Connecting a serial terminal to a Linux host

The electrical connection between terminal is made with a serial cable. Depending on which brand and type of terminal, a straight extension cable, or nullmodem cable is necessary. If you are using a normal PC as terminal, a nullmodem cable is obligatory, real 'dumb' terminals (like my Qume QVT-101) may require a straight serial extension cable.

There are two ways to use a serial terminal with Linux The first is to pass the kernel a boot parameter instructing it to use ttyS0 as system console. The second way is to let the Linux init system run a getty instance on the system device /dev/ttyS0. A getty is a daemon that connects a tty (teletype) device to a program's standard input/output, it is primarily used to to start the "login" prompt (/bin/login) on the virtual consoles (tty0 to tty5) of a Linux computer.

Other uses of a getty are dailin connections (calls made by some computer TO the Linux host), in which case the getty will start a BBS server program or PPP connection when the modem indicates a phone ring.

Finding out what serial ports you have

To see a list of installed (and found) serial ports on your Linux system run this command:

$ dmesg | grep ttyS
ttyS00 at 0x03f8 (irq = 4) is a 16550A
ttyS01 at 0x02f8 (irq = 3) is a 16550A

As you can see my server has two serial ports, ttyS00 and ttyS01 (their device nodes are /dev/ttyS0 and /dev/ttyS1). Most motherboards have two serial ports on them. If you don't get any output, you might have them disabled in the BIOS.

Running an extra getty on ttyS[0..?]

I'm assuming the port used is ttyS0, use the same steps if you want more serial terminals on other ports. The first thing you do is take a backup copy of the file we're going to edit, /etc/inittab

# cp /etc/inittab /etc/inittab.bak

Then, add the folowing line to /etc/inittab with your favorite text editor (Vim, nano, gedit, kwrite or the likes) and save it. This line will make the init system start a getty on ttyS0.

T0:23:respawn:/sbin/getty -L -n -i -l /bin/bash ttyS0 19200 dumb

When looking at the inittab file, you will notice that for every tty device, a line is included that looks like the one you just added. A short explanation of this line will make it easier to understand what we're doing. Each block is separated by a semicolon(:):

• T0 Identifier used by init. It is unique in the inittab file, as long as this entry doesn't appear anywhere else in inittab, you're okay. We named this entry T0 because it's the first terminal. (for the n-th terminal use Tx, eg. T5 for terminal nr 5)
• 23 Runlevels in which the getty is supposed to run.
• respawn Restart the getty if it dies (it does when the user logs out on that console)

Then there is the command to be started: /sbin/getty -L -n -i -l /bin/bash ttyS0 19200 dumb

Here is a list of getty's parameters I used (taken straight from the agetty manual pages):

-L : Force  the line to be a local line with no need for carrier
     detect. This can be useful when you have locally  attached
     terminal where  the serial line does not set the carrier detect
     signal.

-n : Do not prompt the user for a login name.  This  can be  used  in
     connection with -l option to invoke a non-standard login process
     such as  a  BBS  system.

-i : Do  not  display  the  contents  of  /etc/issue other) before
     writing the login  prompt.  Terminals or communications hardware
     may become confused when receiving lots of text  at  the  wrong
     baud  rate

-l /bin/bash : Invoke the  specified  login_program  instead  of
               /bin/login.  This allows the use of a  non-standard
               login  program (for  example,  one that asks for a
               dial-up password or that uses a different password
               file).

ttyS0 :  the serial device the terminal is connected to
19200 :  the communication speed (baudrate)
dumb : the type of terminal attached.

And here follows an explanation why I used these parameters.

• -L : It's a local terminal, so it's necessary.
• -n : used with -l option, see below.
• -i : Don't display /etc/issue, that's not needed...
• -l /bin/bash

Normally getty runs the /bin/login program. This asks for a UNIX username and password. If the login is correct, it starts the user's favourite shell (bash for instance). Typing my password is very hard since it contains many special characters (I'm not used to the US layout) and some keys don't work well. By adding -l program, getty will start an other program like /bin/bash instead of /bin/login.

• ttyS0 : the system device the terminal is connected to.
• 19200 : the maximum speed the qvt101 can handle. Note: the standard Linux console uses 38400 baud, which is much faster. You can use another value here, but remember to set the terminal to that speed.
• dumb : the type of terminal attached. This is used by libtermcap and programs like vi, emacs, lynx and screen who handle the terminal in an intelligent way. Normally I should be able to use "qvt101" or "qvt101+" but for some reason libtermcap doesn't know these.