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>1.2. Why use a serial console?</H1
><P
>For the average user a serial console has no advantage over a
console offered by 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 <SPAN
CLASS="ACRONYM"
>ASCII</SPAN
> text, with limited
support for languages other than English. A new terminal can be
more expensive than an old <SPAN
CLASS="ACRONYM"
>PC</SPAN
>.</P
><P
>There are some scenarios where serial consoles are
useful. These are:</P
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
>Systems administration of remote computers</DT
><DD
><P
><SPAN
CLASS="SYSTEMITEM"
>Linux</SPAN
> is a good
operating system for deployment at unstaffed sites. <SPAN
CLASS="SYSTEMITEM"
>Linux</SPAN
> is also good for hosting
critical network infrastructure such as <SPAN
CLASS="ACRONYM"
>DNS</SPAN
>
and <SPAN
CLASS="ACRONYM"
>DHCP</SPAN
> services. These services are
generally installed at every site of an organisation including
sites which may be too small or too remote to have information
technology staff.</P
><P
>System administration of these remote computers is usually
done using <SPAN
CLASS="APPLICATION"
><SPAN
CLASS="ACRONYM"
>SSH</SPAN
></SPAN
>, 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.</P
><P
>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.</P
></DD
><DT
>High density racks of computers</DT
><DD
><P
>Clusters of personal computers can outperform mainframe
computers and form competitive supercomputers for some
applications. See the <A
HREF="http://www.tldp.org/HOWTO/Cluster-HOWTO.html"
TARGET="_top"
><I
CLASS="CITETITLE"
>Cluster-HOWTO</I
></A
>
for more information on clustering.</P
><P
>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.</P
><P
>A first glance it seems that a monitor and keyboard switch
is the best solution. However the <SPAN
CLASS="ACRONYM"
>VGA</SPAN
> signal
to the monitor is small, so even with the switch the monitor
cannot be placed very far away from the rack of
computers.</P
><P
>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.</P
><P
>A standard <SPAN
CLASS="ACRONYM"
>RS-232</SPAN
> 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 access up to 90 serial consoles.</P
></DD
><DT
>Recording console messages</DT
><DD
><P
>This is useful in two very different cases.</P
><P
>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 <SPAN
CLASS="SYSTEMITEM"
>Linux</SPAN
>
machine can be used as the serial terminal.</P
><P
>Some secure installations require all security events to
be unalterably logged. One way to meet this requirement is to
print all console messages. Connecting the serial console to a
serial printer can achieve this.<A
NAME="AEN140"
HREF="#FTN.AEN140"
><SPAN
CLASS="footnote"
>[1]</SPAN
></A
></P
></DD
><DT
>Embedded software development</DT
><DD
><P
><SPAN
CLASS="SYSTEMITEM"
>Linux</SPAN
> is
increasingly being used as an operating system for embedded
applications. These computers do not have keyboards or
screens.</P
><P
>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.</P
><P
>The shipping product need not present the
<SPAN
CLASS="ACRONYM"
>RS-232</SPAN
> port on an external connector.
Alternatively the <SPAN
CLASS="ACRONYM"
>RS-232</SPAN
> port is often used for
downloading software updates.</P
></DD
><DT
>Craft terminal for telecommunications equipment</DT
><DD
><P
><SPAN
CLASS="SYSTEMITEM"
>Linux</SPAN
> is
increasingly being used as the operating system inside
telecommunications equipment. The <A
HREF="http://www.osdlab.org/projects/cgl/"
TARGET="_top"
>Carrier Grade
Linux</A
> consortia hopes to accelerate and coordinate this
trend.</P
><P
>Most telecommunications equipment is remotely managed from
a distant computer. However, site technicans (called
<I
CLASS="WORDASWORD"
>craft personnel</I
> in telco-speak) need to
access the equipment to test installation changes, check the
status of reported faults, and so on. The terminal used by the
craft personnel is called the <I
CLASS="WORDASWORD"
>craft
terminal</I
>. The craft terminal plugs into the
<I
CLASS="WORDASWORD"
>craft interface</I
> on the equipment. The
serial console makes an ideal craft interface.</P
></DD
></DL
></DIV
><P
>Unlike minicomputer systems, the
<SPAN
CLASS="PRODUCTNAME"
><SPAN
CLASS="ACRONYM"
>IBM</SPAN
>
<SPAN
CLASS="ACRONYM"
>PC</SPAN
></SPAN
> was not designed to use a
serial console. This has two consequences.</P
><P
>Firstly, Power On Self-Test messages and Basic Input/Output
System (<SPAN
CLASS="ACRONYM"
>BIOS</SPAN
>) messages are sent to the screen
and received from the keyboard. This makes it difficult to use the
serial port to reconfigure the <SPAN
CLASS="ACRONYM"
>BIOS</SPAN
> and
impossible to see Power On Self-Test errors.</P
><P
>An increasing number of manufacturers of rackable
<EM
>server</EM
> equipment are altering their
<SPAN
CLASS="ACRONYM"
>BIOS</SPAN
>s to optionally use the
<SPAN
CLASS="ACRONYM"
>RS-232</SPAN
> port for BIOS configuration and test
messages. 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
<SPAN
CLASS="ACRONYM"
>BIOS</SPAN
> from the serial port then there are
hardware solutions such as <A
HREF="http://www.realweasel.com/"
TARGET="_top"
><SPAN
CLASS="PRODUCTNAME"
>PC Weasel
2000</SPAN
></A
>.</P
><P
>Secondly, the <SPAN
CLASS="ACRONYM"
>RS-232</SPAN
> port on the
<SPAN
CLASS="PRODUCTNAME"
><SPAN
CLASS="ACRONYM"
>IBM</SPAN
>
<SPAN
CLASS="ACRONYM"
>PC</SPAN
></SPAN
> is designed for connecting to a
modem. Thus a null modem cable is needed when connecting the PC's
serial port to a terminal.</P
></DIV
><H3
CLASS="FOOTNOTES"
>Notes</H3
><TABLE
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><A
NAME="FTN.AEN140"
HREF="intro-why.html#AEN140"
><SPAN
CLASS="footnote"
>[1]</SPAN
></A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>The <SPAN
CLASS="SYSTEMITEM"
>Linux</SPAN
>
<SPAN
CLASS="PRODUCTNUMBER"
>2.4</SPAN
> kernel also supports the
output of console messages to
<SPAN
CLASS="PRODUCTNAME"
>Centronics</SPAN
> or
<I
CLASS="CITETITLE"
><SPAN
CLASS="ACRONYM"
>IEEE</SPAN
> 1284-2000</I
>
parallel printer interfaces.</P
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