
Area}. Should the sending stop when the current cursor position is
reached? If {Xfer Term} is set to Cursor, only the data on the screen
up to the cursor is sent.
13.15.4. Block/Page terminator
What is the termination symbol to be appended to a block of data?
{Blk End} or at the end of a page {Send Term}ination.
13.16. Locks
There are various types of Locks. One is the Locked keyboard due to
flow control. See ``Keyboard Lock'' Another lock {Feature Lock} is
that which prohibits the host computer from changing the terminal set-
up by sending certain escape sequences to the terminal. Placing such
a lock may result in unexpected behavior as application programs send
escape sequences to the terminals that are ignored. Not all set-up
parameters lock. Unless you have a good reason to do so, you should
not enable such locking.
A Function Key lock will prohibit the computer from redefining what a
programmable function key sends. You may want to use this if you have
something important programmed into the function keys.
13.17. Screen Saver {Scrn Saver}
Also called "CRT Saver". This turns off (or dims) the screen after
the terminal is not used for a period of time. It prolongs the life
of the screen and may save some energy. Hitting any key will usually
restore the screen and may "execute" that key so it's best to hit the
shift-key, etc.
13.18. Printer
For Wyse, if there is no {Printer Attached} set it to Off. It's not
essential to do this, but if you do it any escape sequence to send
text to the printer (instead of the terminal) will be ignored.
Setting up the printer port is about the same (usually simpler) as
setting up the communications on the main port. There are a couple of
options specific to the printer. Is the printer a serial or parallel
printer? If it's parallel it should be designated as such in setup
and connected to the parallel port on the terminal (if there is one).
Should a FF (form feed) be sent to the printer at the end of a print
job? If {Print Term} is set to FF, this will happen.
14. Computer Set-Up (Configure) Details
There are various files to edit to set up the computer for terminals.
If you're lucky, you'll only need to edit /etc/inittab. One does this
by editing at the console (or from any working terminal).
14.1. Getty (in /etc/inittab)
In order to have a login process run on a serial port when the
computer starts up (or switches run levels) a getty command must be
put into the /etc/inittab file. Getty GETs a TTY (a terminal) going.
Each terminal needs its own getty command. There is also at least one
getty command for the console in every /etc/inittab file. Find this
and put the getty commands for the real terminals next to it. This
file may contain sample getty lines for text terminals that are
commented out so that all you need to do is to uncomment them (remove
the leading #) and change a few arguments.
The arguments which are permitted depend on which getty you use:
Two gettys best for directly connected terminals are:
1. agetty (sometimes just called getty): Very easy to set up. No
config files. See ``Agetty''
2. ``getty (part of getty_ps)''
Two gettys best for modems (avoid for terminals) are:
1. mgetty: the best one for modems; works for terminals too but
inferior
2. uugetty: for modems only; part of the getty_ps package
A simple getty to use only where the monitor is the console. Most
Linux users with a text interface have this setup and don't use a
real text-terminal.
1. mingetty: for monitor-consoles only
If you don't have the getty you want check other distributions and the
alien program to convert between RPM and Debian packages. The source
code may be downloaded from Serial Software
<http://sunsite.unc.edu/pub/Linux/system/serial/>.
If you are not using modem control lines (for example if you only use
the minimum number of 3 conductors: transmit, receive, and common
signal ground) you should let getty know this by using a "local" flag.
The format of this depends on which getty you use.
14.1.1. Agetty (may be named getty)
An example line in /etc/inittab:
S1:23:respawn:/sbin/getty -L 19200 ttyS1 vt102
S1 is from ttyS1. 23 means that getty is run upon entering run levels
2 or 3. respawn means that if getty is killed, it will automatically
start up (respawn) again. /sbin/getty is the getty command. The -L
means Local (ignore modem control signals). -h (not shown in the
example) enables hardware flow control (same as stty crtscts). 19200
is the baud rate. ttyS1 means /dev/ttyS1 (COM2 in MS-DOS). vt102 is
the type of terminal and this getty will set the environment variable
TERM to this value. There are no configuration files. Type "init q"
on the command line after editing getty and you should see a login
prompt.
14.1.1.1. Agetty's detection of parity
The agetty program will auto-detect any parity set inside the
terminal. Except it will not work if you use 8-bit data bytes with
1-bit parity. See ``Agetty's parity with 8-bit data bytes''. If you
use stty to set parity, agetty will automatically unset it since it
wants the parity bit to come thru as if it was a data bit. This is
because it needs to get the last bit (possibly a parity bit) as you
type your login-name so that it can auto-detect parity. Thus if you
use parity, enable it only at the terminals and let agetty auto-detect
it and set it at the computer. If your terminal supports received
parity, the login prompt will look garbled until you type something so
that getty can detect the parity. The garbled prompt will deter
visitors, etc. from trying to login. That could be just what you
want.
There is sometimes a problem with auto detection of parity. This
happens because after you first type your login name, agetty uses the
login program to finish logging you in. If the first login attempt
fails, login runs again to handle all future attempts at logging in
(including the type-in of your login-name). The problem is that only
agetty can detect parity while the login program doesn't detect
parity. So if for some reason you wind up in the login program and
the correct parity hasn't yet been detected, you're in trouble since
the login program can't detect the parity. With wrong parity, the
login program can't correctly read what you type and you can't log in.
If your terminal supports received parity, you will continue to see a
garbled screen.
One may get into this "login loop" in various ways. Suppose you only
type a single letter or two for your login name and then hit return.
If these letters are not sufficient for parity detection, then login
runs before parity has been detected. Sometimes this problem happens
if you don't have the terminal on and connected when agetty first
starts up. If you get stuck in this "login" loop a solution is to
just wait about a minute until agetty runs again due to "timeout".
14.1.1.2. Agetty's parity with 8-bit data bytes
Unfortunately, agetty can't detect this parity. It (as of late 1999)
has no option for disabling the auto-detection of parity and thus will
detect incorrect parity. The result is that the login process will be
garbled and parity will be set wrong. Thus it doesn't seem feasible
to try to use 8-bit data bytes with parity.
14.1.2. getty (part of getty_ps)
(This is from the old Serial-HOWTO by Greg Hankins)
Add entries for getty to use for your terminal in the configuration
file /etc/gettydefs if there they aren't already there:
# 38400 bps Dumb Terminal entry
DT38400# B38400 CS8 CLOCAL # B38400 SANE -ISTRIP CLOCAL #@S @L login: #DT38400
# 19200 bps Dumb Terminal entry
DT19200# B19200 CS8 CLOCAL # B19200 SANE -ISTRIP CLOCAL #@S @L login: #DT19200
# 9600 bps Dumb Terminal entry
DT9600# B9600 CS8 CLOCAL # B9600 SANE -ISTRIP CLOCAL #@S @L login: #DT9600
If you want, you can make getty print interesting things in the login
banner. In my examples, I have the system name and the serial line
printed. You can add other things:
@B The current (evaluated at the time the @B is seen) bps rate.
@D The current date, in MM/DD/YY.
@L The serial line to which getty is attached.
@S The system name.
@T The current time, in HH:MM:SS (24-hour).
@U The number of currently signed-on users. This is a
count of the number of entries in the /etc/utmp file
that have a non-null ut_name field.
@V The value of VERSION, as given in the defaults file.
To display a single '@' character, use either '\@' or '@@'.
When you are done editing /etc/gettydefs, you can verify that the
syntax is correct by doing:
linux# getty -c /etc/gettydefs
Make sure there is no getty or uugetty config file for the serial port
that your terminal is attached to (/etc/default/{uu}getty.ttySN or
/etc/conf.{uu}getty.ttySN), as this will probably interfere with
running getty on a terminal. Remove the file if it exits.
Edit your /etc/inittab file to run getty on the serial port
(substituting in the correct information for your environment - port,
speed, and default terminal type):
S1:23:respawn:/sbin/getty ttyS1 DT9600 vt100
Restart init:
linux# init q
At this point, you should see a login prompt on your terminal. You
may have to hit return to get the terminal's attention.
14.1.3. mgetty
The "m" stands for modem. This program is primarily for modems and as
of mid 1999 doesn't always work very well for text-terminals. It's
poorly documented for terminals and you may need to wade thru much
documentation for modems in order to figure out how to use it for
terminals. Look at the last lines of /etc/mgetty/mgetty.config for an
example of configuring it for a terminal. It will only support
software (Xon/Xoff) flow control (used by many terminals) if you
recompile it. This will be hopefully be fixed in the future. It
would be nice to use the same getty for terminals as for modems but
mgetty needs a little fixing to fill the bill.
14.2. Stty & Setserial
There is both a "stty" command and a "setserial" command for setting
up the serial ports. Some (or all) of the needed stty settings can
be done via getty and there may be no need to use setserial so you may
not need to use either command. These two commands (stty and
setserial) set up different aspects of the serial port. Stty does the
most while setserial configures the low-level stuff such as interrupts
and port addresses. To "save" the settings, these commands must be
written in certain files (shell scripts) which run each time the
computer starts up. Distributions of Linux often supply a shell
script which runs setserial but seldom supply one which runs stty
since on seldom need it.
14.3. Setserial
This part is in 3 HOWTOs: Modem, Serial, and Text-Terminal. There are
some minor differences, depending on which HOWTO it appears in.
14.3.1. Introduction
Don't ever use setserial with Laptops (PCMCIA). setserial is a
program which allows you to tell the device driver software the I/O
address of the serial port, which interrupt (IRQ) is set in the port's
hardware, what type of UART you have, etc. It can also show how the
driver is currently set. In addition, it can be made to probe the
hardware and try to determine the UART type and IRQ, but this has
severe limitations. See ``Probing''. Note that it can't set the IRQ
or the port address in the hardware of PnP serial ports.
If you only have one or two built-in serial ports, they will usually
get set up correctly without using setserial. Otherwise (or if there
are problems with the serial port) you will likely need to deal with
setserial. Besides the man page for setserial, check out info in
/usr/doc/setserial.../ or /usr/share/doc/setserial. It should tell
you how setserial is handled in your distribution of Linux.
Setserial is often run automatically at boot-time by a start-up shell-
script for the purpose of assigning IRQs, etc. to the driver.
Setserial will only work if the serial module is loaded (or if the
equivalent was compiled into your kernel). If you should (for some
reason) unload the serial module later on, the changes previously made
by setserial will be forgotten by the kernel. So setserial must be
run again to reestablish them. In addition to running via a start-up
script, something akin to setserial also runs earlier when the serial
module is loaded (or the like). Thus when you watch the start-up
messages on the screen it may look like it ran twice, and in fact it
has.
Setserial can set the time that the port will keep operating after
it's closed (in order to output any characters still in its buffer in
main RAM). This is needed at slow baud rates of 1200 or lower. It's
also needed at higher speeds if there are a lot of "flow control"
waits. See "closing_wait" in the man pg. If your serial port is
Plug-and-Play you may need to consult other HOWTOs such as Plug-and-
Play or Serial.
Setserial does not set either IRQ's nor I/O addresses in the serial
port hardware itself. That is done either by jumpers or by plug-and-
play. You must tell setserial the identical values that have been set
in the hardware. Do not just invent some values that you think would
be nice to use and then tell them to setserial. However, if you know
the I/O address but don't know the IRQ you may command setserial to
attempt to determine the IRQ.
You can see a list of possible commands by just typing setserial with
no arguments. This fails to show you the one-letter options such as
-v for verbose which you should normally use when troubleshooting.
Note that setserial calls an IO address a "port". If you type:
setserial -g /dev/ttyS*
you'll see some info about how that device driver is configured for
your ports. Note that where it says "UART: unknown" it probably means
that no uart exists. In other words you probably have no such serial
port and the other info shown about the port is meaningless and should
be ignored. If you really do have such a serial port, setserial
doesn't recognize it and that needs to be fixed.
If you add -a to the option -g you will see more info although few
people need to deal with (or understand) this additional info since
the default settings you see usually work fine. In normal cases the
hardware is set up the same way as "setserial" reports, but if you are
having problems there is a good chance that "setserial" has it wrong.
In fact, you can run "setserial" and assign a purely fictitious I/O
port address, any IRQ, and whatever uart type you would like to have.
Then the next time you type "setserial ..." it will display these
bogus values without complaint. Of course the serial port driver will
not work correctly (if at all) if you attempt to use such a port.
Thus when giving parameters to "setserial" anything goes. It gives
you no warning if what you tell it is incorrect and will allow you to
create conflicts in IRQs and I/O port addresses that will have
disastrous results later on.
While assignments made by setserial are lost when the PC is powered
off, a configuration file may restore them (or a previous
configuration) when the PC is started up again. In newer versions,
what you change by setserial gets automatically saved to a
configuration file. In older versions, the configuration file only
changes if you edit it manually so the configuration remains the same
from boot to boot. See ``Configuration Scripts/Files''
14.3.2. Probing
With appropriate options, setserial can probe (at a given I/O address)
for a serial port but you must guess the I/O address. If you ask it
to probe for /dev/ttyS2 for example, it will only probe at the address
it thinks ttyS2 is at (2F8). If you tell setserial that ttyS2 is at a
different address, then it will probe at that address, etc. See
``Probing''
The purpose of this is to see if there is a uart there, and if so,
what its IRQ is. Use "setserial" mainly as a last resort as there are
faster ways to attempt it such as wvdialconf to detect modems, looking
at very early boot-time messages, or using pnpdump --dumpregs. To try
to detect the physical hardware use the -v (verbose) and autoconfig
command to setserial. If the resulting message shows a uart type such
as 16550A, then you're OK. If instead it shows "unknown" for the uart
type, then there is supposedly no serial port at all at that I/O
address. Some cheap serial ports don't identify themselves correctly
so if you see "unknown" you still might have a serial port there.
Besides auto-probing for a uart type, setserial can auto-probe for
IRQ's but this doesn't always work right either. In versions of
setserial >= 2.15, the results of your last probe test may be saved
and put into the configuration file /etc/serial.conf which will be
used next time you start Linux. At boot-time when the serial module
loads (or the like), a probe for UARTs is made automatically and
reported on the screen. But the IRQs shown may be wrong. The second
report of the same is the result of a script which usually does no
probing and thus provides no reliable information as to how the
hardware is actually set. It only shows configuration date someone
wrote into the script or data that got saved in /etc/serial.conf.
It may be that two serial ports both have the same IO address set in
the hardware. Of course this is not permitted but it sometimes
happens anyway. Probing detects one serial port when actually there
are two. However if they have different IRQs, then the probe for IRQs
may show IRQ = 0. For me it only did this if I first used setserial
to give the IRQ a ficticious value.
14.3.3. Boot-time Configuration
When the kernel loads the serial module (or if the "module equivalent"
is built into the kernel) then only ttyS{0-3} are auto-detected and
the driver is set to use only IRQs 4 and 3 (regardless of what IRQs
are actually set in the hardware). You see this as a boot-time
message just like as if setserial had been run. If you use 3 or more
ports, this may result in IRQ conflicts.
To fix such conflicts by telling setserial the true IRQs (or for other
reasons) there may be a file somewhere that runs setserial again.
This happens early at boot-time before any process uses the serial
port. In fact, your distribution may have set things up so that the
setserial program runs automatically from a start-up script at boot-
time. More info about how to handle this situation for your
particular distribution might be found in file named "setserial..." or
the like located in directory /usr/doc/ or /usr/share/doc/.
14.3.4. Configuration Scripts/Files
Your objective is to modify (or create) a script file in the /etc tree
that runs setserial at boot-time. Most distributions provide such a
file (but it may not initially reside in the /etc tree). In addition,
setserial 2.15 and higher often have an /etc/serial.conf file that is
used by the above script so that you don't need to directly edit the
script that runs setserial. In addition just using setserial on the
command line (2.15+) may ultimately alter this configuration file.
So prior to version 2.15 all you do is edit a script. After 2.15 you
may need to either do one of three things: 1. edit a script. 2. edit
/etc/serial.conf or 3. run "setserial" on the command line which will
result in /etc/serial.conf automatically being edited. Which one of
these you need to do depends on both your particular distribution, and
how you have set it up.
14.3.5. Edit a script (after version 2.15: perhaps not)
Prior to setserial 2.15 (1999) there was no /etc/serial.conf file to
configure setserial. Thus you need to find the file that runs
"setserial" at boot time and edit it. If it doesn't exist, you need
to create one (or place the commands in a file that runs early at
boot-time). If such a file is currently being used it's likely
somewhere in the /etc directory-tree. But Redhat <6.0 has supplied it
in /usr/doc/setserial/ but you need to move it to the /etc tree before
using it. You might use "locate" to try to find such a file. For
example, you could type: locate "*serial*".
