For debugging purposes it is often useful to have a serial connection to a computer. You have several advantages doing that:

a real console is available from remote
it is possible to use Sysreq calls from another computer
Logs may be just logged instead of typed from the screen
login to the machine is possible without a local keyboard or a network connection

What you need:

a primary computer that should be observed, called server
a null modem cabel
a second computer that is connected with that cabel, called client
a terminal emulation program

First, connect both computers with the cable.

Server Configurations

Several actions are needed to make use of a serial connection to the server:

Configure the Bootloader GRUB

Disable the graphics in /boot/grub/menu.lst by adding a comment mark in front of the gfxmenu line:

#gfxmenu (hd0,4)/boot/message

To configure the serial port, add the following two lines to the global section of the menu file:

serial --unit=0 --speed=115200
terminal serial console

This will make the bootloader visible on the serial line.

Tell the Kernel about the new console

add a parameter

console=tty0 console=ttyS0,115200

to the commandline of the kernel. You may also add several devices, seperated by a comma. After the next reboot, the kernel will accept the device /dev/ttyS0 as a console.

Enable sysrq on the Server

Change the variable ENABLE_SYSRQ in /etc/sysconfig/sysctl to "yes". This may also be done with the "/etc/sysconfig Editor" in YaST. This variable is checkd at boottime, and sets the value of /proc/sys/kernel/sysrq accordingly.

Add a login Prompt

In the file /etc/inittab, remove the # at the beginning of the line

S0:12345:respawn:/sbin/agetty -L 115200 ttyS0 vt102

This will enable you to login as a normal user. To be able to login as root, you still need to add this device to /etc/securetty. Add the line

ttyS0

in securetty to have this possibility. If you don't want to reboot after this, run the command

telinit q

to reload the configuration file.

Redirect system messages

If you alternatively want to view system messages via serial line instead of getting a login prompt, add the following line to your /etc/syslog.conf instead:

*.* /dev/ttyS0

Client

On the client side, you basically need a terminal emulation program. A nifty tool for that is screen. If the serial port on the client is /dev/ttyS0, start the connection as follows:

screen /dev/ttyS0 115200

If all was done correctly, a login prompt should appear.

screen has many useful options. By default, you access its commands by typing CTRL-a <letter>. A small help is displayed with CTRL-A ? . Often used features are:

CTRL-A H
start/stop a log on screenlog.0 in the current directory.
CTRL-A B
send a break to the server to issue a sysrequest. Try that for example with CTRL-A B H to get a small help for sysrequests from the kernel.
CTRL-A C
Start a second screen.
CTRL-A A
Toggle the display of the last used screen.
CTRL-A <NR>
Toggle screen with number <NR>.

Serial Null Modem Connection