Simple circuit tests twisted-pair cables

Using the system shown in Figure 1, a cable containing twisted-wire pairs and detect open or reversed pairs, shorted pairs, and shorts between unrelated pairs can be tested. The tester consists of an active 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.)



A battery or a dc supply delivers 15V to 24V to the test set. The voltage regulator (IC1) is connected as a current regulator to supply a nominal 25mA 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.

A heat sink to the IC1 regulator can be added 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 700mW 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.


This article was originally published in the May 29, 1986 issue of EDN.