The script /etc/rc.d/rc.serial was commonly used in the past. The
Debian distribution used /etc/rc.boot/0setserial. Another file once
used was /etc/rc.d/rc.local but it's not a good idea to use this since
it may not be run early enough. It's been reported that other
processes may try to open the serial port before rc.local runs
resulting in serial communication failure.
If such a file is supplied, it should contain a number of commented-
out examples. By uncommenting some of these and/or modifying them,
you should be able to set things up correctly. Make sure that you are
using a valid path for setserial, and a valid device name. You could
do a test by executing this file manually (just type its name as the
super-user) to see if it works right. Testing like this is a lot
faster than doing repeated reboots to get it right. Of course you can
also test a single setserial command by just typing it on the command
line.
If you want setserial to automatically determine the uart and the IRQ
for ttyS3 you would add something like:
/sbin/setserial /dev/ttyS3 auto_irq skip_test autoconfig
Do this for every serial port you want to auto configure. Be sure to
give a device name that really does exist on your machine. In some
cases this will not work right due to the hardware so if you know what
the uart and irq actually are, may want to assign them explicitly with
"setserial". For example:
/sbin/setserial /dev/ttyS3 irq 5 uart 16550A skip_test
For versions >= 2.15 (provided your distribution implemented the
change, Redhat didn't) it may be more tricky to do since the file that
runs setserial on startup, /etc/init.d/setserial or the like was not
intended to be edited by the user. See ``New configuration method
using /etc/serial.conf''.
14.3.6. New configuration method using /etc/serial.conf
Prior to setserial version 2.15, the way to configure setserial was to
manually edit the shell-script that ran setserial at boot-time. See
``Edit a script (after version 2.15: perhaps not)''. Starting with
version 2.15 (1999) of setserial this shell-script is not edited but
instead gets its data from a configuration file: /etc/serial.conf.
Furthermore you may not even need to edit serial.conf because using
the "setserial" command on the command line may automatically cause
serial.conf to be edited appropriately.
This was intended to make it so that you don't need to edit any file
in order to set up (or change) setserial so it will do the right thing
each time that Linux is booted. But there are serious pitfalls
because it's not really "setserial" that edits serial.conf. Confusion
is compounded because different distributions handle this differently.
In addition, you may modify it so it works differently.
What often happens is this: When you shut down your PC the script
that runs "setserial" at boot-time is run again, but this time it only
does what the part for the "stop" case says to do: It uses
"setserial" to find out what the current state of "setserial" is and
puts that info into the serial.conf file. Thus when you run
"setserial" to change the serial.conf file, it doesn't get changed
immediately but only when and if you shut down normally.
Now you can perhaps guess what problems might occur. Suppose you
don't shut down normally (someone turns the power off, etc.) and the
changes don't get saved. Suppose you experiment with "setserial" and
forget to run it a final time to restore the original state (or make a
mistake in restoring the original state). Then your "experimental"
settings are saved.
If you manually edit serial.conf, then your editing is destroyed when
you shut down because it gets changed back to the state of setserial
at shutdown. There is a way to disable the changing of serial.conf at
shutdown and that is to remove "###AUTOSAVE###" or the like from first
line of serial.conf. In at least one distribution, the removal of
"###AUTOSAVE###" from the first line is automatically done after the
first time you shutdown just after installation. The serial.conf file
will hopefully contain some comments to help you out.
The file most commonly used to run setserial at boot-time (in
conformance with the configuration file) is now /etc/init.d/setserial
(Debian) or /etc/init.d/serial (Redhat), or etc., but it should not
normally be edited. For 2.15 Redhat 6.0 just had a file
/usr/doc/setserial-2.15/rc.serial which you have to move to
/etc/init.d/ if you want setserial to run at boot-time.
To disable a port, use setserial to set it to "uart none". The format
of /etc/serial.conf appears to be just like that of the parameters
placed after "setserial" on the command line with one line for each
port. If you don't use autosave, you may edit /etc/serial.conf
manually.
BUG: As of July 1999 there is a bug/problem since with ###AUTOSAVE###
only the setserial parameters displayed by "setserial -Gg /dev/ttyS*"
get saved but the other parameters don't get saved. Use the -a flag
to "setserial" to see all parameters. This will only affect a small
minority of users since the defaults for the parameters not saved are
usually OK for most situations. It's been reported as a bug and may
be fixed by now.
In order to force the current settings set by setserial to be saved to
the configuration file (serial.conf) without shutting down, do what
normally happens when you shutdown: Run the shell-script
/etc/init.d/{set}serial stop. The "stop" command will save the
current configuration but the serial ports still keep working OK.
In some cases you may wind up with both the old and new configuration
methods installed but hopefully only one of them runs at boot-time.
Debian labeled obsolete files with "...pre-2.15".
14.3.7. IRQs
By default, both ttyS0 and ttyS2 will share IRQ 4, while ttyS1 and
ttyS3 share IRQ 3. But actually sharing serial interrupts (using them
in running programs) is not permitted unless you: 1. have kernel 2.2
or better, and 2. you've complied in support for this, and 3. your
serial hardware supports it. See Serial-HOWTO: Interrupt sharing and
Kernels 2.2+.
If you only have two serial ports, ttyS0 and ttyS1, you're still OK
since IRQ sharing conflicts don't exist for non-existent devices.
If you add an internal modem and retain ttyS0 and ttyS1, then you
should attempt to find an unused IRQ and set it both on your serial
port (or modem card) and then use setserial to assign it to your
device driver. If IRQ 5 is not being used for a sound card, this may
be one you can use for a modem. To set the IRQ in hardware you may
need to use isapnp, a PnP BIOS, or patch Linux to make it PnP. To
help you determine which spare IRQ's you might have, type "man
setserial" and search for say: "IRQ 11".
14.4. Stty
14.4.1. Introduction
stty does much of the configuration of the serial port but since
application programs (and the getty program) often handle it, you may
not need to use it much. It's handy if your having problems or want
to see how the port is set up. Try typing ``stty -a'' at your
terminal/console to see how it's now set. Also try typing it without
the -a (all) for a short listing which shows how it's set different
than normal. Don't try to learn all the setting unless you want to
become a serial guru. Most of the defaults should work OK and some of
the settings are needed only for certain obsolete dumb terminals made
in the 1970's.
Whereas setserial only deals with actual serial ports, stty is used
both for serial ports and for virtual terminals such as the standard
Linux text interface at a PC monitor. For the PC monitor, many of the
stty settings are meaningless. Changing the baud rate, etc. doesn't
appear to actually do anything.
Here are some of the items stty configures: speed (bits/sec), parity,
bits/byte, # of stop bits, strip 8th bit?, modem control signals, flow
control, break signal, end-of-line markers, change case, padding, beep
if buffer overrun?, echo what you type to the screen, allow background
tasks to write to terminal?, define special (control) characters (such
as what key to press for interrupt). See the stty man or info page
for more details. Also see the man page: termios which covers the
same options set by stty but (as of mid 1999) covers features which
the stty man page fails to mention.
With some implementations of getty (getty_ps package), the commands
that one would normally give to stty are typed into a getty
configuration file: /etc/gettydefs. Even without this configuration
file, the getty command line may be sufficient to set things up so
that you don't need stty."')
One may write C programs which change the stty configuration, etc.
Looking at some of the documentation for this may help one better
understand the use of the stty command (and its many possible
arguments). Serial-Programming-HOWTO is useful. The manual page:
termios contains a description of the C-language structure (of type
termios) which stores the stty configuration in computer memory. Many
of the flag names in this C-structure are almost the same (and do the
same thing) as the arguments to the stty command.
14.4.2. Using stty for a "foreign" terminal
Using stty to inspect or configure the terminal that you are currently
using is easy. Doing it for a different (foreign) terminal or serial
port may be tricky. For example, let's say you are at the PC monitor
(tty1) and want to use stty to deal with the serial port ttyS2. Prior
to about 2000 you needed to use the redirection operator "<". After
2000 (provided your version of setserial is >= 1.17 and stty >= 2.0)
there is an alternate method using the -F option. This will work when
the old redirection method fails. Even with the latest versions be
warned that if there is a terminal on ttyS2 and a shell is running on
that terminal, then what you see will likely be deceptive and trying
to set it will not work. See ``Two Interfaces at a Terminal'' to
understand it.
The new method is ``stty -F /dev/ttyS2 ...'' (or --file instead of F).
If ... is -a it displays all the stty settings. The old redirection
method (which still works in later versions) is to type ``stty ...
</dev/ttyS2''. If the new method works but the old one hangs, it
implies that the port is hung due to lack of a modem control line from
being asserted. Thus the old method is still useful for
troubleshooting. See the following subsection.
14.4.2.1. Old redirection method
Here's a problem with the old redirection operator (which doesn't
happen if you use the newer -F option instead). Sometimes when trying
to use stty, the command hangs and nothing happens (you don't get a
prompt for a next command even after hitting <return>). This is
likely due to the port being stuck because it's waiting for one of the
modem control lines to be asserted. For example, unless you've set
"clocal" to ignore modem control lines, then if no CD signal is
asserted the port will not open and stty will not work for it (unless
you use the newer -F option). A similar situation seems to exist for
hardware flow control. If the cable for the port doesn't even have a
conductor for the pin that needs to be asserted then there is no easy
way to stop the hang.
One way to try to get out of the above hang is to use the newer -F
option and set "clocal" and/or "crtscts". If you don't have the -F
option then you may try to run program on the port that will force it
to operate even if the control lines say not to. Then hopefully this
program might set the port so it doesn't need the control signal in
the future in order to open: clocal or -crtscts. To use "minicom" to
do this you have to reconfigure minicom for another ttyS, etc, and
then exit it and restart it. Since you then have to reconfigure
minicom again, it may be simpler to just reboot the PC.
The old redirection method makes ttyS2 the standard input to stty.
This gives the stty program a link to the "file" ttyS2 so that it may
"read" it. But instead of reading the bytes sent to ttyS2 as one
might expect, it uses the link to find the configuration settings of
the port so that it may read or change them. Some people tried to use
``stty ... > /dev/ttyS2'' to set the terminal. This will not do it.
Instead, it takes the message normal displayed by the stty command for
the terminal you are on (say tty1) and sends this message to ttyS2.
But it doesn't change any settings for ttyS2.
14.4.3. Two interfaces at a terminal
When using a shell (such as bash) with command-line-editing enabled
there are two different terminal interfaces (what you see when you
type stty -a). When you type at the command line you have a temporary
"raw" interface (or raw mode) where each character is read by the
command-line-editor as you type it. Once you hit the <return> key,
the command-line-editor is exited and the terminal interface is
changed to the nominal "cooked" interface (cooked mode) for the
terminal. This cooked mode lasts until the next prompt is sent to the
terminal. Note that one never types anything to this cooked mode but
what was typed in raw mode becomes cooked mode as soon as one hits the
<return> key.
When a prompt is sent to the terminal the terminal goes from "cooked"
to "raw" mode (just like it does when you start an editor since you
are starting the command-line editor). The settings for the "raw"
mode are based only on the basic settings taken from the "cooked"
mode. Raw mode keeps these setting but changes several other settings
in order to change the mode to "raw". It is not at all based on the
settings used in the previous "raw" mode. Thus if one uses stty to
change settings for the raw mode, such settings will be lost as soon
as one hits the <return> key at the terminal that has supposedly been
"set".
Now when one types stty to look at the terminal interface, one may
either get a view of the cooked mode or the raw mode. You need to
figure out which one you're looking at. It you use stty from another
terminal to deal with a terminal that is displaying a command line,
then the view is that of the raw mode. Any changes made will only be
made to the raw mode and will be lost when someone presses <return> at
the terminal you tried to "set". But if you type a stty command at
your terminal (without using < for redirection) and then hit <return>
it's a different story. The <return> puts the terminal in cooked
mode. Your changes are saved and will still be there when the
terminal goes back into raw mode (unless of course it's a setting not
allowed in raw mode).
This situation can create problems. For example, suppose you corrupt
your terminal interface and to restore it you go to another terminal
and "stty -F dev/ttyS1 sane" (or the like) to restore it. It will not
work! Of course you can try to type "stty sane ..." at the terminal
that is corrupted but you can't see what you typed. All the above not
only applies to dumb terminals but to virtual terminals used on a PC
Monitor as well as to the terminal windows in X. In other words, it
applies to almost everyone who uses Linux. Luckily, a file that runs
stty at boot-time will likely deal with a terminal (or serial port
with no terminal) that has no shell running on it so there's no
problem.
14.4.4. Where to put the stty command ?
Should you need to have stty set up the serial interface each time the
computer starts up then you need to put the stty command in a file
that will be executed each time the computer is started up (Linux
boots). It should be run before the serial port is used (including
running getty on the port). There are many possible places to put it.
If it gets put in more than one place and you only know about (or
remember) one of those places, then a conflict is likely. So make
sure to document what you do.
One place to put it would be in the same file that runs setserial when
the system is booted. The location is distribution and version
dependent. It would seem best to put it after the setserial command
so that the low level stuff is done first. If you have directories in
the /etc tree where every file in them is executed at boot-time
(System V Init) then you could create a file named "stty" for this
purpose.
14.5. Terminfo & Termcap (brief)
See ``Terminfo and Termcap (detailed)'' for a more detailed discussion
of termcap. Many application programs that you run use the terminfo
(formerly termcap) data base. This has an entry (or file) for each
model or type (such as vt100) of terminal and tells what the terminal
can do, what codes to send for various actions, and what codes to send
to the terminal to initialize it.
Since many terminals (and PC's also) can emulate other terminals and
have various "modes" of operation, there may be several terminfo
entries from which to choose for a given physical terminal. They
usually will have similar names. The last parameter of getty (for
both agetty and getty_ps) should be the terminfo name of the terminal
(or terminal emulation) that you are using (such as vt100).
The terminfo does more than just specify what the terminal is capable
of doing and disclose what codes to send to the terminal to get it to
do those things. It also specifies what "bold" will look like (will
it be reverse video or will it be high intensity, etc.), what the
cursor will look like, if the letters will be black, white, or some
other color, etc. In PC terminology these are called "preferences".
It also specifies initialization codes to send to the terminal
(analogous to the init strings sent to modems). Such strings are not
automatically sent to the terminal by Linux. See ``Init String''. If
you don't like the way the display on the screen looks and behaves you
may need to edit (and then update) the terminfo (or termcap) file.
See ``Terminfo Compiler (tic)'' for how to update.
14.6. Setting TERM and TERMINFO
These are two environment variables for terminals: TERM and TERMINFO,
but you may not need to do anything about them. TERM must always be
set to the type of the terminal you are using (such as vt100). If
you don't know the type (name) see ``What is the terminfo name of my
terminal ?''. TERMINFO contains the path to the terminfo data base,
but may not be needed if the database is in a default location (or
TERMINFO could be set automatically by a file that comes with your
distribution of Linux). You may want to look at `` Compiled database
locations''.
Fortunately, the getty program usually sets TERM for you just before
login. It just uses the terminal type that was specified on getty's
command line (in /etc/inittab). This permits application programs to
find the name of your terminal and then look up the terminal
capabilities in the terminfo data base. See ``TERM Variable'' for
more details on TERM.
If your terminfo data base can't be found you may see an error message
about it on your terminal. If this happens it's time to check out
where terminfo resides and set TERMINFO if needed. You may find out
where the terminfo database is by searching for a common terminfo file
such as "vt100" using the "locate" command. Make sure that your
terminal is in this database. An example of setting TERMINFO is:
export TERMINFO=/usr/share/terminfo (put this in /etc/profile or the
like). If the data for your terminal in this data base is not to your
liking, you may need to edit it. See ``Terminfo & Termcap (brief)''.
14.6.1. What is the terminfo name of my terminal ?
You need the exact name in order to set the TERM environment variable
or to give to getty. The same name should be used by both the termcap
and terminfo databases so you only need to find it once. A terminal
usually has alias names but if more than one name is shown, use the
first one.
To find it, try looking at the /etc/termcap... file (if you have it).
If not, then either look at the terminfo trees (see `` Compiled
database locations'') or try to find the terminfo source code file
(see ``Source-code database locations''.
