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7.0 Diagnostics
7.7.5 E
XTERNAL
A
NALOG
L
OOPBACK
S
ELF
-T
EST
(ALXST)
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. No DTE
or external BERT is necessary. As shown in Figure 7-4, the test circuit includes
the leased lines (the external loopback).
NOTE
The ALXST may result in errors or may fail altogether even where
conditions are satisfactory for normal operation. The test channel
presented to the modem during the ALXST includes all impairments of
both the inbound and outbound channels (amplitude distortion, delay
distortion, noise, etc.). The main purpose of the ALXST is to verify that
the modem is connected to the leased-line and the leased line circuit is
complete. The test is not meant to assess the suitability of the leased
line.
Figure 7-4. External Analog Loopback Self Test (ALBST).
TRANSMITTER
PATTERN
GENERATOR
RECEIVER
PATTERN
DETECTOR