C Loopback Tests

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.

3.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.

4.Note that the CD indicator is on. If it is not on, there could be a problem with your modem.

5.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.

6.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.

7.Your modem passes this test if the data received on your monitor is the same as the data entered from your keyboard. If different data appears on your monitor, your modem is probably causing the problem, though it could also be your computer. If your modem passes this test, but you are receiving errors while on line, the remote modem or the phone line could be at fault.

Remote Digital Loopback Test (V.54 Loop 2)

The remote digital loopback test tests the phone lines and the circuits of both your modem and a remote modem. In this test, your modem must be on line with another modem that is set up to respond to a request for remote digital loopback. (Note that some modems might not support remote digital loopback or might have it disabled.) Data from your computer or terminal is transmitted through your modem and over the phone line to the remote modem, where it is then looped back to your modem.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

AT&T6

 

 

 

 

 

 

 

 

 

 

 

 

 

AT&T4

 

 

 

 

CONNECT 9600

 

 

 

 

 

 

 

 

 

 

 

 

 

OK

 

 

 

 

UUUUUUUUUUU

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

UUUUU

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Digital Analog

Analog Digital

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Computer or Terminal Local MultiModem

Remote MultiModem Computer or Terminal

Figure C–2. Remote digital loopback test.

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