Engineers have long known how to test a cable for continuity by simply connecting all conductors in series and checking with an ohmmeter. This method is sometimes impractical, however, because it cannot check for short circuits. The method shown in Figure 1 solves the short-circuit problem. Connecting LED indicators at each shorting loop provides a visual indication. The beauty of this scheme is that any short circuit causes at least one LED to extinguish. You can then diagnose the malfunction by the visual signature of the LEDs.
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| Figure 1. | This simple cable-testing method tests for continuity as well as short circuits. | |
Taking the method one step further, let the LEDs be the emitters of photocouplers. Connecting the phototransistors in series provides a simple go-no-go test that requires no visual observation. Note that the 2 V LED forward-voltage drop presents a challenge when you're testing a cable with a large number of conductors, so be sure to apply a high enough voltage.
