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MT5634ZBA-USB User Guide
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 loopback, remote
digital loopback, and local digital loopback. These ITU-T V.54 loopback tests isolate
telephone circuit and transmission problems.
In a loopback test, data from your computer loops through the circuits of your modem
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 modem
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 transmitter,
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 Local MultiModem
Digital Analog
AT&T1
CONNECT
UUUUUUUUU
UUU
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&T1 and press ENTER. This places your modem in analog loopback mode
in the originate mode. A
CONNECT
message should appear on your display. The
modem is now out of command mode and in a pseudo-online mode.
3. Note that the CD LED is on. If you are set for 14,400 bps or higher, a speed LED
should be on. If the CD LED is not on, there is a defect in your modem.
4. Enter characters from your keyboard. For this test, typing multiple uppercase
U
characters is a good way to send an alternating test pattern of binary ones and
zeros. The characters entered should be displayed on your monitor. The TD and RD
LEDs should flash when a character is entered.
5. To exit the test, type the escape sequence +++AT and press ENTER. This puts the
modem in online command mode. Then type either AT&T or ATH to return to
command mode.