Simple circuit tests twisted-pair cables

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

Originally published in the May 29, 1986, issue of EDN

Using the system shown in Fig 1, 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.)

Simple circuit tests twisted-pair cables figure 1

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 (D_A through D_H, 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 D_A, D_B, 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.