14.7. Rarely Needed /etc/ttytype File
The configuration file /etc/ttytype is used to map /dev/ttySn's to
terminal names per terminfo. tset uses it, but if the TERM
environment variable is already set correctly, then this file is not
needed. Since the Linux getty sets TERM for each tty, you don't need
this file. In other Unix-like systems such as FreeBSD, the file
/etc/ttys maps ttys to much more, such as the appropriate getty
command, and the category of connection (such as "dialup"). An
example line of Linux ttytype: vt220 ttyS1
14.8. Login Restrictions
By default, the root user may not login from a terminal. To permit
this you must create (or edit) the file /etc/securetty per the manual
page "securetty". But use of this may be be distribution specific as
the Suse distribution doesn't use /etc/securetty. To restrict logins
of certain users and/or certain terminals, etc. edit /etc/login.access
(this replaces the old /etc/usertty file ??). /etc/login.def
determines if /etc/securetty is to be used and could be edited so as
to make /etc/securetty not needed (or not used). /etc/porttime
restricts the times at which certain ttys and users may use the
computer. If there are too many failed login attempt by a user, that
user may be prohibited from ever logging in again. See the man page
"faillog" for how to control this.
14.9. Run Command Only If TERM=my_term_type
Sometimes there are commands that one wants to execute at start-up
only for a certain type of terminal. To do this for the stty command
is no problem since one uses the redirection operator < to specify
which terminal the command is for. But what about shell aliases or
functions? You may want to make a function for the ls command so it
will color-code the listing of directories only on color terminals or
consoles. For monochrome terminals you want the same function name
(but a different function body) which will use symbols as a substitute
for color-coding. Where to put such function definitions that are to
be different for different terminals?
You may put them inside an "if" statement in /etc/profile which runs
at startup each time one logs on. The conditional "if" statement
defines certain functions, etc. only if the terminal is of a specified
type.
14.9.1. Example for ls Function
While much of what this if statement does could be done in the
configuration file for dircolors, here's an example for the case of
the bash shell:
______________________________________________________________________
if [ "$TERM" = linux ]; then
eval `dircolors`;
elif [ "$TERM" = vt220 ]; then
ls () { command ls -F $* ; }# to export the function ls():
declare -xf ls
else echo "From /etc/profile: Unknown terminal type $TERM"
fi
______________________________________________________________________
15. Terminfo and Termcap (detailed)
15.1. Intro to Terminfo
Terminfo (formerly Termcap) is a database of terminal capabilities and
more. For every (well almost) model of terminal it tells application
programs what the terminal is capable of doing. It tells what escape
sequences (or control characters) to send to the terminal in order to
do things such as move the cursor to a new location, erase part of the
screen, scroll the screen, change modes, change appearance (colors,
brightness, blinking, underlining, reverse video etc.). After about
1980, many terminals supported over a hundred different commands (some
of which take numeric parameters).
The normal way in which terminfo gives the its information to an
application program is via the "ncurses" functions that a programmer
may put into a C program. For example if a program wants to move the
cursor to row 3, col 6 it simply calls: move(3,6). The move()
function (part of ncurses) knows how to do this for your terminal (it
can read terminfo). So it sends the appropriate escape sequence to
the terminal to make this particular move for a certain terminal.
The terminfo abbreviations are usually longer than those of termcap
and thus it's easier to guess what they mean. The manual pages for
terminfo are more detailed (and include the old termcap
abbreviations). Also, the termcap entries had a size limitation which
is not present for terminfo. Thus, unless you are already committed
to working with termcap, it's suggested you use terminfo.
15.2. Terminfo Database
15.2.1. Introduction
The terminfo database is compiled and thus has a source part and a
compiled part. The old termcap database has only a source part but
this source can, by a single command, be both converted to terminfo
source and then compiled. Thus you may get by without having any
terminfo source since the termcap source can create the compiled
terminfo database. To see a display of the database for the terminal
you're now using (including a PC monitor) type "infocmp" and you
should see the source terminfo "file" for it.
To see if your terminal (say vt100) is in the terminfo data base type
"locate vt100". If you don't know what your terminal name is, explore
the listing of files in the compiled database or see ``What is the
terminfo name of my terminal ?''
15.2.2. Where is the database located ?
15.2.2.1. Compiled database locations
Typing "locate vt100" may show /usr/lib/terminfo/v/vt100,
/usr/share/terminfo/v/vt100, /home/you/.terminfo/v/vt100, and/or
/etc/terminfo/v/vt100. All these are possible locations of the
compiled terminfo files. Although the /etc/terminfo directory is not
a standard location for it, having a few terminal types there could be
useful in case the /usr directory is not accessible. For example /usr
could be on a separate disk or partition that failed to mount.
Normally, programs that use your main terminfo data base are able to
find it if it's in at least one of the locations mentioned above.
Otherwise the environment variable TERMINFO may be set to the path to
this database. Example: TERMINFO=/usr/share/terminfo
If the compiled terminfo is in more than one location, everything is
usually OK until someone gets a new terminfo file(s) (from a newer
distribution, from the net, by editing the old one, etc.). The newer
terminfo needs to be put in all the existing locations (or redundant
locations need to be abolished). If you don't ensure this gets done,
then some application programs could wind up still finding and using
the old (and buggy) terminfo data that sill exists in a "usual"
location. Setting the environment variable TERMINFO to the up-to-date
location (as mentioned above) may help avoid this problem.
15.2.2.2. Source-code database locations
The source code you use may reside in /etc/termcap and/or in
terminfo.src (or another name). See the man pages: terminfo(5) or
termcap(5) for the format required to create (or modify) these source
files. The file terminfo.src may be in various locations on your
computer or it may not be included with your Linux distribution. Use
the locate command to try to find it. It is available for downloading
(under the name termtypes.ti) from
<http://sagan.earthspace.net/terminfo>
15.2.3. Terminfo Compiler (tic)
The data in the source files is compiled with the "tic" program which
is capable of converting between termcap format and terminfo format.
Thus you can create a compiled terminfo data base from termcap source.
The installation program which was used to install Linux probably
installed the compiled files on your hard disk so you don't need to
compile anything unless you modify /etc/termcap (or terminfo.src ).
"tic" will automatically install the resulting compiled files into a
terminfo directory ready to be used by application programs.
15.2.4. Look at Your Terminfo
It's a good idea to take a look at the terminfo entry for the terminal
you are using (source code of course) and read the comments. A quick
way to inspect it without comments is to just type "infocmp". But the
comments may tell you something special about the terminal such as how
you need to set it up so that it will work correctly with the terminfo
database.
15.2.5. Deleting Data Not Needed
In order to save disk space, one may delete all of the database except
for the terminals types that you have (or might need in the future).
Don't delete any of the termcaps for a "Linux terminal" (the console)
or the xterm ones if you use X-Windows. The terminal type "dumb" may
be needed when an application program can't figure out what type of
terminal you are using. It would save disk space if install programs
only installed the terminfo for the terminals that you have and if you
could get a termcap for a newly installed terminal over the Internet
in a few seconds.
15.3. Bugs in Existing Terminfo Files (and Hardware)
Unfortunately, there are a number of bugs in the terminfo and termcap
files. In addition, many of these definitions are incomplete and do
not define certain features available on the terminals. Sometimes you
can get by without modifying the terminfo but in other cases you need
to modify it or possibly use another emulation that has a good
terminfo.
The sad state of the supplied terminfo files is due to a number of
reasons. One is that during the 1980's when many of them were written
(often in termcap format), application programs did not utilize more
advanced terminal features. Thus if such feature were not in the
termcap (or terminfo) file, no one complained. Today, programs such
as vim use "context highlighting" and minicom uses the terminal's
graphics character set. These often need more definitions to be added
to the old termcap. This may (or may not) have already been done.
Most terminals had hardware bugs (in their firmware) and sometimes
they were "fixed" by modifying the termcap. Then the manufacturer
might send out replacement chips which would fix the bug. Not all
owners would bother to get the replacement chips. Thus there may be 2
or more terminfos for your terminal, depending on what firmware chips
it has in it. This situation was often not noted in the termcap and
only one termcap may be supplied with Linux. Some hardware bugs which
existed for features that were almost never used in the past likely
never did get fixed. Also, some reported hardware bugs may never have
been fixed since they were not of much significance at the time or
because the company went out of business, etc.
15.4. Modifying Terminfo Files
To do this you need a manual for your terminal showing what escape
sequences it uses. Newer manuals from the 1990's often don't show
this. You also need a terminfo manual (or the like). For example, in
order to add graphic capability you must assign values to the terminfo
variables: enacs, rmacs, and smacs by editing a source file. Then by
using "tic" you may compile it. "tic" should automatically put the
compiled terminfo file in the correct directory reserved for it.
If you would like to find a better terminfo entry for a certain
terminal than the one supplied, you might try searching the Internet
(but what you find may be worse). If your new terminfo entry is
better than the old one and it seems stable (you've used it for a
while with no problems) then you should send a copy to the maintainer
of terminfo as noted at the start of the source file for terminfo (or
termcap).
15.5. Init String
Included in the terminfo are often a couple of initialization strings
which may be sent to the terminal to initialize it. This may change
the appearance of the screen, change what mode the terminal is in,
and/or make the terminal emulate another terminal. An initialization
string is not automatically sent to the terminal to initialize it.
One might expect that the getty program should do this but if it did,
one could make a change to the set-up at the terminal and this change
wouldn't be implemented because the init string would automatically
cancel it. You must use a command given on the command line (or in a
shell script) to send the init string such. Such commands are:
"tset", "tput init", or "setterm -initialize". Sometimes there is no
need to send the init string since the terminal may set itself up
correctly when it is powered on (using options/preferences one has set
up and saved in non-volatile memory of the terminal).
15.6. TERM Variable
The Environment variable TERM should be set to the name of terminal
which you are using. If TERM hasn't been set yet and you don't know
the name of your terminal see ``What is the terminfo name of my
terminal ?''. It is normally set by the terminal_type parameter
passed to the getty program (look at it in the /etc/inittab file).
This name must be in the Terminfo data base. Just type "set" at the
command line to see what TERM is set to (or type: tset -q). At a
console (monitor) TERM is set to "linux" which is the PC monitor
emulating a fictitious terminal model named "linux". Since "linux" is
close to a vt100 terminal and many text terminals are also, the
"linux" designation will sometimes work as a temporary expedient with
a text terminal.
If more than one type of terminal may be connected to the same port
(/dev/tty...) (for example, if there is a switch to permit different
terminal types to use the same serial port, or if the port is
connected to a modem to which people call in from different types of
terminals) then TERM needs to be set each time someone connects to the
serial port. There is often a query escape sequence so that the
computer may ask the terminal what type it is. Another way is to ask
the user to type in (or select) the type of terminal s/he is using.
You may need to use tset for this or write a short shell script to
handle this.
One way to do this is to use "tset" (see the manual page). tset tries
to determine the terminal name of the terminal you are using. Then it
looks up the data in terminfo and sends your terminal an init string.
It can also set the value of TERM. For example, a user dials in and
logs in. The .profile login script is executed which contains within
it the following statement: eval `tset -s ?vt100`. This results in:
The user is asked if s/he is using a vt100. The user either responds
yes or types in the actual terminal type s/he is using. Then tset
sends the init string and sets TERM to this terminal name (type).
15.7. Terminfo/Termcap Documents
�
� manual pages for terminfo(5) (best) and/or termcap(5). The Termcap
Manual <http://www.delorie.com/gnu/docs/termcap/termcap_toc.html>
(2nd ed.) by Richard M. Stallman is a GNU manual which is somewhat
obsolete since it doesn't include terminfo.
� the files: terminfo.src and /etc/termcap have info about various
versions of termcap files, naming conventions for terminals, and
special capabilities code named u6-u9. If you don't have one, go to
<http://sagan.earthspace.net/terminfo>
� "Termcap and Terminfo" is a book published by O'Reilly in 1988.
16. Using the Terminal
16.1. Intro to Using Terminal
This section is about controlling the terminal-computer interface
and/or changing the terminal set-up while using the terminal. It
explains (or points to explanations of) how the user of a terminal can
control and inspect the interface and how to use various commands
provided by the device driver. It does not explain how to use the
many application programs, shells or most Linux utilities. Two
commands commonly used at the terminal are:
� clear (to clear the screen)
� reset (to reset the terminal
� setterm -reset (alternative for "reset" in case of bug)
16.2. Starting Up the Terminal
Of course the power must be on for the terminal to work. If you don't
see a login prompt hit the "return" (or "enter") key a few times.
Then type your account name (followed by a return/enter) and your
password when prompted for it (also followed by return/enter). Make
sure not to type all capital letters. If you do, the computer may
think that you have an old terminal that can't send lowercase letters
and the serial driver may set itself up to send only capital letters
to the terminal.
If nothing happens, make sure that both the host computer and the
terminal are OK. If the host computer is shut down (no power) what
you type at the terminal keyboard may appear on the screen since the
transmit and receive pins at the computer may be connected together
resulting in echoing of characters by an "off" computer. If you can't
log in when the host computer is running, see ``Trouble-Shooting''.
16.3. Terminal (Serial) Device Driver
When typing at the command line, the shell (such as the Bash shell) is
reading what you type and reacting to it. What you type first passes
thru the terminal driver part of your operating system. This driver
may translate certain characters (such as changing the "return"
character generated by the "return" key into a "new-line" character
for Linux files). It also recognizes certain control codes which you
may type at the keyboard such as ^C to interrupt the execution of a
program. It also normally echoes what you type back to the display.
``Stty'' may be used to configure how this terminal driver behaves,
including disabling some (or all) of its functionality.
16.4. Problems with Editors
There may be some problems with using both emacs and vi on some
terminals.
16.4.1. emacs and ^S, ^Q
If software flow control exists, then the ^S command in emacs will
freeze the display. The ^Q command will unfreeze the display. The
fix is to map this to another key-press by configuring emacs that way.
16.4.2. vi and Cursor-Keys
Vi uses the esc-key as a command to exit insert mode. If one hits an
arrow-key (cursor-key) an escape sequence (starting with the ESC
character) is sent to the host. Vi must distinguish between these two
meanings of ESC. A smart vi (such as vim if configured for it) is
able to detect the difference by noting the time between the ESC and
the next key. If it's a short time, then it's likely that a cursor
key was pressed. Use "help cursor-keys" in vim to find out more.
Here's another way to fix this. On VT terminals the left-arrow-key
may be either set to send ESC [ D or ESC O D. The other arrow keys
are similar but use A, B, and C instead of D. If you're having
problems, choose ESC [ D since the "O" in the other alternative could
be interpreted by vi as a command to "Open a line". The "[" should be
interpreted by vi to mean that an arrow-key has been pressed. ESC [ D
will be sent provided "Cursor Key Application Mode" has not been set.
ESC [ D is normally the default so everything is seemingly OK. Except
that many termcaps contain a string (not the init string) which sets
what you want to avoid: "Application Mode". Editors may send this
string to the terminal when the editor starts up. Now you are in
trouble.
This string has the termcap code "ks" (smkx in terminfo) meaning
enable the function (and related) keys (including the arrow keys). An
application enables these keys by sending the "ks" string to the
terminal. Whoever wrote the termcap reasoned that if an application
wants to enable these keys, then they should be put into "Application
Mode" since this is an "application", but you don't want this.
The Linux console has no "ks" string so you can't fall into this trap
at the console. For other terminals you may need to edit the termcap
(or terminfo) or use another termcap entry. You need to change not
only the "ks" string but also the termcap definitions of what they
send: kd, kl, kr, ku. Then run tic to install it.
For vim (vi iMproved) there is a way to set it up to work OK with ESC
O D (so you don't need to edit termcap): See vim help for
"vt100-cursor-keys". You may run "gitkeys" and then press your cursor
keys to see what they send but they may be set to send something
different when you're in an editor.
16.5. Color ls Corruption
If ls is corrupting your terminal emulation with the color feature,
turn it off. ls --color, and ls --colour all use the color feature.
Some installations have ls set to use color by default. Check
/etc/profile, etc. for ls aliases. See ``Example for ls Function''
for how to have ls do color for the console and do monochrome for
terminals.
16.6. Display Freezes (hung terminal)
The symptom of a hung terminal is where what you type doesn't display
on the terminal (or in some cases displays but doesn't do anything).
If what you type is invisible (or does nothing) type ^Q to restart
flow (if flow control stopped it). Hanging may also be due to:
``Sent Terminal Binary'' or ``Abnormally Exited a Program''
If you didn't do any of these two, then your program could by buggy or
you interaction with it fatally illegal.
