Simple Circuit Tests Twisted-Pair Cables - Try 2

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.

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