5.3.1 Using Local Analog Loopback (LAL)
The Local Analog Loopback (LAL) test checks the operation of the
local Model 2400, and is performed separately on each unit. Any data
sent to the local Model 2400 from the DTE in this test mode will be
echoed (returned) back to the DTE. For example, characters typed on
the keyboard of a terminal will appear on the terminal screen. To
perform a LAL test, follow these steps:
1.
Activate LAL. This may be done in one of two ways: First, by
moving the front panel toggle switch DOWN to “LAL.” ;
second, by raising the appropriate pin on the terminal
interface (pin 18 for Model 2400/A, pin L for Model 2400/C).
Make sure DIP switches S1-6 and S1-7 are OFF. Once LAL is
activated, the Model 2400 transmitter output is connected to
its own receiver. The test LED should be lit.
2.
Verify that the data terminal equipment is operating properly
and can be used for a test. If a fault is indicated, call a
technician or replace the unit.
3. Perform a V.52 BER (bit error rate) test as described in
Section 5.3.3. If the BER test equipment indicates no faults,
but the data terminal indicates a fault, follow the
manufacturer’s checkout procedures for the data terminal.
Also, check the interface cable between the terminal and the
Model 2400.
5.3.2 Using Remote Digital Loopback (RDL)
The Remote Digital Loopback (RDL) test checks the performance
of both the local and remote Model 2400s, and the communication link
between them. Any characters sent to the remote Model 2400 in this
test mode will be returned back to the originating device. For example,
characters typed on the keyboard of the local terminal will appear on
the local terminal screen after having been passed to the remote Model
2400 and looped back. To perform an RDL test, follow these steps:
1. Activate RDL. This may be done in two ways: first, by moving
the front panel toggle switch UP to “RDL”; second, by raising
the appropriate pin on the terminal interface (pin 21 for Model
2400/A, pin N for Model 2400/C). Make sure S1-6 and S1-7
are OFF.
NOTE: to activate RDL by pin 21 on the Model 2400/A, Jumper
JP2 must be set in the “NORMAL” setting (see Section 3.2.1)
2. Perform a V.52 BER test as described in Section 5.3.3. If the
BER test equipment indicates a fault, and the Local Analog
Loopback test was successful for both Model 2400s, you may
have a problem with the twisted pair line between the
modems. You should then check the twisted pair line for
proper connections and continuity.
5.3.3 Using the V.52 BER Test
To use the V.52 BER tests in conjunction with the V.54 loopback
tests, follow these instructions:
1. Locate the 511/511E toggle switch on the front panel of the
2400 and move it DOWN. This activates the V.52 BER test
mode and transmits a 511 test pattern into the loop. If any
errors are present, the local modem’s red “Error” LED will blink
sporadically.
2. If the above test indicates no errors are present, move the
V.52 toggle switch UP, activating the “511/E” test with
errors present. If the test is working properly, the local
modem's red BERT LED will glow. A successful “511/E” test
will confirm that the link is in place, and that the Model 2400’s
built-in 511 generator and detector are working properly.
NOTE: The above V.52 BER tests can be used independently of
the V.54 loopback tests. This requires two operators: one to initiate
and monitor the tests at the local Model 2400, and one to do the
same at the remote Model 2400. In this case, the test pattern sent
by each Model 2400 will not be looped back, but will be transmitted
down the line to the other Model 2400. Both operators must initiate
and monitor the tests simultaneously.
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