If you want to quit the program you were running and you can't do it
by the usual methods (some programs have special keys you must hit to
exit) then try killing it from another terminal using "top" or "kill".
If the process refuses to die, you may try sending it signal 9 from
top which should force it to exit. The "9" type of forced exit may
leave some temporary files lying around as well as a corrupted
interface. Killing the login shell should result in a startup of
getty with a new login prompt.
People new to Linux may unintentionally press Ctrl-S (^S) (or the "No
Scroll" key) which mysteriously freezes the screen (although that is
what this key is supposed to do if you use software flow-control). To
restore normal screen interaction, press Ctrl-Q (^Q). Note that
everything typed during the "freeze" gets executed but you don't see
any report of this until you hit ^Q. Thus when it's frozen, don't
type anything drastic that might destroy files, etc. One argument for
using hardware flow-control is to prevent such freezes.
16.7. Corrupted Terminal Interface
This includes the case of a "frozen display" = "hung terminal" of the
previous section.
16.7.1. Symptoms and Some Fixes
When the display doesn't look right, or when what you type doesn't
display correctly (if at all), or nothing happens when you type a
command, you may have a corrupted terminal interface. In rare cases
when the serial port hardware gets itself corrupted, the only fix may
be to cycle power (turn off the PC and reboot). Sometimes logging in
again will solve the problem. To do this kill the shell process
running on the terminal (or kill getty if it's running). You do this
from another terminal. Once killed, a new getty process respawns
which hopefully will end the corruption. Recycling power (or
resetting) for the terminal may help too.
The corruption may be due to things such as bug in the program you're
using, a hardware failure (including an obscure hardware defect that
you can normally live with), or possibly an incorrect configuration.
If everything was working normally but it suddenly goes bad, it may be
that the interface got corrupted by something you did. Three mistakes
you might have made to corrupt the interface are:
� ``Sent Terminal Binary''
� ``Abnormally Exited a Program''
� ``Typed ctrl-S by mistake''
16.7.2. Sent Terminal Binary Characters
Your terminal will change its characteristics if sent certain escape
sequences or control characters. It you inadvertently try to display
a binary file, it might by chance contain such sequences which may put
your terminal into some strange mode of operation or even make it
unusable. Always view or edit a binary file with programs designed
for that purpose so that this doesn't happen. Most editors and pagers
will handle binary OK so as not to corrupt the interface. Some may
display a message telling you that they can't edit binary. But using
"cat ...." or "cp .... /dev/tty.." where .... is a binary file, will
send the binary to the terminal and likely corrupt things.
Corruption it can also happen when using a communications program
where a remote computer may send binary to your screen. There are
numerous other ways it can happen so be prepared for it. Even a
supposed ASCII file may contain unwanted control codes.
To fix this problem reset the terminal. Type either just "reset" (may
be broken) or "setterm -reset" (followed by a <return> of course).
You may not be able to see what you're typing. This will send the
reset string from the terminfo entry to the terminal. As an
alternative to this, if the correct set-up has been saved inside the
terminal then pressing a special key(s) (perhaps in setup mode) may
restore the settings. Then you might still need to use "tset" to send
the init string if you use it to set up your terminal.
Note that the "reset" command appears to be broken for terminals that
need "clocal" set since "reset" seems to unset "clocal". In this case
instead of fixing the problem, "reset" only compounds it by disabling
communication between the terminal and computer. You will likely need
to log in again and hope the getty sets "clocal". If you see a login
prompt without asking for it, you're in luck. Otherwise see
``Symptoms and Some Fixes'' for how to get a login prompt. You should
try out "reset" before you need it and use "setterm -reset" if "reset"
logs you out or otherwise doesn't work right. I submitted a bug
report in Mar. 2000 so "reset" could be fixed by now.
``Symptoms and Some Fixes''
16.7.3. Abnormally Exited a Program
Large application programs (such as editors) often use the stty
command (or the like) in their code to temporarily change the stty
configuration when you are running the program. This may put the
device driver into "raw" mode so that every character you type goes
directly thru to the application program. Echoing by the driver is
disabled so that everything you see on the screen comes directly from
the application program. Thus many control commands (such as ^C) may
not work within such applications.
When you tell such an application to quit, the application program
first restores the stty settings to what they were before the
application program started. If you abnormally exit the program (you
may guess this has happened when what you type no longer displays on
the screen) then you may still be in "raw mode" on the command line.
To get out of raw mode and restore the normal stty settings type "stty
sane". However, you must type this just after a "return" and end it
with a "return". But hitting the "return" key doesn't do the job
since the "return" code no longer gets translated to the new-line
characters that the shell is waiting for. So just type new-line (^J)
instead of "return". The "sane" terminal interface may not be exactly
the same as the normal one but it usually works. "stty sane" may also
be useful to get out of a corrupted interface due to other causes.
16.8. Special (Control) Characters
A number of control characters which you may type at the keyboard are
"caught" by the terminal driver and perform various tasks. To see
these control commands type: stty -a and look at lines 2-4. They are
tersely explained in the stty manual pages. They may be changed to
different control characters or disabled using the stty command. Thus
your control characters might be different than those described below.
They are used for command-line editing, interrupting, scrolling, and
to pass the next character thru transparently.
16.8.1. Command-Line Editing
While the terminal driver has a few commands for command-line editing,
some shells have a built-in real editor (such as "readline" in the
Bash shell). Such an editor is normally on by default so you don't
need to do anything to enable it. If it's available you don't need to
learn many of the following commands although they often still work
along with the command-line editor. The most important to learn are
^C (interrupt), ^D, and how to stop scrolling.
� Delete-key (shown by stty as ^?) erases the last character
� ^U kills (deletes) the line
� ^W deletes a word backwards
� ^R reprints the line. Useful mainly on hard copy terminals ??
16.8.2. Interrupting (& Quit, Suspend, EOF, Flush)
� ^C interrupts. Exits the program and returns you to the command-
line prompt.
� ^/ quits. Same as interrupt ^C but weaker. Also dumps a "core"
file (which you likely have no use for) into your working
directory.
� ^Z suspends. Stops the program and puts it in the background.
Type fg to restart it.
� ^D end of file. If typed at the command-line prompt, exits the
shell and goes to where you were before the shell started.
� ^O flush. Not implemented in Linux. Sends output to /dev/null.
16.8.3. Stop/Start Scrolling
If what you want to see scrolls off the bottom of the screen, you may
prevent this by sending a "stop" signal (^S or Xoff) to the host
(provided Xon-Xoff ``Flow Control'' is enabled). Send a "start signal
to resume (^Q or Xon). Some terminals have a "No Scroll" key which
will alternately send Xoff and Xon or possibly send the hardware flow
control signals ?? Here's what ctrl-S (^S) and ctrl-Q (^Q) do:
� ^S stops scrolling (Xoff)
� ^Q resume scrolling (Xon)
If you want to both stop scrolling and quit, use ^C. If you want to
stop scrolling to do something else but want to keep the program that
was generating the output in memory so you can resume scrolling later,
use ^Z suspend.
An alternative scrolling method is to pipe the output thru a pager
such as more, less, or most. However, the output might not be
standard output but could be error output which the pager doesn't
recognize. To fix this you may need to use redirection "2>&1" to get
the pager to work OK. It is often simpler to just use ^S and ^Q
unless you need to scroll backwards.
At a PC console (emulating a terminal) you may scroll backwards by
using Shift-PageUp. This is frequently needed since the scrolling is
often too fast to stop using ^S. Once you've scrolled backwards
Shift-PageDown will scroll forward again.
16.8.4. Take Next Character Literally
^V sends the next character typed (usually a control character)
directly thru the device driver with no action or interpretation.
Echoed back are two ASCII characters such as ^C.
16.9. Viewing Latin-1 Files on a 7-bit Terminal
Some "text" files are 8-bit Latin1 (see ``Character-Sets''). If you
have a terminal that will not display Latin1 (or don't have the Latin1
character set selected), then a bullet symbol will display as a 7,
etc. When viewing manual pages (they are Latin1) you may give the
option -7 to man so as to translate the 7's, etc. to something close
to a bullet (in ASCII). Are there some pagers that make these
translations ??
16.10. Inspecting the Interface
These utility programs will provide information about the terminal
interface:
� gitkeys: shows what byte(s) each key sends to the host.
� tty: shows what tty port you are connected to.
� set: shows the value of TERM (the terminfo entry name)
� stty -a: shows all stty settings.
� setserial -g /dev/tty?? (you fill in ??) shows UART type, port
address and IRQ number.
� infocmp: shows the current terminfo entry (less comments)
16.11. Changing the Terminal Settings
The terminal settings are normally set once when the terminal is
installed using the setup procedures in the terminal manual. However,
some settings may be changed when the terminal is in use. You
normally would not give any "stty" of "setserial" commands when the
terminal is in use as they are likely to corrupt the terminal
interface. However, there are changes you may make to the appearance
of the terminal screen or to its behavior without destroying the
integrity of the interface. Sometimes these changes are made
automatically by application programs so you may not need to deal with
them.
One direct method of making such changes is to use the setup key (or
the like) at the terminal and then use menus (or the like) to make the
changes. To do this you may need to be familiar with the terminal.
The other 3 methods send an escape sequence from the computer to the
terminal to make the changes. These 3 examples show different methods
of doing this to set reverse video:
1. setterm -reverse
2. tput -rev
3. echo ^[[7m
16.11.1. setterm
This is the easiest command to use. It uses long options (but doesn't
use the normal -- before them). It consults the terminfo database to
determine what code to send. You may change the color, brightness,
linewrap, keyboard repeat, cursor appearance, etc.
16.11.2. tput
The "tput" command is similar to "setterm" but instead of using
ordinary words as arguments, you must use the abbreviations used by
terminfo. Many of the abbreviations are quite terse and hard to
remember.
16.11.3. echo
In the example "echo ^[[7m" to set reverse video, the ^[ is the escape
character. To type it type ^V^[ (or ^V followed by the escape key).
To use this "echo" method you must find out what code to use from a
terminal manual or from terminfo or termcap. It's simpler to use
setterm or tput if you are typing on the command line. Since "echo
..." will execute faster (since it doesn't do any lookups) it's good
for using in shell scripts which run at start-up, etc.
16.11.4. Saving Changes
When you turn off the terminal the changes you made will be lost
(unless you saved them in non-volatile terminal memory by going into
set-up mode and saving it). If you want to use them again without
having to retype them, put the commands in a shell script or make it a
shell function. Then run it when you want to make the changes. One
way to make the changes semi-permanent is to put the commands in a
file that runs each time you login or start up the computer.
16.12. Make a Terminal the Console
This is also called a "serial console". Many messages from the system
are normally only sent to the console (the PC monitor). Some of the
messages sent to the console at boot-time may also be seen on any
terminal after the boot succeeds by typing the command: dmesg. If the
boot failed this will not be of any use since the terminal can't work
without an operating system. It's possible to modify the Linux kernel
so as to make a terminal serve as the console and receive all the
messages from Linux intended for the console. Unfortunately, the
messages from the BIOS (which display on the monitor when a PC is
first started) will not display on this terminal (but still display on
the monitor).
Some people do this when they run a PC without a monitor or keyboard.
Normally, a PC will not start up without a keyboard and video card but
some BIOSs permit it. If you are lucky enough to have such a BIOS
that supports "console re-direct" you will likely then need to setup
the BIOS using the CMOS menus when you start your PC.
16.12.1. For Kernels 2.2 or higher
The instructions for creating a serial-console are included with
source code documentation in the file: serial-console.txt. Normally,
the device /dev/console is linked to tty0 (the PC console). For a
serial-console you create a new /dev/console which is a true device
(and not linked to something else). You must also put a statement
regarding the serial-console into /etc/lilo.conf and then run lilo.
This is because the equivalent of "setserial" needs to be run to set
up your serial-console before the kernel is loaded. See the above
mentioned documentation and the man page for lilo.conf for more
details.
Here is an example /etc/lilo.conf file contents (for ttyS0):
prompt
timeout=50
boot = /dev/sda
vga = normal # force sane state
install = /boot/boot.b
message = /boot/message
image = /vmlinuz
root = /dev/sda1
label = console
serial = 0,9600n8
append = "console=ttyS0"
16.12.2. For Kernels before 2.2
The Linux Journal in April 1997 had an article on patching the Linux
kernel. You add a couple of #defines at the start of
src/linux/drivers/char/console.c:
#define CONFIG_SERIAL_ECHO
#define SERIAL_ECHO_PORT 0x2f8 /* Serial port address */
The following was not in the Linux Journal article.
In kernel 2.+ (and earlier ??) you need to also set the baud
rate (unless 9600 is OK). Find these 2 lines:
serial_echo_outb(0x00, UART_DLM); /* 9600 baud */
serial_echo_outb(0x0c, UART_DLL);
Change 0x0c in the line above (depending on the baud rate you want):
115200 baud: 0x01 19200 baud: 0x06 2400 baud: 0x30
57600 baud: 0x02 9600 baud: 0x0c 1200 baud: 0x60
38400 baud: 0x03 4800 baud: 0x18
If you currently use the console to select which operating system to
boot (using LILO), but would like to do this from a terminal, then you
need to add a line to the /etc/lilo.conf file. See the manual page
for lilo.conf and search for "serial=".
16.12.3. Can I Run Linux without a Monitor (PC Console) ?
Yes, you use a terminal and make it behave like the console per above.
You will likely still need a video card since most BIOSs require one
to get the PC started. Your BIOS may also require a keyboard to get
started or it may have an option where you can set the BIOS not to
require a keyboard.
16.13. Multiple Sessions
The "screen" package runs multiple sessions something like virtual
terminals on the console: See ``The Console: /dev/tty?''. However,
this is not like "pages" (``Pages'') since the image of the pages are
stored in the host computer and not inside the terminal as they are
with "pages".
16.14. Logging Out
To log out type either "logout" or "exit". Under some circumstances
your request will be refused, but you should be told why. One reason
for refusal is if you are not in the same shell into which you logged
into. Another way to log out is to press ^D. Since ^D is also used
for other purposes, you may not want it to log you out. If you set
IGNOREEOF in the Bash shell then ^D will no longer log you out.
16.15. Chatting between Terminals, Spying
If two persons logged into terminals on the same host computer want to
chat with each other they may use the "write" or the "talk" program.
On the Internet, chatting may be done using the "lynx" browser.
For spying on what someone else is doing at their terminal use the
"ttysnoop" program. In "ttysnoop" the two terminals have the same
status and anything typed on either keyboard appears on both screens
(in the same location). So if you're really spying and don't want to
be detected, you shouldn't type anything.
"ttysnoop" can be used for training with instructor and trainee
sitting side-by-side, each at their own terminal. The instructor may
watch what the trainee is typing and can correct any mistakes by
typing it correctly. The trainee can watch what the instructor types
and then try typing it. It's just like they used one terminal with
both people having their hands on the keyboard at the same time.
There's one shortcoming to "ttysnoop" and that is that the terminals
involved should all be (or emulate) the same type of terminal (such as
vt100). Since the "Linux" emulation done by a console (monitor) and
the "minicom" emulation are close to vt100 one may use ttysnoop using
two PCs, one running "minicom" which emulates a terminal.
There's a non-free program called "DoubleVision" that is something
like ttysnoop but does much more. Terminals may be of different types
and it can remember and playback sessions on terminals so you can
observe what happened in the past. The webpage is at
<http://www.tridia.com>.
16.16. Sharing the Serial Port
If you have another serial device (a printer, modem, etc.) that you
want to connect to the same serial port that a terminal is on, then
there is more involved than just plugging in the other device. This
is mainly because when the getty program or shell is listening on the
port, they (or the serial driver) are likely to give a response to any
bytes sent to the port. Getty will respawn even if it's killed and
will keep sending login prompts to the serial port (which may now be
connected to another device). One way to work around this problem is
to switch runlevels so that no getty program or shell is running on
the port. But if your other use for the port only uses the port for
output to an external device, then you may live dangerously by using
the port for the other use with the shell or getty (intended for a
terminal) simultaneously running on it.
For the runlevel fix, you may create another runlevel in which no
getty program runs on the port. Then you enter that new runlevel and
use the serial port for something else. To set this up you need to
edit /etc/inittab and check and/or set the runlevels at which getty
runs. For example the line: "S1:23:respawn:/sbin/getty ..." means
that getty will run (on ttyS1) in runlevels 2 and 3. Now you could
have it only run in level 2 (by deleting "3") and then go to runlevel
3 when you want to use the other serial device. Thus to use the
serial port for something else, the super-user would type "init 3" to
switch to runlevel 3. Type "init 2" to get back to runlevel 2. Note
that each runlevel may have a different set of initialization scripts
but you can change this if you want, so that the same scripts are run
in both runlevels. How the scripts and runlevels work are different
for each Linux distribution. In Debian, the explanation of this is in
/usr/doc/sysvinit but also look in /usr/share/doc.
