For the average user a serial console has no advantage over
using a directly attached keyboard and screen. Serial consoles are
much slower, taking up to a second to fill a 80 column by 24 line
screen. Serial consoles generally only support non-proportional
ASCII text, with limited support for languages
other than English. A new terminal can be more expensive than an
old PC.
There are some scenarios where serial consoles are
useful. These are:
- Systems administration of remote computers
Linux is an good operating
system for deployment at unstaffed sites.
Linux is also good at hosting
critical network infrastructure such as DNS and
DHCP servers. These servers are generally
installed at every site of an organisation including sites which
may be too small or too remote to have IT
staff.
System administration of these remote computers is usually
done using SSH, but
there are times when access to the console is the only way to
diagnose and correct software failures. Major upgrades to the
installed distribution may also require console access.
In these cases the serial console is attached to a modem.
Access to the console is gained from a remote computer by
dialing into the modem. This allows the console to be reached
from any telephone socket.
- High density racks of computers
Clusters of personal computers can outperform mainframe
computers and form competitive supercomputers for some
applications. See the Cluster-HOWTO
for more information on clustering.
These clusters are typically assembled into 19-inch
telecommunications equipment racks and the system unit of each
computer is typically one rack unit (or 1.75 inches) tall. It
is not desirable to put a keyboard and monitor on each computer,
as a small cathode ray tube monitor would consume the space used
by sixteen rack units.
A first glance it seems that a monitor and keyboard switch
is the best solution. However the VGA signal
to the monitor is small, so even with the switch the monitor
cannot be placed very far away from the rack of
computers.
It is desirable to allow the consoles to be monitored in
the operators' room of the computer center, rather than in the
very expensive space of the machine room. Although monitor
switches with remote control and fiber optical extensions are
available, this solution can be expensive.
A standard RS-232 cable can be 15 meters
in length. Longer distances are easily possible. The cabling is
cheap. Terminal servers can be used to allow one terminal to be
access up to 90 serial consoles.
- Recording console messages
This is useful in two very different cases.
Kernel programmers are often faced with a kernel error
message that is displayed a split second before the computer
reboots. A serial console can be used to record that
message. Another Linux
machine can be used as the serial terminal.
Some secure installations require all security events to
be unalterably logged. A way to meet this requirement is to
print all console messages. Connecting the serial console to a
serial printer can achieve this.[1]
- Embedded software development
Linux is
increasingly being as the operating system in embedded
applications. These computers do not have keyboards or
screens.
A serial port is a cheap way to allow software developers
to directly access the embedded computer. This is invaluable
for debugging. Most chip sets designed for embedded computers
have a serial port precisely for this purpose.
The shipping product need not present the
RS-232 port on an external connector.
Alternatively the RS-232 port is often used for
downloading software updates.
Unlike minicomputer systems, the
IBM
PC was not designed to use a
serial console. This has two consequences.
Firstly, Power On Self-Test messages and Basic Input/Output
System (BIOS) messages are sent to the screen
and received from the keyboard. This makes it difficult to
reconfigure the BIOS and seeing makes it
impossible to see Power On Self-Test errors.
An increasing number of manufacturers of rackable
server equipment are altering their
BIOSs to optionally use the
RS-232 port as the console. If you are buying a
machine specifically for use with serial console you should seek
this feature. If you have an existing machine that definitely
requires access to the BIOS from the serial port
then there are hardware solutions such as PC Weasel
2000.
Secondly, the RS-232 port on the
IBM
PC is designed for connecting to a
modem. Thus a null modem cable is needed when connecting the PC's
serial port to a terminal.