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PC Hardware in a Nutshell, 3rd Edition
book

PC Hardware in a Nutshell, 3rd Edition

by Robert Bruce Thompson, Barbara Fritchman Thompson
July 2003
Beginner to intermediate content levelBeginner to intermediate
874 pages
38h 13m
English
O'Reilly Media, Inc.
Content preview from 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)

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Publisher Resources

ISBN: 059600513XErrata Page