Simple circuit for testing twisted pair cables

Using the system shown in Figure 1, a cable containing twisted pairs can be quickly tested to detect open or inverted pairs, shorts between pairs, and shorts between unrelated pairs. The tester consists of an active test piece and passive terminations that are inserted into both ends of the cable. (The following uses an RS-449 cable as an example.)

The test piece is powered by a battery or a 15V to 24V DC power supply. The voltage regulator (IC1) is connected as a current regulator to provide a nominal 25mA current to the LED strings at each end of the cable. The cable in this example contains eight twisted pairs. If the cable is good, all eight LEDs on the test piece (DA to DH are segments in series of a bar display) and all eight LEDs on the terminal piece (D1 to D8) will light up. If a twisted pair is open or reversed, the corresponding LED on the terminal piece will turn off; if a pair is shorted, the corresponding LEDs on both ends will turn off; if any unrelated wires in any two different pairs are shorted, all the middle LEDs in series at both ends will turn off. For example, if wires 4 and 6 are shorted, LEDs DA, DB, D1, and D2 will turn off.

For safety reasons, a heat sink can be added to the regulator IC1, but a properly functioning tester is well within the power consumption limits of the regulator. Even with many line pairs shorted, the junction temperature generated by 700mW power consumption does not exceed 60°C, while the IC is guaranteed to shut down at 160°C. The entire tester costs less than $50 to make.

Figure 1. This system can quickly test a cable containing twisted pairs.