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RS-232 TO FIBER OPTIC MULTIPLEXOR
5.3.3 L
OCAL
L
OOPBACK
T
ESTING
If the remote loop test shows one or more channels of the multiplexor system
not operating or generating data errors, the problem can be isolated to one
multiplexor by a local loop test.
A local loop test is performed by connecting the optical transmitter of the
multiplexor back to its own optical receiver, which causes all signals fed into
any channel of the multiplexor to be looped back to the corresponding
outputs of the same channel. Proceed as follows:
1. Connect the optical transmitter’s output to the receiver’s input with
a short fiberoptic cable.
2. Verify that the SDCD indicator comes ON; test the individual channels
in the same way described in the previous sections.
3. If your data becomes swamped with continuous errors, the cable may be
too short to avoid overloading the optical receiver. If you have a longer
cable, replace the short cable and try again. If you don’t have a longer
cable, unscrew or untwist the cable connector at the end attached to the
mux’s transmitter, and withdraw the cable partway out of the transmitter’s
socket. Now try again; the light coming through the cable should be
much weaker.
5.4 No Data Transfer on All Channels
If data cannot be transmitted on any channel and the multiplexor pair
doesn’t pass a remote loop test (see
Section 5.3.2
), perform a local loop test
(see
Section 5.3.3
). If neither mux passes the local loop test, this would tend
to indicate that the data is being garbled before it gets to the muxes. Check
your software, your terminal hardware, and your RS-232 cables carefully.
If at least one multiplexor passed the local loop test, or if your data is good
when it comes out of each RS-232 cable, the problem must be in either the
