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CHAPTER 5: Troubleshooting
verify the waveform with an oscilloscope at both the transmitting and
receiving ends. (At the receiving end, there will be varying timing distortion—
that is, jitter—of up to 13 µs, caused by the sampling rate of the multiplexing
circuits.) Connect the internal transmit clock from Pin 15 at one multiplexer
to the Transmit Data input (Pin 2) and verify that the same signal appears at
the Receive Data output (Pin 3) of the other multiplexor. Repeat this test,
connecting the transmit clock to the Data Terminal Ready input (Pin 20)
and verify that the same signal appears at the Clear to Send output (Pin 5)
of the other multiplexor.
5.3.2 R
EMOTE
L
OOPBACK
T
ESTING
The only way to perform a complete and totally reliable test of an multiplexor
channel or channels is an end-to-end test using two bit-error-rate testers. If
only one tester is available, a remote loop test is almost as reliable as an end-
to-end test.
A remote loop test sends data from one multiplexor to the other, where it
is externally jumpered back into the second multiplexor and returned to the
first multiplexor, where the tester is located. Since data passes through the
multiplexing/demultiplexing circuits twice in a remote loop test, the
maximum data rate is only half the maximum that can be transmitted in an
end-to-end test. For internally clocked synchronous tests, the maximum data
rate would be 9600 bps. For asynchronous tests, the maximum data rate is
dependent on the amount of timing distortion the tester can tolerate.
The remote loop test is run in the same manner as described previously for
the end-to-end test, except that what happens at the other end of the system
in the end-to-end test happens at the same end in the remote loopback test.
For example, data fed into one multiplexor’s transmit data input is received
at the same multiplexor’s receive data output.
To put a channel in remote loop, proceed as follows:
1. Connect the tester (or breakout box) to one multiplexor’s channel’s
connector, and at the other multiplexor set the corresponding channel’s
STD switch position ON and all other positions OFF.
2. Insert jumper wires into the connector, or make up a jumper plug, to
connect Pin 2 to 3, 14 to 17, and 5 to 20. A test run in this way tests all
functions of the multiplexor except the Data Carrier Detect, Data Set
Ready, and Internal Transmit Clock outputs at the remote multiplexor
(the one that has the jumpers). For a total system test using only a single
tester and a remote loop mode, the test must be run from each end of the
system.
