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. 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). 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 one 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