IBM System/34 Maintenance Manual page 487

31-340 Transmit Level Limiting Check
The level control circuit of the CBS data coupler (or
similar coupler) can be set to a lower signal level than
the transmit level on the modem. When this is the case,
overloading of the level control circuit causes slow
recovery after change of direction, arid a possible loss
of the received signal. The modem transmit level must
be decreased enough to prevent overloading the CBS
coupler.
The following steps give the procedure to check
transmit level limiting.
1. Connect a decibel meter across the DT (data tip)
and DR (data ring) terminals of the modem at the
CBS coupler interface. Set the meter to Bridging
(no terminating resistance).
2. Set the Mode Selector switch to the Proc Run
3.
4.
5.
6.
7.
"position (CE panel).
Set"the Address/Data switches to 0000.
Set the MSIPL switch to the diskette position (CE
panel).
Set the CSIPL switch to the diskette position (CE
panel).
Set all other CE panel switches to their down
positions.
Insert diskette DIAGB1.
8. Press the Load switch (operator panel).
9. Select the data communications exerciser test
(transmit test).
10. Select 16 or 32 as data to be transmitted. Select
the Scope Loop A Cmnd as the test run option.
11. If this is a new installation, dial the system from a
local telephone. If you are troubleshooting a
problem, have your system dialed from the remote
system. Leave the telephone handset off the
cradle (off the hook).
12. Observe the meter. The transmit signal level meter
reading should equal the level of the coupler ± 1
dB. If the reading decreases in less than three
seconds, the coupler is limiting the transmit level.
If the reading remains constant, that level is being
transmitted to the line.
13. If the coupler is limiting the transmit signal level,
adjust the transmit level switches on the modem
card until the transmit level is below the controlled
level in the coupler. (Limiting no longer takes
place.) The correct level is one decibel below the
lower level reading in step 12. Stop here if this is
a new installation.
14. Check to see that another modem receives a
suitable signal level when this modem transmits by
doing the following:
a. Have another person at the remote station
connect a decibel meter across the DT (data tip)
and DR (data ring) terminals of the modem at
the CBS coupler interface.
b. Ensure the meter is set to Bridging (no
terminating resistance).
c. Observe the meter. The receive level at the
remote station should be 16 dB less than the
transmit level. However, an acceptable level
could be from -31 dB up to the same level as
the transmit level.
28
31-340
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