Chapter 3 Operation
3-13
3.21.1 Quick Test
See if the LCD display on the Loop-V 4200 has normal text. If not, Loop-V 4200 has failed.
Remove all line connections to Loop-V 4200. Remove power. After a few seconds, re-apply power. Observe
the power-up self-test sequence. If this fails, then Loop-V 4200 has failed.
See if the LEDs show any abnormal displays. If yes, use the LCD indications to guide the user to test other
parts of the network.
Especially during initial installation, excessive errors may be due to (a) incorrect configuration of either Loop-
V 4200 or of the equipment at the other end of the line, or (b) due to faulty line installation, which results in
excessive noise, cross talk, or impedance mismatch. Especially in electrically noisy environments, such as
central offices, use of shielded cables are mandatory.
3.21.2 Substitution
If a spare Loop-V 4200 is available, then replace the working one with the spare. The user must carefully
configure the spare exactly as the working one. If the substitution clears the problem, then the original
working one is suspect. Note that this is not definitive as other reasons may cause the same symptom. A
good practice is to reconfigure the original one and swap once more.
If both units behave the same, then the problem is probably elsewhere.
3.21.3 Using Loopback Plugs
Without a spare, loopback plugs are handy for diagnosis. Note that internal loopback facilities of the Loop-V
4200 does not include the interface circuitry. Thus a set of plugs, one for each of the interfaces types, are
needed for complete tests. These plugs are wired such that signals from the Loop-V 4200 are looped back
by hard wire to the receive pin of the interface.
Replace the line connector with a loopback plug. Observe if the line is in sync. If not then Loop-V 4200 has
failed. Then perform a PRBS/QRSS test towards the line. If this fails, then Loop-V 4200 has failed.
Note that if a far end terminal is available, the first test should be a local line loopback to see if the line is
good.
If tests with loopback plugs all pass, then the problem is probably elsewhere.
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