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The ANALOG LOOP(EXT)
screen will be displayed, with ON
and OFF as the possible selections.
Select ON to activate the test. (In
general, the DSR during ALB test
option discussed in chapter should
be left ON—the default selection—
to avoid disrupting the DTE and
DTE software operations.)
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 (H-01,
etc.), followed by the modem
speed. Make sure that test data is
being generated. Watch for data
errors. By transmitting test data
and comparing it with the received
copy, the DTE and modem are
checked for proper operation.
During the ALBX test, RD (receive
data) may appear on the status
screen even though no data is
being received—this is normal.
On a flat line, the MODEM
modem in ALBX test mode will
train and remain running across
the following range of signal loss:
in V.32 mode, 0–51 dBm (0–52
dBm at 12.0 Kbps); for V.33
operation, 0-47 dBm; and for Bell
208A/B operation, 0–37 dBm. If
signal loss exceeds these limits, the
test may not start or may be halted.
CHAPTER 7: Diagnostics
During the ALBX test, the
modem will attempt to retrain
upon loss of carrier. In switched
carrier mode, the modem will
attempt to retrain upon loss of
RTS.
To end the test, press the ENT
pushbutton to return to the
ANALOG LOOP(EXT) screen,
and then select OFF.
7.8.5 E
XTERNAL
S
-T
(ALXST)
ELF
EST
The external analog loopback self
test is available for 4-wire leased
line mode only.
The ALXST places the modem in
analog loopback and causes data to
be sent and received by an internal
511-bit pattern generator and
receiver; therefore, no DTE or
external BERT is necessary. As
shown in Fig. 7-4, the test circuit
includes the leased lines (the
external loopback).
A
L
NALOG
OOPBACK
149
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