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The ALXST can be initiated
from the modem's front panel,
as follows:
From screen 2 of the LCD MAIN
MENU, select TEST. Go to screen 2
of the LOCAL TEST menu and
select ALXST.
The ANALOG ST (EXT) screen
will be displayed, with ON and OFF
as the possible selections. Select
ON to activate the test.
The EIA status screen will be
displayed while the test is being
conducted. If no external loopback
is present, the status screen will
display an "IDLE" message. If a
loopback is present, handshake
sequence codes will appear,
followed by the modem speed. If
"ER" is present on the front panel,
data errors have occurred. The ER
indicator may flash on when the
test is first turned on, but it should
quickly go out and remain off until
the end of the test.
The information at the end of
the previous section (for the ALBX
test) concerning signal loss, loss of
carrier and loss of RTS also apply to
the ALXST.
CHAPTER 7: Diagnostics
To end the test, press the ENT
pushbutton to return to the
ANALOG ST (EXT) screen,
and then select OFF.
7.8.6 D
L
IGITAL
OOPBACK
Digital loopback testing determines
the operational performance of the
local modem, the remote modem,
and the telephone line as a
composite system. For this test, data
must be sent and received at the
remote modem. This test
regenerates the data and loops it
back to the remote modem. As with
the ALB test, a comparison of the
results of the DLB test can indicate
whether or not the DTE is
performing satisfactorily. A
comparison of the results of the
ALB and DLB tests can help
determine whether the modem or
the telephone line is defective.
In the DLB test, the data to be
sent and received is originated and
checked by the DTE or external
BERT. Fig. 7-5 shows the data flow
for this test.
T
(DLB)
EST
151
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