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256-KBPS LINE DRIVER (V.35, RS-530, OR X.21)
Figure 6-5. Remote digital loopback.
6.6 Local Digital Loopback
Activate the local digital loopback test by pressing the DIG button on the line
driver’s front panel. This test involves creating an outbound loopback at the local
line driver (see Figure 6-6 on the next page). It works the same way as if the remote
operator had initiated a remote digital loopback at the remote site. With this test,
the remote operator can check the performance of the remote and local line
drivers and the line between them:
1. Press DIG; the local line driver will signal the remote unit and will start local
digital loopback. The TEST LED should light on both units. The local line
driver’s receive input should be connected to its own transmitter.
2. Perform a Bit Error Rate Test (BERT). Either:
• Hook up an external BERT unit in place of the remote DTE; or
• Use the remote line driver’s internal BERT. Press the PATT button. The ERR
LED should light briefly just to confirm that the LED is working, and then the
test will proceed. If errors are detected, the ERR LED will be continuously lit
(if the errors are continuous) or will blink (for intermittent errors).
3. If the BERT test indicates a fault, but both line drivers pass their modem self-
tests (see Section 6.3), there is a problem somewhere in the communication
line between the units.
REM
DEPRESSED
DATA
CLK
RCVR
CLK
DATA
XMTR
LOCAL
DTE
XMTR
RCVR
REMOTE
DTE