There's also the problem of the stty configuration of the port. When
the port is being used for the terminal it has certain configurations
but when say "init 3" is used to switch runlevels and disable getty on
the port, the original (boot-time) configuration of the port is not
restored. You are likely to wind up with the port configured in a
"raw" mode. This means that the serial port likely needs to be fully
reconfigured by stty, either by a script you write or by the next
application which runs on the port. Some applications such as
printing from the serial port are not capable of fully reconfiguring
(the /etc/printcap can only partially reconfigure). Thus you may need
to write a script to do it. The stty configuration of a terminal will
be different depending on whether a shell is running on it, whether
it's at the "login" prompt, etc. Thus the stty configuration after
switching runlevels may vary.
17. Trouble-Shooting (software)
If you suspect that the problem is a hardware problem, see the
``Repair and Diagnose'' section. If the problem involves the serial
port itself see the Serial-HOWTO.
Here is a list of possible problems:
� ``Is the Terminal OK ?'' Suspect the terminal is defective.
� ``Missing Text'' Either skips over some text or displays some text
OK and hangs.
� ``Getty Respawning Too Rapidly'' (console error message)
� ``Fails Just After Login''
� ``Can't Login'' but login prompt is OK.
� ``Garbled Login Prompt''
� ``No Sign of any Login Prompt''
There are two cases where the terminal goes bad. One is when it's
been recently working OK and suddenly goes bad. This is discussed in
the next sub-section. The other case is where things don't work right
just after you install a terminal. For this case you may skip over
the next section.
17.1. Terminal Was Working OK
When a formerly working terminal suddenly goes bad it is often easy to
find the problem. That's because (except for hardware failures) the
problem is likely due to something that you did (or something the
software you used did).
The problem may be obvious such as an error message when the terminal
is first turned on. If it makes a strange noise it likely needs
repair. See ``Repair & Diagnose''. First, think about what has been
done or changed recently as it's likely the cause of the problem. Did
the problem happen just after new system software was installed or
after a change in the configuration?
If the terminal isn't responding correctly (if at all) to what you
type to it, you may have a ``Corrupted Terminal Interface''.
17.2. Terminal Newly Installed
If you've just connected up a terminal to a computer per instructions
and it doesn't work this section is for you. If a terminal that
formerly worked OK doesn't work now then see ``Terminal Was Working
OK'' If you suspect that the serial port on your computer may be
defective you might try running a diagnostic test program on it. At
present (June 1998) it seems that Linux doesn't yet have such a
diagnostic program so you may need to run diagnostics under MS
DOS/Windows. There are some programs to monitor the various serial
lines such at DTR, CTS, etc. and this may help. See ``Serial
Monitoring/Diagnostics''
One approach is to first see if the the terminal will work by trying
to copy a file to the terminal (cp my_file /dev/ttyS?) under the most
simple situation. This means with the modem control lines disabled
and at a show speed that doesn't need flow control (make sure that any
hardware flow control is disabled). If this copy works, then make the
situation a little more complicated and see if it still works, etc.,
etc. When the trouble appears just after you made a change, then that
change is likely the source of the trouble. Actually, its more
efficient (but more complex) to jump from the simple situation to
about half way to the final configuration so that the test eliminates
about half of the remaining possible causes of the problem. Then
repeat this methodology for the next test. This way it would only
take about 10 tests to find the cause out of a thousand possible
causes. You should deviate a little from this method based on hunches
and clues.
17.3. Is the Terminal OK ?
A good terminal will usually start up with some words on the screen.
If these words convey no error message, its probably OK. If there is
no sign of power (no lights on, etc.) re-plug in the computer power
cord at both ends. Make sure there is power at the wall jack (or at
the extension cord end). Try another power cord if you have one.
Make sure the terminal is turned on and that its fuse is not blown. A
blank (or dim) screen may sometimes be fixed by just turning up the
brightness and contrast using knobs or a keyboard key in set-up mode.
If it still won't work See ``Repair & Diagnose'' for tips on repairing
it.
If the terminal starts up OK, but you suspect that something may be
wrong with it, go into "local mode" where is works like a typewriter
and try typing on it. See ``Local Mode''.
17.4. Missing Text
If some text displays on the terminal OK and then it stops without
finishing (in the middle of a word, etc.) or if chunks of text are
missing, you likely have a problem with flow control. If you can't
figure out right away what's causing it, decrease the speed. If that
fixes it, it's likely a flow control problem. It may be that flow
control is not working at all due to failure to configure it
correctly, or due to incorrect cable wiring (for hardware flow
control). See ``Flow Control''
If single characters are missing, perhaps the serial port is being
overrun by too fast a speed. Try a slower baud rate.
If you are using a baud rate under 1200 (very slow, mostly used for
old hard-copy terminals and printers) and the text gets truncated,
then the problem may be in the serial device driver. See Printing-
HOWTO under "Serial devices" on how to fix this.
17.5. Getty Respawning Too Rapidly
17.5.1. Serial Module Not Loaded
Use the "lsmod" command to see if the serial module is loaded.
17.5.2. No Modem Control Voltage
If getty can't open and/or use a port because of the lack of a
positive modem control voltage on one of the pins, then getty might be
killed. Then, per the instructions in inittab, getty respawns and
tries again, only to be killed again, etc., etc. You may see an error
message indicating that due to getty respawning too rapidly it has
been temporarily disabled. Try using the "local" option with getty
and/or check the modem control settings and voltages.
17.5.3. Key Shorted
Another possible cause of getty respawning is if a keyboard key is
shorted, giving the same result as if the key was continuously held
down. With auto-repeat enabled, this "types" thousands characters to
the login prompt. Look for a screen filled with all the same
character (in some cases with 2 or more different characters).
17.6. Fails Just After Login
If you can login OK but then you can't use the terminal it may be
because the starting of the login shell has reconfigured the terminal
(to an incorrect setting) by a command which someone put into one of
the files that are run when you login and a shell starts. These files
include /etc/profile and /.bashrc. Look for a command starting with
"stty" or "setserial" and make sure that it's correct. Even if it's
done OK in one initialization file, it may be reset incorrectly in
another initialization file that you are not aware of. Ways to get
into the systems to fix it are to use another terminal or console, use
a rescue diskette, or type: "linux single" at the lilo prompt which
puts you into single user mode without running startup files.
17.7. Can't Login
If you get a login prompt but get no response (or perhaps a garbled
response) to your login attempts a possible cause is that the
communication is bad one-way from the terminal to the computer. It
could be a bad or mis-wired cable/connector. If you're not already
using the "local" option with getty, do so to disable the modem
control lines. See ``Getty (in /etc/inittab)''. You might also
disable hardware flow control (stty -crtscts) if it was enabled. If
it now works OK then your modem control lines are likely either not
wired correctly or there's a mistake in your set-up. Some terminals
allow setting different values (such as baud rate) for send and
receive so the receive could be OK but the send bad.
You should also (at the console) try "stty < /dev/ttyS1" (if you use
ttyS1) to see that it's set up correctly. It will often be in raw
mode (and this is probably OK) with -icanon and -echo etc. If the
terminal incorrectly set at half-duplex (HDX), then one set of the
characters you see when you type are coming from the terminal itself.
If the characters are doubled, then the echos from the computer are OK
and you may switch to full-duplex to fix this. But if half-duplex is
set and you only see what looks like normal "echos", then they are not
coming from the computer as they should be.
If you get a message saying something like "login failed" then if
there is no error in typing or in the password, there may be some
restrictions on logins which will not allow you to log in.
Unfortunately, this message, may not tell you why it failed. See
``Login Restrictions''
17.8. Garbled Login Prompt
This may be due to using the wrong character set, transmission errors
due to too high of a baud rate, incompatible baud rates, incompatible
parity, or the wrong number of bits per byte. If it's a variety of
strange characters you have the wrong character set or a high order
bit is being set by mistake. If words are misspelled, try a lower
baud rate. For baud, parity, or bits/character incompatibilities you
see a lot of the same "error character" which represents a real
character that can't be displayed correctly due to an error in parity
or baud rate.
If you are using agetty (often just named getty), the agetty program
will detect and set parity and/or bits/character if you type
something. Try it with a return to see if it fixes it.
17.9. No Sign of any Login Prompt
This is when nothing at all happens at the terminal, but the terminal
seems to be working OK. One of the first things to do is to make sure
that all cable connections are tight and connected to the correct
connector on both the computer and terminal. Other causes include
defective hardware or cables (must be null modem), getty not running,
terminal setup at wrong baud-rate, terminal in local mode, etc.. At
this point two different avenues of approach are (you may pursue more
than one at a time).
� ``Diagnose Problem from the Console''
� Measure Voltages ``Measure Voltages''
17.9.1. Diagnose Problem from the Console
At the console (or another working terminal), use "top" or "ps -al" to
see if getty is running on the port. Don't confuse it with getty
programs running on other ports or on the virtual consoles. You will
not get a login prompt unless getty runs.
One test is to try to copy a short file to the terminal (It might be a
good idea to try this near the start of the installation process
before setting up getty). Use the Linux copy command such as: cp
file_name /dev/ttyS1. If it doesn't work, use stty to make the
interface as simple as possible with everything disabled (such as
hardware flow control: -crtscts; parity, and modem control signals:
clocal). Be sure the baud rates and the bits/byte are the same. If
nothing happens verify that the port is alive with a voltmeter per the
next section.
17.9.2. Measure Voltages
If you have a voltmeter handy check for a negative voltage (-4v to
-15v) at pin 3 (receive data) at the terminal side of the null modem
cable. The positive lead of the meter should be connected to a good
ground (the metal connectors on the ends of cables are often not
grounded). If there is no such negative voltage then check for it at
the transmit pin (TxD) on the computer (see ``DB9-DB25'' for the pin-
out). If it's present there but not at the receive pin (RxD) at the
terminal, then the cable is bad (loose connection, broken wire, or not
a null modem). If voltage is absent, then the serial port on the
computer is dead. Test it with a software diagnostic test or replace
it.
If the serial port is alive, you may want to send a file to it (with
modem controls disabled) and see if anything gets to it. To check for
a transmitted signal with an analog voltmeter, look at the needle at
-12 V when the line is idle. Then start sending a file (or start
getty). You should see the needle dropping to 0 and fluttering about
0 as it measures short-run averages of the bit stream. You can see
this also on the AC scale provided that your meter has a capacitor to
block out DC voltages when on the AC scale. If it doesn't, then the
idle DC of -12 V will cause a high false AC reading. Without a meter,
you could connect a known good device (such as another terminal or an
external modem) to the serial port and see if it works OK.
17.10. Slow: pauses of several seconds between bursts of characters
You likely have mis-set interrupts> See the Serial-HOWTO section
starting with "Slow:"
17.11. Terminal doesn't scroll
One reason is that something is wrong with terminfo. Another reason
could be that you are outside the scrolling region set for the
terminal. Some programs just assume that your terminal has 24 lines
and set the scrolling region for 24 lines (by an escape sequence sent
to the terminal) without consulting terminfo to see how many lines
there actually are. Then when you use another program it may leave
the cursor on line 25 where it becomes trapped and the terminal will
not scroll. To avoid this problem, create an environment variable
"export LINES=25" and then the programs that assume 24 lines will
hopefully use 25 lines set the scrolling region accordingly. An
alternate way to fix this problem is to use the command "stty rows
25".
17.12. Serial Monitoring/Diagnostics
A few Linux programs will monitor the modem control lines and indicate
if they are positive (1) or negative (0).
� statserial (in Debian distribution)
� serialmon (doesn't monitor RTS, CTS, DSR but logs other functions)
� modemstat (only works on Linux PC consoles. Will coexist with the
command line)
You may already have them. If not, go to Serial Software
<http://sunsite.unc.edu/pub/Linux/system/serial/>. When using
these, bear in mind that what you see is the state of the lines at
the host computer. The situation at the terminal will be different
since some wires are often missing from cables while other wires
cross over. As of June 1998, I know of no diagnostic program in
Linux for the serial port.
17.13. Local Mode
In local mode, the terminal disconnects from the computer and behaves
like a typewriter (only it doesn't type on paper but on the screen).
Going back into on-line mode reconnects to the computer allowing you
to resume activities at the same point where you left off when you
went into "local". This is useful both for testing the terminal and
for educational purposes. Some terminals use "block mode" as the
"local mode".
When in local mode you may type escape sequences (starting with the
ESC key) and observe what they do. If the terminal doesn't work
correctly in local mode, it's unlikely that it will work correctly
when connected to the computer. If you're not exactly sure what an
escape sequence does, you can try it out in local mode. You might
also use it for trying out a terminal that is for sale. To get into
local mode on some terminals you first enter set-up mode and then
select "local" from a menu (or press a certain key). See ``Getting
Into Set-Up (Configuration) Mode''.
17.14. Serial Electrical Test Equipment
17.14.1. Breakout Gadgets, etc.
While a multimeter (used as a voltmeter) may be all that you need for
just a few terminals, simple special test equipment has been made for
testing serial port lines. Some are called "breakout ... " where
breakout means to break out conductors from a cable. These gadgets
have a couple of connectors on them and insert into the serial cable.
Some have test points for connecting a voltmeter. Others have LED
lamps which light when certain modem control lines are asserted
(turned on). Still others have jumpers so that you can connect any
wire to any wire. Some have switches.
Radio Shack sells (in 1998) a "RS-232 Troubleshooter" or "RS-232 Line
Tester" which checks TD, RD, CD, RTS, CTS, DTR, and DSR. A green
light means on (+12 v) while red means off (-12 v). They also sell a
"RS-232 Serial Jumper Box" which permits connecting the pins anyway
you choose.
17.14.2. Measuring Voltages
Any voltmeter or multimeter, even the cheapest that sells for about
$10, should work fine. Trying to use other methods for checking
voltage is tricky. Don't use a LED unless it has a series resistor to
reduce the voltage across the LED. A 470 ohm resistor is used for a
20 ma LED (but not all LED's are 20 ma). The LED will only light for
a certain polarity so you may test for + or - voltages. Does anyone
make such a gadget for automotive circuit testing?? Logic probes may
be damaged if you try to use them since the TTL voltages for which
they are designed are only 5 volts. Trying to use a 12 V incandescent
light bulb is not a good idea. It won't show polarity and due to
limited output current of the UART it probably will not even light up.
To measure voltage on a female connector you may plug in a bent paper
clip into the desired opening. The paper clip's diameter should be no
larger than the pins so that it doesn't damage the contact. Clip an
alligator clip (or the like) to the paper clip to connect up.
17.14.3. Taste Voltage
As a last resort, if you have no test equipment and are willing to
risk getting shocked (or even electrocuted) you can always taste the
voltage. Before touching one of the test leads with your tongue, test
them to make sure that there is no high voltage on them. Touch both
leads (at the same time) to one hand to see if they shock you. Then
if no shock, wet the skin contact points by licking and repeat. If
this test gives you a shock, you certainly don't want to use your
tongue.
For the test for 12 V, Lick a finger and hold one test lead in it.
Put the other test lead on your tongue. If the lead on your tongue is
positive, there will be a noticeable taste. You might try this with
flashlight batteries first so you will know what taste to expect.
18. Repair & Diagnose
Repairing a terminal has much in common with repairing a monitor
and/or keyboard. Sometimes the built-in diagnostics of the terminal
will tell you what is wrong on the screen. If not, then by the
symptoms, one may often isolate the trouble to one of the following:
bad keyboard, CRT dead, terminal digital electronics failure. It's
best to have a service manual, but even if you don't have one, many
terminals may still be repaired.
18.1. Repair Books & Websites
18.1.1. Books
Bigelow, Stephen J.: Troubleshooting & Repairing Computer Monitors,
2nd edition, McGraw-Hill, 1997. Doesn't cover the character
generation electronics nor the keyboard.
