C Loopback Tests
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Appendix C: Loopback Tests
Introduction
Each time you turn on your modem, it performs an automatic self-test to ensure
proper operation. Your modem also has three diagnostic tests: local analog loop-
back, remote digital loopback, and local digital loopback. These ITU-T V.54 loop-
back tests isolate telephone circuit and transmission problems.
In a loopback test, data from your computer loops through the circuits of your mo-
dem and/or a remote modem before it appears on your monitor. When the loop has
been completed, the data on your PC’s monitor should match the original data.
The local analog loopback test allows you to verify that the modem’s transmitter
and receiver circuits are functioning properly.
The local digital loopback allows you to verify that the local computer or terminal,
the two modems, and the transmission line between them are functioning properly.
The remote digital loopback test allows you to verify that the remote computer or
terminal, the remote modem, the serial ports, the telephone line, and the local mo-
dem are functioning properly.
Note: All loopback tests operate at all speeds except 300 bps.

Local Analog Loopback Test (V.54 Loop 3)

In this test, data from your computer or terminal is sent to your modem’s transmit-
ter, converted into analog form, looped back to the modem’s receiver, converted into
digital form, and then sent to your monitor for verification. No connection to the
phone line is required.
Computer or Terminal
Digital Analog
AT&T1
CONNECT
UUUUUUUUUUUU
Local MultiModem
Figure C-1. Local analog loopback test.
Test procedure
1. Connect the modem to your computer. Using your communication program, set
the desired baud rate and go into terminal mode.
2. Type AT and press ENTER; you should get an OK message. Type AT\N and
press ENTER to disable error correction.