PC Hardware in a Nutshell, 3rd Edition

Determining Pinouts on an Unknown Cable

At some point, you may find yourself looking at a cable with no idea how it’s pinned out or whether it’s usable for what you want to do. When that happens, proceed as follows:

Just try it

You can connect any RS-232 pin to any other pin without damaging anything. The worst that can happen is that nothing will work.

Look for a label

Labels may be unambiguous. One labeled “HP LaserJet” is probably just what you need to connect a serial HP LaserJet. Short labels may mislead. A cable labeled “null modem” may not have the pinouts you need for your “null modem” cable. Also, some cables are asymmetric—both ends may use the same connector, but the pinouts differ and the two ends are not interchangeable.

Identify the active pins visually

Custom-made cables often have pins only for active lines. By determining which pins are connected on each end, you can often make a reasonable guess about the actual pinouts. If the visible pins correspond to a “standard” cable type, chances are good that it is that type of cable.

Disassemble the connectors

Remove the connector hood and note the connections, including local loopbacks within the connector body. By noting which pin number is connected to which color wire, you can trace the pinouts end to end. Mass-produced cables ordinarily use molded connectors, making this method impossible to use.

Use a continuity tester or DVM

Assuming a male-to-female cable, insert one probe into hole 1 of the female connector, and then brush ...