18.1.2. Websites
The FAQ <http://www.repairfaq.org> for the newsgroup:
sci.electronics.repair is long and comprehensive, although it doesn't
cover terminals per se. See the section "Computer and Video
Monitors". Much of this information is applicable to terminals as are
the sections: "Testing Capacitors", "Testing Flyback Transformers",
etc. Perhaps in the future, the "info" on repair in this HOWTO will
consist mainly of links to the above FAQ (or the like). Shuford's
repair archive
<http://www.cs.utk.edu/~shuford/terminal/repair_hints_news.txt> of
newsgroup postings on terminal repair is another source of info.
18.2. Safety
CRT's use high voltage of up to 30,000 volts for color (less for
monochrome). Be careful not to touch this voltage if the set is on
and the cover off. It probably won't kill you even if you do since
the amount of current it can supply is limited. But it is likely to
badly burn and shock you, etc. High voltage can jump across air gaps
and go thru cracked insulation so keep your hands a safe distance from
it. You should notice the well-insulated high voltage cable connected
to one side of the picture tube. Even when the set is off, there is
still enough residual voltage on the picture tube cable connection to
give you quite a shock. To discharge this voltage when the set is
unplugged use a screwdriver (insulated handle) with the metal blade
grounded to the picture tube ground cable with a jumper wire. Don't
use chassis ground.
The lower voltages (of hundreds of volts) can be even more dangerous
since they are not current limited. It is even more dangerous if your
hands are wet or if you are wearing a metal watchband, ring or the
like. In rare cases people have been killed by it so be careful. The
lowest voltages of only several volts on digital circuitry are fairly
safe but don't touch anything (except with a well insulated tool)
unless you know for sure.
18.3. Appearance of Display
If the display is too dim, turn up the brightness and/or contrast.
using knobs on the exterior of the unit (if they exist). If the
width, height or centering is incorrect, there are often control knobs
for these. For some older terminals one must press an arrow key (or
the like) in set-up mode.
You may need to remove the cover to make adjustments, especially on
older models. You could arrange things so that a large mirror is in
front of the terminal so as to view the display in the mirror while
making adjustments. The adjustments to turn may be on a printed
circuit board. While a screwdriver (possibly Phillips-head) may be
all that's needed, inductors may require special TV alignment tools
(plastic hex wrenches, etc.). The abbreviated name of the adjustment
should be printed on the circuit board. For example, here are some
such names:
� V-Size adjusts the Vertical height (Size)
� H-Size adjusts the Horizontal width (Size). It may be an inductor.
� V-Pos adjusts the Vertical Position
� H-Pos adjusts the Horizontal Position
� V-Lin adjusts Vertical Linearity (Use if width of scan lines
differs at the top and bottom of the screen)
� V-Hold adjusts Vertical Hold (Use if screen is uncontrollable
scrolling)
� Bright adjusts brightness (an external knob may also exist)
� Sub-Bright adjusts brightness of subdued intensity mode (often the
normal mode: dimmer than bold or bright mode).
Changing linearity may change the size so that it will need to be
readjusted. A terminal that has been stored for some time may have a
small display rectangle on the screen surrounded by a large black
border. If it's difficult to adjust, wait a while before adjusting it
since it will likely recover some with use (the black borders will
shrink).
18.4. Diagnose
18.4.1. Terminal Made a Noise
If the terminal made some noise just before it failed (or when you
turn it on after it failed) that noise is a clue to what is wrong. If
you hear a sparking noise or see/smell smoke, immediately turn the
terminal off to prevent further damage. The problem is likely in the
high voltage power supply of several thousand volts. Remove the cover
and if the bad spot is not evident, turn it on again for a short time
in a dimly lit room to look for arcing. The high voltage cable (runs
between the flyback transformer and the side of the picture tube) may
have broken insulation that arcs to ground. Fix it with high-voltage
insulating dope, or special electrical tape designed say for 10,000
volts.
The flyback transformer (high voltage) may make only a faint clicking
or sparking noise if it fails. You may not hear it until you turn the
terminal off for a while to rest and then turn it back on again. To
track down the noise you may use a piece of small rubber tubing (such
as used in automobiles) as a stethoscope to listen to it. But while
you are listening for the noise, the terminal is suffering more damage
so try find it fast (but not so fast as to risk getting shocked).
A short in the power supply may cause a fuse to blow with a pop.
Replacing a blown fuse may not solve the problem as the same short may
blow the fuse again. Inspect for any darkened spots due to high heat
and test those components. Shorted power transistors may cause the
fuse to blow. They may be tested with a transistor checker or even
with an ohm-meter. Use the low ohm scale on an ohm-meter so that the
voltage applied by the meter is low. This will reduce the possible
damage to good components caused by this test voltage.
If the terminal has been exposed to dampness such as being stored in a
damp place or near a kitchen with steam from cooking, a fix may be to
dry out the unit. Heating a "failed" flyback transformer with a blow
dryer for several minutes may restore it.
18.4.2. Terminal Made No Noise
A blank screen may be due to someone turning the brightness control to
the lowest level or to aging. The next thing to do is to check the
cables for loose or broken connections. If there is no sign of power,
substitute a new power cord after making sure that the power outlet on
the wall is "hot".
If the keyboard is suspected, try it on another terminal of the same
type or substitute a good keyboard. Wiggle the keyboard cable ends
and the plug. Wires inside cables may break, especially near their
ends. If the break is verified by wiggling it (having the problem go
on and off in synchronization with the wiggles), then one may either
get a new cable or cut into the cable and re-solder the breaks, etc.
One of the first things to do if the keyboard works is to put the
terminal into ``Local Mode''. If it works OK in local, then the
problem is likely in the connection to the host computer (or incorrect
interface) or in the UART chips of the terminal.
By carefully inspecting the circuitry, one may often find the cause of
the problem. Look for discoloration, cracks, etc. An intermittent
problem may sometimes be found by tapping on components with a ball-
point pen (not the metal tip of course). A break in the conductor of
a printed circuit board may sometimes be revealed by flexing the
board. Solder that looks like it formed a drop or a solder joint with
little solder may need re-soldering. Soldering may heat up
transistors (and other components) and damage them so use a heat sink
if feasible.
If you have a common brand of terminal, you may be able to search
newsgroup postings on the Internet to find out what the most frequent
types of problems are for your terminal and perhaps information on how
to fix them.
To see if the digital electronics work, try (using a good keyboard)
typing at the bad terminal. Try to read this typing at a good
terminal (or the console) using the copy command or with a terminal
communication program such as minicom. You may need to hit the return
key at the terminal in order to send a line. One may ask the bad
terminal for its identity etc. from another terminal. This will show
if two-way communication works.
18.5. Error Messages on the Screen
You are in luck if you see an error message on the screen. This
usually happens when you first turn the terminal on.
18.5.1. Keyboard Error
This usually means that the keyboard is not plugged in, or that the
connection is loose. For more serious problems see ``Keyboards''
18.5.2. Checksum Error in NVR
NVR is "Non-Volatile RAM". This means that the NVR where the set-up
information is stored has become corrupted. The terminal will likely
still work but the configuration that was last saved when someone last
configured the terminal has likely been lost. Try configuring again
and then save it. It might work. On very old terminals (early
1980's) there was a battery-powered CMOS to save the configuration so
in this case the problem could be just a dead battery. Sometimes the
EEPROM chip (no battery needed) goes bad after too many saves. It may
be hard to find. If you can't fix it you are either stuck with the
default configuration or you may have escape sequences sent to the
terminal when you start it up to try to configure it.
18.6. Capacitors
Electrolytic capacitors have a metal shell and are may become weak or
fail if they set for years without being used. Sometimes just leaving
the terminal on for a while will help partially restore them. If you
can, exercise any terminals you have in storage by turning them on for
a while every year or two.
18.7. Keyboards
18.7.1. Interchangeability
The keyboards for terminals are not the same as keyboards for PC's.
The difference is not only in the key layout but in the codes
generated when a key is pressed. Also, keyboards for various brands
and models of terminals are not always interchangeable with each
other. Sometimes one get an "incompatible" keyboard to partially work
on a terminal. All the ASCII keys will work OK, but special keys such
as set-up and break will not work correctly.
18.7.2. How They Work
Most keyboards just make a simple contact between two conductors when
you press a key. Electronics inside a chip in the keyboard converts
this contact closure into a code sent over the keyboard's external
cable. Instead of having a separate wire (or conductor) going from
each key to the chip, the following type scheme is used. Number the
conductors say from 1-10 and A-J. For example: conductor 3 goes to
several keys and conductor B goes to several keys, but only one key
has both conductors 3 and B going to it. When that key is pressed, a
short circuit is established between 3 and B. The chip senses this
short and knows what key has been pressed. Such a scheme reduces the
number of conductors needed (and reduces the number of pins needed on
the chip). It's a similar scheme to what is called a "crossbar
switch".
18.7.3. One Press Types 2 Different Characters
If, due to a defect, conductors 3 and 4 become shorted together then
pressing the 3-B key will also short 4 and B and the chip will think
that both keys 3-B and 4-B have been pressed. This is likely to type
2 different characters when all you wanted was one character.
18.7.4. Modern vs Old Keyboards
While the modern keyboard and the old fashioned type look about the
same, the mechanics of operation are different. The old ones have
individual key switches under the key-caps with each switch enclosed
in a hard plastic case. The modern ones use large flexible plastic
sheets (membrane) the size of the keyboard. A plastic sheet with
holes in it is sandwiched between two other plastic sheets containing
printed circuits (including contact points). When you press a key,
the two "printed" sheets are pressed together at a certain point,
closing the contacts printed on the sheets at that point.
18.7.5. Keyboard Doesn't Work At All
If none of the keys work try another keyboard (if you have one) to
verify that the keyboard is the problem. The most likely cause is a
broken wire inside the cord (cable) that connects it to the terminal.
The most likely location of the break is at either end of the cord.
Try wigging the ends of the cord while tapping on a key to see if it
works intermittently. If you find a bad spot, you may carefully cut
into the cord with a knife at the bad spot and splice the broken
conductor. Sometimes just a drop of solder will splice it. Seal up
the cord with electrical tape, glue, or caulk.
18.7.6. Typing b Displays bb, etc. (doubled)
If all characters appear double there is likely nothing wrong with the
keyboard. Instead, your terminal has likely been incorrectly set up
for half-duplex (HDX or local echo=on) and every character you type is
echoed back both from the electronics inside your terminal and from
your host computer. If the two characters are not the same, there may
be a short circuit inside your keyboard. See ``One Press Types 2
Different Characters''
18.7.7. The Keyboard Types By Itself
If a key is shorted out it is likely to type out a large number of the
same character if auto-repeat is enabled. If more than one key is
shorted out then repeating sequences of a few characters will be
typed. This may cause getty to respawn too fast if it happens at the
login prompt. See ``Key Shorted''. The fix is to clean the contacts
per ``Cleaning Keyboard Contacts''.
18.7.8. Liquid Spilled on the Keyboard
If water or watery liquid has been spilled on the keyboard (or if it
was exposed to rain, heavy dew, or dampness) some keys may not work
right. The dampness may cause a key to short out (like it was pressed
down all the time) and you may see the screen fill up with that letter
if auto-repeat is enabled. If it's gotten wet and then partially (or
fully) dried out, certain keys may not work due to deposits on the
contact surfaces. For the modern type of keyboard, one may readily
take apart the plastic sheets inside and dry/clean them. For the old
type one may let it dry out in the sun or oven (low temp.). When it's
dry it may still need contact cleaner on some keys as explained below.
18.7.9. Cleaning Keyboard Contacts
18.7.9.1. Keyboards with Membranes
On some newer keyboards, the plastic sheets (membranes) are easy to
remove for inspection and cleaning if needed. You only need to remove
several screws to take apart the keyboard and get to the sheets. On
some old IBM keyboards the sheets can't be removed without breaking
off many plastic tabs which will need to be repaired with glue to put
back (probably not worthwhile to repair). Such a keyboard may
sometimes be made to work by flexing, twisting, and/or pounding the
assembly containing the plastic sheets.
18.7.9.2. Keyboards with Individual Switches
What follows is for older keyboards that have separate hard plastic
switches for each key. Before going to all the work of cleaning
electrical contacts first try turning the keyboard upside-down and
working the bad keys. This may help dislodge dirt, especially if you
press the key hard and fast to set up vibration. Pressing the key
down and wiggling it from side to side, etc. often helps.
Often the key-caps may be removed by prying them upward using a small
screwdriver as a lever while preventing excessive tilting with a
finger. There exists a special tool known as keycap puller but you
can get by without it. (Warning: Key-caps on modern keyboards don't
pry up.) The key-cap may tilt a bit and wobble as it comes loose. It
may even fly up and onto the floor. Then you have two choices on how
to clean the contacts: Use contact cleaner spray directly on top of
the key switch, or take the key switch apart and clean it. Still
another choice is to replace the key switch with a new or used one.
Directly spraying contact cleaner or the like (obtained at an
electronics store) into the top of the key switch is the fastest
method but it may not work. Before spraying, clean the area around it
a little. With the keyboard live (or with the key contacts connected
to an ohm-meter) use the tube which came with the spray to squirt
cleaner so it will get inside the key switch. Don't let the cleaning
liquid get under nearby keys where it may pick up dust and then seep
(with the dust) into adjacent key switches. If you make this mistake
you may fix one key but damage nearby keys. If this should happen,
immediately work (repeatedly press) the affected nearby keys until
they work OK.
You might tilt the keyboard so that the cleaner flows better into the
contacts. For the CIT101e terminal with an Alps keyboard, this means
tilting the digit row up toward the ceiling. Work the key switch up
and down with a pen or small screwdriver handle to avoid getting the
toxic cleaner liquid on your skin (or wear gloves). You might try
turning the keyboard upside-down while working the key to drain off
remaining cleaner. I don't usually do this. The more cleaner you
squirt in the more likely it will fix it but it is also more likely to
do more damage to the plastic or contaminate adjacent keys, so use
what you think is just enough to do the job. Once the key works OK,
work it up and down a little more and test it a half minute later,
etc. to make sure it will still work OK.
Sometimes a key works fine when the contacts inside are saturated with
contact cleaner liquid. But when the liquid dries a few minutes later
then the resulting scale deposit left from the evaporation of the
cleaning liquid on the contacts, prevents good contact. Then the key
may work erratically (if at all). Operating the key when the liquid
is drying inside may help. Some switches have the contacts nearly
sealed inside so little if any contact cleaner reaches the contacts.
The cleaner that does get to the contacts may carry contamination with
it (cleaning around the tops before spraying helps minimize this).
If you need to disassemble the key switch, first inspect it to see how
it is installed and comes apart. Sometimes one may remove the cover
of the switch without removing the switch from the keyboard. To do
this pry up (or pull up) the top of the key switch after prying apart
thin plastic tabs that retain it. Don't pry too hard or you may break
the thin plastic. If this can't be done, you may have to unsolder the
switch and remove it in order to take it apart (or replace it). Once
the switch has been taken apart you still may not be able to see the
contacts if the contact surfaces are sandwiched together (nearly
touching). You may get contact cleaner on the contacts by slightly
prying apart the conducting surfaces and squirting cleaner between
them. There may be some kind of clip holding the contact surfaces
together which needs to be removed before prying these surfaces apart.
With cleaner on the contacts, work them. Tilting the keyboard or
inverting it may help. Take care not to loose small parts as they may
fly up into the air when taking apart a key switch.
19. Appendix A: General
19.1. List of Linux Terminal Commands
19.1.1. Sending a Command to the Terminal
� ``setterm'': long options
� ``tput'': terse options
� tset: initializes only
� clear: clears screen
� setterm -reset: sends reset string
19.1.2. Configuring the Terminal Device Driver
� ``Setserial'':
� ``Stty''
19.1.3. Terminfo
� ``Terminfo Compiler (tic)'' terminfo compiler & translator
� toe: shows list of terminals for which you have terminfo files
� ``infocmp'' compares or displays terminfo entries
19.1.4. Other
� gitkeys: shows what bytes each key sends to the host.
� tty: shows what tty port you are connected to.
� set (or tset -q): shows the value of TERM, the terminfo entry name
� ``tset'': sets TERM interactively and initializes
19.2. The Internet and Books
19.2.1. Terminal Info on the Internet
� Shuford's Website
<http://www.cs.utk.edu/~shuford/terminal_index.html> at the
University of Tennessee has a great deal of useful information
about text terminals.
� VT terminal information and history <http://www.vt100.net/>
� Boundless <http://www.boundless.com/textterm/> purchased the VT and
Dorio terminal business from DEC. To get Specs select either ADDS,
VT, or DORIO links. Then select a "data sheet" link. Then on the
data sheet select the "Go to Specs" link.
