Create Your Own Test Fixture To Check Cableform Wiring Quickly And Accurately

Dec. 13, 2009

Many companies face the problem of checking cableforms to see that they are correctly wired up before installing them. I have seen two ways of doing this, both with disadvantages for smaller companies with lower volumes.

One test requires two technicians standing beside the cableform with an ohmmeter or other continuity checker checking each connection. Besides being labor-intensive and slow, this technique is often inaccurate. It doesn’t easily pick up dry joints or shorts. The other technique requires a computerized cableform checker. The unit under test is plugged in and the computer performs the check. This is quick and accurate, but often prohibitively expensive.

I developed the technique described here for use in companies that tested enough cableforms to make manual checking expensive, but not enough to justify the cost of a computerized checker. Figure 1 shows a very simple example of a cableform with two plugs and three sockets. The test apparatus in Figure 2 uses two matching sockets and three matching plugs, 18 resistors, an ohmmeter (external or built-in), a single-pole two-position switch (SW1), and a multi-position switch (SW2), all mounted in a box.

The user plugs the cableform into the checker (the test apparatus) and connects the ohmmeter. The checker connects each wire in the cableform in series with (for example) a 10-Ω resistor between each wire. In the case where one wire goes to several plugs or sockets—for example, wires “a” and “b”—all of the pins are connected to a corresponding number of resistors of a value to give 10 Ω when connected in parallel.

If the cableform under test is correctly wired and the switches are set to the Test position, the resistance will be approximately 140 Ω (plus the resistance of the wires). To calibrate the checker, plug in a known-good cableform and note the exact resistance shown. If the unit under test (UUT) is incorrectly wired or has a high-resistance (dirty) pin, or a dry joint, or a short between two wires, the measured resistance will be either infinite or greater than or less than the value shown by the known-good cableform. If the resistance isn’t what it should be, simply step through the switch settings to check each wire in turn.

Starting at “a,” the resistance should increase in steps of approximately 10 Ω. The fault is located where it does not. It’s a simple matter to make a list of the connections corresponding to the switch settings. The checker should use high-stability resistors.

Although I haven’t tried it, it should be possible to substitute a controlled current source for the ohmmeter and LEDs for the resistors and see which LED doesn’t light up. You can also put an earphone across the ohmmeter and listen for dry joints.

Some supervisors have said that there must be a snag in this technique somewhere, as it can’t be that simple, and refused to try it. But no one has yet shown me the snag, nor have I known it to fail.

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