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11.4. 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.
The RS-232 standard will drive at least 15 meters of shielded cable. Longer distances are possible with better cable; 100 meter cables are advertised by some specialty firms. Distances longer than 15m are also be possible with the high-quality unshielded twisted pair used for 100Base-TX ethernet. Be wary of long unshielded cables, as the RS-232 signals are not balanced and thus pick up noise easily. For distances beyond 100m use an RS-232 line driver; these will typically drive up to 2000 meters over category 3 UTP cable. For greater distances consider using fiber optical modems, the global telephony system, the mobile telephony system, satellite or radio.
If the environment has a lot of radio frequency noise then use shielded cable and connectors. 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 body 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.