� Wyse text-terminals kbase
<http://www.wyse.com/service/support/kbase/wyseterm.asp> is a major
manufacturer of terminals. For new models see Wyse terminal
<http://www.wyse.com/terminal/> See also Old Wyse terminal Specs
<http://www.wyse.com/service/faq/wysetter.htm>
� Escape Seqs.; N. America <http://www.pericom-
usa.com/twdocs/doc/twproae.htm> or Escape Seqs.; Europe
<http://www.pericom.co.uk/teemworld/doc/twproae.htm> is a list of
escape sequences (and control codes) for some terminal emulations
(including VT 100, 300, 420, and Wyse).
� comp.terminals is the newsgroup for terminals
19.2.2. Books Related to Terminals
� EIA-232 serial port see ``EIA-232 (RS-232) Books''.
� Repair see ``Repair Books & Websites''.
� Terminfo database see ``Termcap Documents''
19.2.3. Entire Books on Terminals
As far as I know, there is no satisfactory book on text terminals
(unless you are interested in antique terminals of the 1970's).
� Handbook of Interactive Computer Terminals by Duane E. Sharp;
Reston Publishing Co. 1977. (mostly obsolete)
� Communicating with Display Terminals by Roger K. deBry; McGraw-Hill
1985. (mostly on IBM synchronous terminals)
The "HANDBOOK ... " presents brief specifications of over 100
different models of antique terminals made in the early 1970's by over
60 different companies. It also explains how they work physically but
incorrectly shows a diagram for a CRT which uses electrostatic
deflection of the electron beam (p. 36). Terminals actually used
magnetic deflection (even in the 1970's). This book explains a number
of advanced technical concepts such as "random scan" and "color
penetration principle".
The "COMMUNICATING ... " book in contrast to the "Handbook ... "
ignores the physical and electronic details of terminals. It has an
entire chapter explaining binary numbers (which is not needed in a
book on terminals since this information is widely available
elsewhere). It seems to mostly cover old IBM terminals (mainly the
3270) in block and synchronous modes of operation. It's of little use
for the commonly used ANSI terminals used today on Unix-like systems.
Although it does discuss them a little it doesn't show the various
wiring schemes used to connect them to serial ports.
19.2.4. Books with Chapters on Terminals
These chapters cover almost nothing about the terminals themselves and
their capabilities. Rather, these chapters are mostly about how to
set up the computer (and its terminal driver) to work with terminals.
Due to the differences of different Unix-like systems, much of the
information does not not apply to Linux.
� Unix Power Tools by Jerry Peck et. al. O'Reilly 1998. Ch. 5
Setting Up Your Terminal, Ch. 41: Terminal and Serial Line
Settings, Ch. 42: Problems With Terminals
� Advanced Programming in the Unix Environment by W. Richard Stevens
Addison-Wesley, 1993. Ch. 11: Terminal I/O, Ch. 19: Pseudo
Terminals
� Essential System Administration by Aleen Frisch, 2nd ed. O'Reilly,
1998. Ch. 11: Terminals and Modems.
The "UNIX POWER TOOLS" book has 3 short chapters on text terminals.
It covers less ground than this HOWTO but gives more examples to help
you.
The "ADVANCED PROGRAMMING ... " Chapter 11 covers only the device
driver included in the operating system to deal with terminals. It
explains the parameters one gives to the stty command to configure the
terminal.
The "ESSENTIAL SYSTEM ..." book's chapter has more about terminals
than modems. It seems well written.
19.3. Non-Linux OS's
The configuration of the host computer for terminals for non-Linux
operating systems is usually significantly different than for Linux.
Here are some links to on-line manuals for Unix-like systems.
� SCO's OpenServer Adding Serial Terminals
<http://www2.sco.com:1996/HANDBOOK/serial_terminal_adding.html> in
SCO OpenServer Handbook.
� Hewlett-Packard's HP-UX Configuring Terminals and Modems
<http://www.software.hp.com/OS_transition/DOCS/PERIPH/TERMS3.HTM>
20. Appendix B: Escape Sequence Commands Terminology
These are sometimes called "control sequences". This section of Text-
Terminal-HOWTO is incomplete (and may never be complete as there are
such a huge number of control sequences). This section is for
reference and perhaps really belongs in something that would be called
"Text-Terminal-Programming-HOWTO".
An example of an ANSI standard escape sequence is ESC[5B which moves
the cursor down 5 lines. ESC is the Escape character. The parameter
5 is included in the sequence. If it were 7 the cursor would move
down 7 lines, etc. A listing for this sequence as "move cursor down x
lines: ESC[xB" is easy to to understand. But command jargon such as:
"tertiary device attribute request" is less comprehensible. This
section will try to explain some of the more arcane jargon used for
escape sequence commands. A full listing (including the escape
sequence codes for the ANSI standard) is a "wish list" project. Since
many escape sequences do the same thing as is done when setting up the
terminal with ``Set-Up Options'', such escape sequences options will
not be repeated here.
20.1. Esc Sequence List
For a list of many (but not all) escape sequences for various
terminals see Escape Seqs.; N. America <http://www.pericom-
usa.com/twdocs/doc/twproae.htm> or Escape Seqs.; Europe
<http://www.pericom.co.uk/teemworld/doc/twproae.htm>. These are used
for terminal emulation and are not always the same as on the
corresponding real terminal.. A list for VT (not maintained) may be
found at Emulators FAQ <http://www.cs.ruu.nl/wais/html/na-
dir/emulators-faq/part3.html>. Search for "VT".
20.2. 8-bit Control Codes
Table of 8-bit DEC control codes (in hexadecimal). Work on VT2xx or
later. CSI is the most common.
ACRONYM FULL_NAME HEX REPLACES
IND Index (down one line) 84 ESC D
NEL Next Line 85 ESC E
RI Reverse Index (one line up) 8D ESC M
SS2 Single Shift 2 8E ESC N
SS3 Single Shift 3 8F ESC O
DCS Device Control String 90 ESC P
CSI Control Sequence Introducer) 9B ESC [
ST String Terminator 9C ESC \
20.3. Printer Esc
� Auto Print on/off: When on, data from the host is also teed (sent)
to the printer port of the terminal (and also shows on the terminal
screen).
� Print Controller on/off: When on, data from the host is sent only
to the printer (nothing shows on the terminal screen).
20.4. Reports
These sequences are usually a request sent from the host to request a
report from the terminal. The terminal responds by sending a report
(actually another escape sequence) to the host which has embedded in
it certain values telling the host about the current state of the
terminal. In some cases a report may be sent to the host even if it
wasn't asked for. This sometimes happens when set-up is exited. By
default no unsolicited reports should be sent.
� Request for Status (Report Operating Status): Meaning of replies
for VT100 is either "I'm OK" or "I'm not OK"
� Request for Device Attributes: The "device" is usually the
printer. Is there a printer? Is it ready?
� Reqest for Tertiary Device Attributes (VT): Reply is report that
was entered during set-up. The tertiary device is the 3rd device
(the printer or auxiliary port device ??). The 1st device may be
the host computer and the 2nd device the terminal.
� Request for Terminal Parameters: What is the parity, baud rate,
byte width, etc. This request doesn't seem to make much sense,
since if the host didn't already know this it couldn't communicate
with the terminal or send a reply.
20.5. Cursor Movements
The cursor is where the next character received from the host will be
displayed. Most of the cursor movements are self-explanatory. "index
cursor" means to move the cursor down one line. Cursor movements may
be relative to the current position such as "move 4 spaces left" or
absolute such as "move to row 3, column 39". Absolute is called
"Direct Cursor Positioning" or "Direct Cursor Addressing".
The home position is row 1 col. 1 (index origin is 1). But where this
home position is on the physical screen is not completely clear. If
"Cursor Origin Mode" = "Relative Origin Mode" is set, then home is at
the top of the scrolling region (not necessarily the top of the
screen) at the left edge of the screen. If "Absolute Origin Mode" is
set (the same as unsetting any of the two modes in the previous
sentence) then home is at the upper left corner of the screen. On
some old terminals if "Cursor Origin Mode" is set it means that it's
relative.
20.6. Pages (definition)
See ``Pages'' for an explanation of pages. There are a number of
escape sequences to deal with pages. Text may be copied from one page
to another and one may move the cursor from page to page. Switching
pages may or may not be automatic: when the screen becomes full (page
1) then more data from the host goes to page 2. The cursor may only
be on one page at a time and characters which are sent to the terminal
go there. If that page is not being displayed, new text will be
received by the terminal and go into display memory, but you will not
see it (until the terminal is switched to that page).
21. Appendix C: Serial Communications on EIA-232 (RS-232)
21.1. Intro to Serial Communication
(Much of this section is now found in Serial-HOWTO.) Text terminals
on Unix-like systems (and on PC's) are usually connected to an
asynchronous 232 serial port of a computer. It's usually a RS-232-C,
EIA-232-D, or EIA-232-E. These three are almost the same thing. The
original RS prefix became EIA (Electronics Industries Association) and
later EIA/TIA after EIA merged with TIA (Telecommunications Industries
Association). The EIA-232 spec provides also for synchronous (sync)
communication but the hardware to support sync is almost always
missing on PC's. The RS designation is obsolete but is still in use.
EIA will be used in this article.
The serial port is more than just a physical connector on the back of
a computer or terminal. It includes the associated electronics which
must produce signals conforming to the EIA-232 specification. The
standard connector has 25 pins, most of which are unused. An
alternative connector has only 9 pins. One pin is used to send out
data bytes and another to receive data bytes. Another pin is a common
signal ground. The other "useful" pins are used mainly for signalling
purposes with a steady negative voltage meaning "off" and a steady
positive voltage meaning "on".
The UART (Universal Asynchronous Receiver-Transmitter) chip does most
of the work. Today, the functionality of this chip is usually built
into another chip.
21.2. Voltages
21.2.1. Voltage for a Bit
At the EIA-232 serial port, voltages are bipolar (positive or negative
with respect to ground) and should be about 12 volts in magnitude
(some are 5 or 10 volts). For the transmit and receive pins +12
volts is a 0-bit (sometimes called "space") and -12 volts is a 1-bit
(sometimes called "mark"). This is known as inverted logic since
normally a 0-bit is both false and negative while a one is normally
both true and positive. Although the receive and transmit pins are
inverted logic, other pins (modem control lines) are normal logic with
a positive voltage being true (or "on" or "asserted") and a negative
voltage being false (or "off" or "negated"). Zero voltage has no
meaning (except it usually means that the unit is powered off).
A range of voltages is allowed. The specs say the magnitude of a
transmitted signal should be between 5 and 15 volts but must never
exceed 25 V. Any voltage received under 3 V is undefined (but some
terminals will accept a lower voltage as valid). One sometimes sees
erroneous claims that the voltage is commonly 5 volts (or even 3
volts) but it's usually 11-12 volts. If you are using a EIA-422 port
on a Mac computer as an EIA-232 (requires a special cable) or EIA-423
then the voltage will actually be only 5 V. The discussion here
assumes 12 V. There is much confusion about voltages on the Internet.
Note that normal computer logic normally is just a few volts (5 volts
was once the standard) so that if you try to use test equipment
designed for testing 3-5 volt computer logic (TTL) on the 12 volts of
a serial port, it may damage the test equipment.
21.2.2. Voltage Sequence for a Byte
The transmit pin (TxD) is held at -12 V (mark) at idle when nothing is
being sent. To start a byte it jumps to +12 V (space) for the start
bit and remains at +12 V for the duration (period) of the start bit.
Next comes the low-order bit of the data byte. If it's a 0-bit
nothing changes and the line remains at +12 V for another bit-period.
Then comes the next bit, etc. Finally, a parity bit may be sent and
then a -12 V (mark) stop bit. The line remains at -12 V (idle) until
the next start bit. Note that there is no return to 0 volts and thus
there is no simple way (except by a synchronizing signal) to tell
where one bit ends and the next one begins for the case where 2
consecutive bits are the same polarity (both zero or both one).
A 2nd stop bit would also be -12 V, just the same as the first stop
bit. Since there is no signal to mark the boundaries between these
bits, the only effect of the 2nd stop bit is that the line must remain
at -12 V idle twice as long. The receiver has no way of detecting the
difference between a 2nd stop bit and a longer idle time between
bytes. Thus communications works OK if one end uses one stop bit and
the other end uses 2 stop bits, but using only one stop bit is
obviously faster. In rare cases 1 1/2 stop bits are used. This means
that the line is kept at -12 V for 1 1/2 time periods (like a stop bit
50% wider than normal).
21.3. Parity Explained
Characters are normally transmitted with either 7 or 8 bits (of data).
An additional parity bit may (or may not) be appended to this
resulting in a byte length of 7, 8 or 9 bits. Some terminal emulators
and older terminals do not allow 9 bits. Some prohibit 9 bits if 2
stop bits are used (since this would make the total number of bits too
large: 12 bits total).
The parity may be set to odd, even or none (mark and space parity may
be options on some terminals). With odd parity, the parity bit is
selected so that the number of 1-bits in a byte, including the parity
bit, is odd. If a such a byte gets corrupted by a bit being flipped,
the result is an illegal byte of even parity. This error will be
detected and if it's an incoming byte to the terminal an error-
character symbol will appear on the screen. Even parity works in a
similar manner with all legal bytes (including the parity bit) having
an even number of 1-bits. During set-up, the number of bits per
character usually means only the number of data bits per byte (7 for
true ASCII and 8 for various ISO character sets).
A "mark" is a 1-bit (or logic 1) and a "space" is a 0-bit (or logic
0). For mark parity, the parity bit is always a one-bit. For space
parity it's always a zero-bit. Mark or space parity only wastes
bandwidth and should be avoided when feasible. "No parity" means that
no parity bit is added. For terminals that don't permit 9 bit bytes,
"no parity" must be selected when using 8 bit character sets since
there is no room for a parity bit.
21.4. Forming a Byte (Framing)
In serial transmission of bytes via EIA-232 ports, the low-order bit
is always sent first. Serial ports on PC's use asynchronous
communication where there is a start bit and a stop bit to mark the
beginning and end of a byte. This is called framing and the framed
byte is sometimes called a frame. As a result a total of 9, 10, or 11
bits are sent per byte with 10 being the most common. 8-N-1 means 8
data bits, No parity, 1 stop bit. This adds up to 10 bits total when
one counts the start bit. One stop bit is almost universally used.
At 110 bits/sec (and sometimes at 300 bits/sec) 2 stop bits were once
used but today the 2nd stop bit is used only in very unusual
situations (or by mistake since it seemingly still works OK that way).
21.5. Limitations of EIA-232
21.5.1. Low Speed & Short Distance
The conventional EIA-232 serial port is inherently low speed and is
severely limited in distance. Ads often read "high speed" but it can
only work at high speed over very short distances such as to a modem
located right next to the computer. All of the wires use a common
ground return so that twisted-pair technology (needed for high speeds)
can't be used without additional hardware. However some computers
have more modern interfaces. See ``Successors to EIA-232''.
It is somewhat tragic that the RS-232 standard from 1969 did not use
twisted pair technology which could operate about a hundred times
faster. Twisted pairs have been used in telephone cables since the
late 1800's. In 1888 (over 110 years ago) the "Cable Conference"
reported its support of twisted-pair (for telephone systems) and
pointed out its advantages. But over 80 years after this approval by
the "Cable Conference", RS-232 failed to utilize it. Since RS-232
was originally designed for connecting a terminal to a low speed modem
located nearby, the need for high speed and longer distance
transmission was apparently not recognized.
21.5.2. Successors to EIA-232
A number of EIA standards have been established for higher speeds and
longer distances using twisted-pair (balanced) technology. Balanced
transmission can sometimes be a hundred times faster than unbalanced
EIA-232. For a given speed, the distance (maximum cable length) may
be many times longer with twisted pair. But PC-s keep being made with
the "obsolete" EIA-232 since it works OK with modems connected to slow
telephone lines, and it works OK with mice.
One exception is Apple's Mac computer with its EIA-232/EIA-422 GeoPort
which provides twisted-pairs (balanced) for transmit and receive. It
uses a small round "mini-DIN" connector. It also provides
conventional EIA-232 but only at 5 volts (which is still legal
EIA-232). However, due to the fact that Macs cost more than PC's,
they are seldom used as a host computer for terminals. Some newer
terminals use EIA-423 but this is just like the unbalanced EIA-232 and
can be connected to a EIA-232 port. This EIA-423 is only 5 volts, but
the specs call for higher speeds than the EIA-232 (which will be of no
help on a long run where it's the unbalance that causes interference).
