PC Hardware in a Nutshell, 3rd Edition

Loopback Plugs

In days of yore, every PC technician carried a set of loopback plugs for testing 9-pin serial ports, 25-pin serial ports, and parallel ports. Nowadays, it’s pretty rare to see a loopback plug, but they can still be invaluable in diagnosing port problems.

In essence, a loopback plug (also called a wrap plug) is simply a connector of the appropriate type with internal wiring that connects paired pins such as TD and RD. When the loopback plug is connected to a serial port for testing, outbound signals from the serial port (e.g., TD) are looped back to the corresponding inbound pin (e.g., RD). If a signal does not appear as expected, either the outbound pin or the inbound pin is not doing its job.

High-end diagnostic software (such as CheckIt) often includes loopback plugs, either standard or as an option. You can also buy a combination DB25/DB9 loopback plug from most electronics distributors for $5 or so. If you have the components on hand to make cables, it’s easy enough to make your own loopback plugs, using an empty D-shell connector and crimp-on, solder-on, or wirewrap pins. The exact pinouts required differ according to the diagnostic utility you are using, but most utilities can use a loopback plug which uses the following standard pinouts:

DB25F loopback plug

• 2 to 3 (TD to RD)

• 4 to 5 to 22 (RTS to CTS to RI)

• 6 to 8 to 20 (DSR to CD to DTR)

DB9F loopback plug

• 2 to 3 (RD to TD)

• 7 to 8 to 9 (RTS to CTS to RI)

• 6 to 1 to 4 (DSR to CD to DTR)