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Appendices
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.