Simple Circuit Tests Twisted-Pair Cables - Try 2

An EDN Design Idea - Originally published in the May 29, 1986, issue of EDN

Mark D Braunstein, Contel Information Systems, Fairfax, VA; Edited by Paul Rako and Fran Granville - January 19, 2012

Using the system shown in below, you can quickly test a cable containing twisted-wire pairs and detect open or reversed pairs, shorted pairs, and shorts between unrelated pairs. The tester consists of an active test set that plugs into one end of the cable, and a passive terminator that plugs into the other end. (An RS-449 cable is used as an example.)

EDN Cable Tester
A battery or a dc supply delivers 15 to 24V to the test set. The voltage regulator (IC1) is connected as a current regulator to supply a nominal 25 mA to the LED strings at each end of the cable. The cable in this example contains eight twisted pairs, and for a good cable, all eight LEDs in the test set (DA through DH, which are series-connected segments of a bar-graph display) and all eight LEDs in the terminator (D1 through D8) will light. If a twisted pair is open or reversed, the corresponding LED on the terminator will be extinguished; if a pair is shorted, corresponding LEDs at both ends will be extinguished; and if any two unrelated wires of different pairs are shorted, all intervening LEDs in the strings at both ends will be extinguished. For example, if pins 4 and 6 are shorted, LEDs DA, DB, D1, and D2 will not light.

You can add a heat sink to the IC1 regulator as a safety precaution, but normal tester operation is well within the regulator’s power-dissipation limits. Even with many shorted pairs, a dissipation of 700 mW would cause no more than 60°C junction temperature, and the IC is guaranteed to turn itself off at 160°C. The complete tester costs less than $50 to build.

Simple Circuit Tests Twisted-Pair Cables

This circuit should prove useful to anyone using multi-pair cables.  The idea would work well for an RJ-45 network cable tester.


Posted: 6 years 8 months ago by Jon Chandler #12835
Jon Chandler's Avatar
Sorry if you've noticed this article appear and disappear several times. My quick copy and paste job turned out to be more troublesome than expected.

Happy new year all!
Posted: 6 years 8 months ago by Jon Chandler #12836
Jon Chandler's Avatar
Looking at this in a little more detail, it will work well to detect opens, shorts and pairs connected backwards are one end. It will not detected pairs connected out of order...say pair 1 at one end connects to pair 4 at the other. Therefore, it's not effective to detect RJ-45 cross-over cables...

Maybe we can do better?
Posted: 6 years 8 months ago by majenko #12874
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Why not use the bi-directional red/green LEDs (two pin, connected in opposite directions, you know the ones) for the terminator end? Green = good, off = open / short, and Red = connected backwards. Better diagnostics then

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