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11.5. Making serial cables

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 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.

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 DB9 or DB25 connector.

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.

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.

For structured cabling systems, space is tight within DB9/RJ-45 backshells, so the jumpering is better done behind the patch panel. The DB9/RJ-45 connectors present the IBM PC pinout at the DB9 connector and present the Yost or Cisco pinout at the RJ-45 connector.

Caution Incompatible devices in structured cabling systems
 

Take care to connect only RS-232 devices to RS-232 devices when patching structured cabling systems. Other cables may be carrying ethernet, ISDN, telephony, alarm and DC power voltages. Connecting incompatible voltages may destroy equipment.