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OPERATE Switch
Test 1 (T1, Local Loop)
Transmit data is internally clamped to mark, and
the scramble pattern is transmitted. The line
driver is internally connected to the receive pre-
amplifier and the transmit pattern is demodu-
lated and descrambled. The result is a steady
mark, shown by the OPERATE indicator remain-
ing on steadily. If a space is detected (failure), the
OPE RATE indicator turns off for 150 ms.
Test 2 (T2, Local Loop)
This test loops the transmitter to the receiver
with the DTE controlling the modem interface.
Data must be sent and checked by the DTE.
(The function of the DTE interface 'test' line
is identical to the test 2 function.)
Test 3 (T3, Transmit)
This is a communications channel test and should
be used only if the local loop test was success-
ful. Transmit data is internally clamped to mark
and the scramble pattern is transmitted. The
remote receiving modem descrambles the data
and should produce a steady mark (see test 4).
A duplex modem on test 3 can simultaneously
check a received signal for erroneous spaces.
Test 4 (T4, Receive, Remote Loop)
This is a communications channel test and should
be used only if the local loop test was success-
ful. The signal received from the remote modem
on test 3 is demodulated and descrambled and
should produce a steady mark. If a space is
detected, the OPE RATE indicator turns off
for 150 ms.
A duplex modem on test 4 rescrambles and re-
transmits the data, providing the capability for
a remote loop from the transmitting modem.
The transmitting modem (remote from the modem
on test 4) can be set on test 3 or can be set on
OPERATE with the DTE sending marks. In the
latter case, sending spaces indicates errors
at the receiving modem.
10-18
Note:
The DTE interface of the modem is clamped
4.
otf by tests 1, 3, or 4. There is no interaction
If the coupler is limiting the transmit sig-
nal level, increase the attenuation on the
preamp or SNBU card until the transmit
level is below the controlled level in the
coupler (limiting no longer takes place).
(One db lower than the final steady level
in step 3 should be the proper adjust-
ment.) Strap the answer tone to the
same level.
between the DTE and the modem when the
interface is clamped.
CADUCEE Feature
A compromise equalizer is part of the CADUCEE
attachment feature. It is disabled in T1, T2,
or DTE test operation; it is active in T3 or T4
5.
Check to see that an appropriate signal
level is received by another modem when
this modem transmits.
operation (except at 1200 bps).
Transmit Level Limiting and Signal Meter
Adjustment
Transmit Signal Level Limiting and Receive
Sensitivity Adjustments
The level control circuit of the CBS (or equiv-
alent) data coupler can
be
set to a lower sig-
nal level than the transmit level that is strapped
on the modem. When this is the case, saturation
of the level control circuit causes slow recovery
after turnaround, and the first part of the re-
ceived signal might be lost. The modem transmit
level (normal, SNBU, and answer tone) must be
attenuated enough to avoid saturation in the
CBS coupler.
To check for transmit level limiting, proceed
as follows:
1.
Connect a db meter (P/N 453545) across
2.
3.
the DT (data tip) and DR (data ring) terminals
of the modem at the CBS coupler interface.
Set the meter to BRIDGING (no termination
resistance).
Set this modem to T4 (receive) and estab-
lish a telephone connection to another
modem set to T3 (transmit).
Switch this modem to T3 (transmit) and
watch the db meter. If the reading goes
down from the initial value (which should
be the level strapped on the preamp or
SNBU card) within about three seconds,
the coupler "is limiting the transmit level.
If the reading remains at the initial level,
that level is being transmitted to the line.
Signal Quality Meter Adjustment
When the post-processor card N4 is replaced,
readjust the signal quality meter as follows:
1.
Turn modem power on and turn the
test/operate switch to T1.
2.
3.
Assuming that there are no other indica-
tions of failure in test 1, adjust the poten-
tiometer on the post-processor card until
the meter reads perceptibly above zero
(half of the smallest division on the meter
scale).
Recheck the meter reading against the
benchmark measurement made at instal-
lation time. Make this measurement with
the modem in operate mode.
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