EIA-485 is also balanced and is used as a "bus" like ethernet and USB.
One device connected to it is the "master" and polls the "slaves" for
input. Since many computers may share the twisted pair its sometimes
called "multidrop". Another twisted pair is used for traffic from the
master to the slaves. The output voltage on the pins must be tristate
with the third state being open circuit to permit other units to use
the "bus". It's been claimed that protocols are not standardized for
multidrop and that's bad news. See
<http://www.hw.cz/english/docs/rs485/rs485.html>
EIA-530-A (balanced but can also be used unbalanced) at 2Mbits/s
(balanced) was intended to be a replacement for EIA-232 but few have
been installed. It uses the same 25-pin connector as EIA-232.
The High Speed Serial Interface ( HSSI = EIA-612/613) uses a 50-pin
connector and goes up to about 50 Mbits/s but the distance is limited
to only several meters. The Universal Serial Bus (USB) is being built
into PCI chips. It is 12 Mbits/s over a twisted pair with a 4-pin
connector (2 wires are power supply) but it also is limited to short
distances of at most 5 meters (depends on configuration).
21.5.3. Line Drivers
For a text terminal, the EIA-232 speeds are fast enough but the usable
cable length is often too short. Balanced technology could fix this.
The common method of obtaining balanced communication with a text
terminal is to install 2@ line drivers in the serial line to convert
unbalanced to balanced (and conversely). They are a specialty item
and are expensive if purchased new.
21.6. Synchronization & Synchronous
21.6.1. How "Asynchronous" is Synchronized
Per EIA-232 there are only two states of the transmit (or receive)
wire: mark (-12 V) or space (+12 V). There is no state of 0 V. Thus
a sequence of 1-bits is transmitted by just a steady -12 V with no
markers of any kind between bits. For the receiver to detect
individual bits it must always have a clock signal which is in
synchronization with the transmitter clock. Such clocks generate a
"tick" in synchronization with each transmitted (or received) bit.
For asynchronous transmission, synchronization is achieved by framing
each byte with a start bit and a stop bit (done by hardware). The
receiver listens on the line for a start bit and when it detects one
it starts its clock ticking. It uses this clock tick to time the
reading of the next 7, 8 or 9 bits. (It actually is a little more
complex than this since several samples of a bit are often taken and
this requires additional timing ticks.) Then the stop bit is read,
the clock stops and the receiver waits for the next start bit. Thus
async is actually synchronized during the reception of a single byte
but there is no synchronization between one byte and the next byte.
21.6.2. Defining Asynchronous vs Synchronous
Asynchronous (async) means "not synchronous". In practice, an async
signal is what the async serial port sends and receives which is a
stream of bytes each delimited by a start and stop bit. Synchronous
(sync) is most everything else. But this doesn't explain the basic
concepts.
In theory, synchronous means that bytes are sent out at a constant
rate one after another in step with a clock signal tick. There is
often a separate wire or channel for sending the clock signal.
Asynchronous bytes may be sent out erratically with various time
intervals between bytes (like someone typing characters at a
keyboard).
There are borderline situations that need to be classified as either
sync or async. The async serial port often sends out bytes in a
steady stream which would make this a synchronous case but since they
still have the start/stop bits (which makes it possible to send them
out erratically) its called async. Another case is where data bytes
(without any start-stop bits) are put into packets with possible
erratic spacing between one packet and the next. This is called sync
since the bytes within each packet must be transmitted synchronously.
21.6.3. Synchronous Communication
Did you ever wonder what all the unused pins are for on a 25-pin
connector for the serial port? Most of them are for use in
synchronous communication which is seldom implemented on PC's. There
are pins for sync timing signals as well as for a sync reverse
channel. The EIA-232 spec provides for both sync and async but PC's
use a UART (Universal Asynchronous Receiver/Transmitter) chip such as
a 16450, 16550A, or 16650 and can't deal with sync. For sync one
needs a USART chip or the equivalent where the "S" stands for
Synchronous. Since sync is a niche market, a sync serial port is
likely to be quite expensive.
Besides the sync part of the EIA-232, there are various other EIA
synchronous standards. For EIA-232, 3 pins of the connector are
reserved for clock (or timing) signals. Sometimes it's a modem's task
to generate some timing signals making it impossible to use
synchronous communications without a synchronous modem (or without a
device called a "synchronous modem eliminator" which provides the
timing signals).
Although few serial ports are sync, synchronous communication does
often take place over telephone lines using modems which use V.42
error correction. This strips off the start/stop bits and puts the
date bytes in packets resulting in synchronous operation over the
phone line.
21.7. Block Mode
21.7.1. Intro to Block Mode
Block mode is seldom used with Linux. In block mode when one types at
a terminal, the results are saved in the terminal memory and are not
sent just yet to the host computer. Such terminals often have built-
in editing capabilities. When the user presses certain keys (such as
the send key) what has been saved in the terminal memory is sent to
the host computer. Now the Linux editors vi and emacs, react
instantly to pressing certain keys but in the above situation such
keys will be pressed and nothing will happen since nothing is sent
when a key is pressed. Thus using a block mode terminal will not
allow the use of such interactive programs. The old IBM mainframe
interface uses block mode (see ``IBM Terminals '' so many IBM
terminals are block-mode only and also synchronous (see Section
``Synchronization & Synchronous'').
21.7.2. Types of Block Modes, Forms
Block mode may itself have various sub-modes such as "page" (a page at
a time) and "line" (a line at a time). Some terminals have both block
transmission modes and conventional character modes and may be
switched from one mode to another. Async terminals which have block
modes include HP2622A, VT130, VT131, VT330, VT340, and Visual500.
Many later model terminals can emulate block mode. Block modes may
include a forms capability where the host computer sends a form to the
terminal. Then the user fills it out and hits the send key which
sends only the data in the form back to the host computer. The form
itself (not the data) is displayed on the screen in protected fields
which don't get transmitted to the host.
21.7.3. Efficiency
Block mode takes a great deal of load off the host computer,
especially if the host computer's hardware is designed for block modes
(as IBM mainframes were). In character mode every character typed is
sent immediately to the serial port and usually causes an interrupt at
the host computer. The host that receives the byte must stop whatever
it is doing and fetch that character from the port hardware. Even
with UART's that have FIFO hardware buffers, the hardware timeout is
normally only the transmission time of 3 bytes so that an interrupt is
usually issued for every character typed.
In true block mode a long block of characters is received using only
one interrupt. If block mode is used with conventional async FIFO
serial ports, an interrupt is needed only every 14 bytes since they
have 16-byte hardware buffers. Thus much of the load and overhead of
interrupt handling is eliminated and the computer has more time to do
other tasks when block mode is used.
A significant savings for block mode occurs if the terminal is
connected to its host via a network. Without block mode, every
character (byte) typed is sent in its own packet including all the
overhead bytes (40 in a TCP/IP packet as used on the Internet). With
block mode, a large number of characters are sent in a single packet.
21.8. EIA-232 (RS-232) Books
(Note: The first book covers much more than just EIA-232.)
� Black, Uyless D.: Physical Layer Interfaces & Protocols, IEEE
Computer Society Press, Los Alamitos, CA, 1996.
� Campbell, Joe: The RS-232 Solution, 2nd ed., Sybex, 1982.
� Putnam, Byron W.: RS-232 Simplified, Prentice Hall, 1987.
� Seyer, Martin D.: RS-232 Made Easy, 2nd ed., Prentice Hall, 1991.
21.9. Serial Software
See Serial Software <ftp://sunsite.unc.edu/pub/Linux/system/serial/>
for Linux software for the serial ports including getty and port
monitors.
22. Appendix D: Notes by Brand Name
Here are notes by brand name that were too specific to a certain
terminal to be put elsewhere in this HOWTO. If you have some info to
contribute on a certain terminal that is not covered elsewhere, it
could go here. Various models and brands often have much in common
which only need be written about in one place. It would be nice to
have for each terminal model, a large set of links linking to the
documentation relevant to that model (including escape codes). There
are so many models of terminals that such a task would be quite
onerous and I, David Lawyer (as of 1998), have no intention of
attempting this. If terminal manufacturers would only make their
manuals available on the net, then all this might not be needed.
22.1. CIT
CIT terminals were made in Japan in the 1980's for CIE Terminals.
They ceased to be imported in the late 1980's. The company, CIE,
still makes CItoh printers (in 1997) but has no parts for its
abandoned terminals. Ernie at (714) 453-9555 in Irvine CA sells (in
1997) some parts for models 224, 326, etc. but has nothing for the 80
and 101. (The document you are now reading was written mostly on the
101e.)
To save the Set-Up parameters press ^S when in Set-Up mode. cit80:
Contrast: knob on rear of terminal, cit101e: Brightness: use up/down
arrow keys in Set-Up mode.
22.2. IBM Terminals
Don't confuse IBM terminals with IBM PC monitors. Many IBM terminals
don't use ASCII but instead use an 8-bit EBCDIC code. It's claimed
that in EBCDIC the bit order of transmission is reversed from normal
with the high-order bit going first. The IBM mainframe communication
standards are a type of synchronous communication in block mode (sends
large packets of characters). Two standards are "BISYNC" and "SNA"
(which includes networking standards). Many of their terminals
connect with coax cable (RG62A/U) and naive persons may think the
"BNC" connecter on the terminal is for ethernet (but it's not).
While this IBM system is actually more efficient than what is
normally used in Linux, terminals meeting this IBM standard will not
currently work with Linux. However, some IBM terminals are
asynchronous ASCII terminals and should work with Linux on PC's. The
numbers 31xx may work with the exception that 317x and 319x are not
ASCII terminals. Before getting an IBM terminal, make sure there is a
termcap (terminfo) for it. If their isn't, it likely will not work
with Linux. Even if there is a terminfo, it may not work. For
example, there is a termcap for 327x but the 3270 is an EBCDIC
synchronous terminal.
The 3270 series includes the 3278 (late 1970's), 3279 with color and
graphics, and the 3274 terminal controller (something like the 3174).
They may be used for both BISYNC and SNA. The 3290 has a split screen
(splits into quarters).
The synchronous IBM terminals don't connect directly to the IBM
mainframe, but connect to a "terminal controller" (sometimes called
"cluster controller" or "communication controller"). Some of these
controllers can convert a synchronous signal to asynchronous so that
in this case a synchronous terminal could indirectly connect to a
Unix-like host computer via its serial port. But there is still a
major problem and that is block transmission. See section ``Block
Mode''.
22.2.1. IBM 3153
It's claimed that the Aux port is DCE and uses a straight-thru cable.
22.3. Teletypes
These are antiques and represent the oldest terminals. They are like
remotely controlled typewriters but are large and noisy. Made by the
Teletype Corp., the first models were made in the 1920's and predate
the computer by over 30 years. Early models used electro-mechanical
relays and rotating distributors instead of electronics. Their Baudot
code was only 5-bits per character as compared to 7-bit ASCII. See
the book "Small Computer Systems Handbook" by Sol Libes, Hayden Books,
1978: pp. 138-141 ("Teletypes").
22.4. VT (DEC)
Digital Equipment Corporation made the famous VT series of terminals
including the commonly emulated VT100. In 1995 they sold their
terminal business to SunRiver which is now named Boundless
Technologies. Detailed VT terminal information, some manuals, and
history is at <http://www.vt100.net/>. Other information is
available at Shuford's Website
<http://www.cs.utk.edu/~shuford/terminal_index.html>. Information on
current products is available from the Boundless website. See
``Terminal Info on the Internet''.
VT220: Some have a BNC connector for video output (not for input).
Sometimes people erroneously think this is for an ethernet connection.
VT520: Supports full DTR/DSR flow control.
22.5. Wyse
Wyse has some FAQ's for terminal numbers under 100 (such as WY60).
See <http://www.wyse.com/service/faq/wysetter.htm> For the specs on
more recent terminals see See <http://www.wyse.com/terminal/>.
22.5.1. Wyse 60
Display adjustments (must remove cover): Brightness VR202, Height
VR302, Width VR101 (also affects height).
22.5.2. Wyse 85
Can emulate VT52/T100/VT200. Press F3 for setup. Scroll thru setup
with up/down keys.
22.5.3. Wyse 99-GT
Here is the setup Menus of the Wyse99GT (late 1980's). Note that TERM
means "termination" (character) and not "terminal".
WYSE 99-GT Terminal Set-Up as used at the University of CA, Irvine
by David Lawyer, April 1990
F1 DISP:
COLUMNS=80 LINES=24 CELL SIZE=10 X 13
STATUS LINE=STANDARD BACKGROUND=DARK SCROLL SPEED=JUMP
SCREEN SAVER=OFF CURSOR=BLINK BLOCK DISPLAY CURSOR=ON
ATTRIBUTE=CHAR END OF LINE WRAP=ON AUTO SCROLL=ON
----------------------------------------------------------------------------
F2 GENERAL:
PERSONALITY=VT 100 ENHANCE=ON FONT LOAD=OFF
COMM MODE=FULL DUPLEX RCVD CR=CR SEND ACK=ON
RESTORE TABS=ON ANSWERBACK MODE=OFF ANSWERBACK CONCEAL=OFF
WIDTH CHANGE CLEAR=OFF MONITOR=OFF TEST=OFF
----------------------------------------------------------------------------
F3 KEYBRD:
KEYCLICK=OFF KEYLOCK=CAPS KEY REPEAT=ON
RETURN=CR ENTER=CR FUNCT KEY=HOLD
XMT LIMIT=NONE FKEY XMT LIMIT=NONE BREAK=170MS
LANGUAGE=US MARGIN BELL=OFF PRINTER RCV=OFF
----------------------------------------------------------------------------
F4 COMM:
DATA/PRINTER=AUX/MODEM MDM RCV BAUD RATE=9600 MDM XMT BAUD RATE=9600
MDM DATA/STOP BITS=8/1 MDM RCV HNDSHAKE=NONE MDM XMT HNDSHAKE=NONE
MDM PARITY=NONE AUX BAUD RATE=9600 AUX DATA/STOP BITS=8/1
AUX RCV HNDSHAKE=NONE AUX XMT HNDSHAKE=NONE AUX PARITY=NONE
(There is a main port (Modem=MDM) and an Auxiliary Port (AUX)
----------------------------------------------------------------------------
F5 MISC 1:
WARNING BELL=ON FKEY LOCK=OFF FEATURE LOCK=ON
KEYPAD=NUMERIC DEL=DEL/CAN XFER TERM=EOS
CURSOR KEYS=NORMAL MARGIN CTRL=0 DEL FOR LOW Y=ON
GIN TERM=CR CHAR MODE=MULTINATIONAL
----------------------------------------------------------------------------
F6 MISC 2:
LOCAL=OFF SEND=ALL PRINT=NATIONAL
PORT=EIA DATA SEND AREA=SCREEN PRINT AREA=SCREEN
DISCONNECT=60 MSEC SEND TERM=NONE PRINT TERM=NONE
PRINT MODE=NORMAL VT100 ID=VT100 POUND=US
----------------------------------------------------------------------------
F7 TABS: You should see several "T" characters spaced 8 dots apart.
If you don't, hit backspace.
F8 F/KEYS: Normally you will see no definitions for the Function Keys
here (unless someone has set them up and saved them). This means that
they will normally generate their default settings (not displayed here).
<ctrl><F5> shows the "user defined definition" of the F5 key, etc.
F9 A/BACK: Normally not defined: ANSWERBACK =
F10 EXIT: Selecting "DEFAULT ALL" will make the factory default settings
the default.
HINTS on use of WY-99GT User's Guide: Note that much that is missing
from this Guide may be found in the WY-99GT Programmer's Guide. The
VT100 emulation (personality) is known as ANSI and uses ANSI key codes
per p. A-10+ even though the keyboard may be ASCII. A sub-heading on
p. A-13 "ASCII Keyboard" also pertains to VT100 because it has an
"ANSI KEY ..." super-heading a few pages previously. But not all
ASCII keyboard headings pertain to VT100 since they may fall under a
non-ANSI personality super-heading which may found be a few pages
previously. Appendix H is the "ANSI Command Guide" except for the
VT52 (ANSI) personality which is found in Appendix G.
22.5.4. Wyse 150
When exiting set-up using F12, hitting space changes "no" to "yes" to
save the set-up. The sentence to the left of this no/yes is about
"vertical alignment" and has nothing to do with this no/yes for saving
the set-up (confusing menu design).
END OF Text-Terminal-HOWTO