Black Box ME444A-R3, ME444AE-R3, ME445C-X.21 manual Local Digital Loopback, Remote digital loopback

256-KBPS LINE DRIVER (V.35, RS-530, OR X.21)

LOCAL

DTE

REM

DEPRESSED

DATA
CLK	XMTR

CLK

DATA RCVR

RCVR

XMTR

REMOTE

DTE

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.

44