Black Box BRTS-100 manual Pair Reversal

CHAPTER 4: Test Procedures

4.6 Pair Reversal

When you use the passive-bus configuration, it’s imperative that all the transmit pairs (from terminal) be wired the same. That is to say, pins 3 and 6 from each terminal on the bus must be wired to pins 3 and 6 of the NT1 respectively. On a point-to-point configuration, the circuit will operate properly even if the pair is reversed (pin 3 [terminal] to pin 6 [NT1], pin 6 [terminal] to pin 3 [NT1]). However, on a passive-bus configuration, if two terminals are wired differently, their signals, (having opposite polarity) will cancel each other.

This problem, known as pair reversal, usually occurs when the installer accidentally transposes the pairs in the S/T interface cable. A common symptom of pair reversal is that a device works when it is by itself on the bus, but doesn’t work when other devices are present on the bus.

To identify this condition, an additional Test Set or any terminal that initializes itself automatically is required (check with terminal manufacturer). Refer to the Pair Reversal Flow Chart in Figure 4-9 and the diagram in Figure 4-10 for an outline of the functions performed during this test.

This procedure assumes that LINE TEST passes on each individual port on the bus.

1.Remove all terminals from the passive bus.

2.Place the first Test Set or ISDN terminal on the last port of the passive bus. This will be referred to as terminal A.

3.Terminate terminal A at 100 ohms. If you’re using the Test Set as terminal A, the TERMINATE button should be out.

4.If terminal A is:

a.The Test Set, put it in the LINE TEST test mode.

b.An ISDN terminal, power it on.

5.Take the second Test Set, which will be known as terminal B, and connect it to another port on the bus.

6.Unterminate terminal B by pressing the TERMINATE button.

7.Put terminal B in the LINE TEST Test Mode.

8.Press the INITIATE button.

9.a. If the Green Test Status light goes on, terminals A and B are using the same polarity.

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