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>11.5. Making serial cables</H1
><P
>If you use a serial console for densely-racked computers you
will end up making a lot of null-modem serial cables. This section
has some hints on making serial cables. If you are making more
than ten cables and live in a city you will probably find it
economic to have the cables made by a specialty cabling
firm.</P
><P
>Attempt to minimise noise in your cabling design. Many BIOSs
and boot loaders will wait forever if they receive a single
character of line noise. You might choose to use shielded UTP
cables (these require special RJ-45 plugs but use standard RJ-45
sockets).</P
><P
>If the environment has a lot of radio frequency noise then
use traditional shielded cable and metal RS-232 connector shells.
Connect the shield in the cable to the computer at
<EM
>one</EM
> end. This can be done by connecting the
drain wire of the shield it to the Protective Ground (if present)
or by soldering the drain wire to the shell of the connector. If
there is a substantial amount of noise also place a ferrite core
over the shielded cable at both ends of the cable. Follow the
usual good practices of making the cable to the correct length and
screwing home the D connectors into the chassis.</P
><P
>If you are making one of these cables and have some soldering
skill, you can easily do the jumpering of the signal wires within
the backshell of the <SPAN
CLASS="ACRONYM"
>DB9</SPAN
> or
<SPAN
CLASS="ACRONYM"
>DB25</SPAN
> connector.</P
><P
>If you are making a large number of cables then crimping
systems are much faster than soldering. Again, pin jumpering can
be done within the backshell.</P
><P
>No matter what system is adopted, use the Resistance setting
of a multimeter to check for dead and shorted pins. A minute here
can save hours later.</P
><P
>For structured cabling systems, space is tight within
<SPAN
CLASS="ACRONYM"
>DB9/RJ-45</SPAN
> backshells, so the jumpering is better
done behind the patch panel. The <SPAN
CLASS="ACRONYM"
>DB9/RJ-45</SPAN
>
connectors present the <SPAN
CLASS="PRODUCTNAME"
><SPAN
CLASS="ACRONYM"
>IBM</SPAN
>
<SPAN
CLASS="ACRONYM"
>PC</SPAN
></SPAN
> pinout at the DB9 connector and
present the Yost or Cisco pinout at the <SPAN
CLASS="ACRONYM"
>RJ-45</SPAN
>
connector.</P
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>Incompatible devices in structured cabling systems</B
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><P
>Take care to connect only <SPAN
CLASS="ACRONYM"
>RS-232</SPAN
> devices
to <SPAN
CLASS="ACRONYM"
>RS-232</SPAN
> devices when patching structured
cabling systems. Other cables may be carrying ethernet,
<SPAN
CLASS="ACRONYM"
>ISDN</SPAN
>, telephony, alarm and
<SPAN
CLASS="ACRONYM"
>DC</SPAN
> power voltages. Connecting incompatible
voltages may destroy equipment.</P
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