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Chapter 6 - Troubleshooting

MT4X56USB 63

Local Digital Loopback Test (V.54 Loop 2)

The local digital loopback test is identical to the remote digital loopback test with one exception.

Instead of using your modem to signal a remote modem to place itself in digital loopback mode, your

modem is placed in digital loopback mode while the remote modem is not. Data is entered and

transmitted from the remote modem, sent across the phone line to your modem, and looped back to

the remote modem.

AT&T3

Digital Analog

Analog Digital

Computer or Terminal

Local MultiModemUSB

Remote MultiModemUSB

Computer or Terminal

UUUUUUUUU

UUUUU

Figure 6-3. Local digital loopback test

Test Procedure

1. Open your communications software and go into terminal mode. Type AT and press ENTER;

you should get an OK message. Type AT\N and press ENTER to disable error correction.

2. Dial the remote modem and establish your online connection.

3. Type the escape sequence +++AT and press ENTER to bring your modem into online

command mode.

4. Type AT&T3 and press ENTER. Once you receive an OK message from your modem (if

responses are enabled), your modem is placed in digital loopback mode.

5. Have someone enter data from the remote 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

data received by your modem enters its analog receiver, is converted to digital data, is

reconverted into analog, and then is transmitted back to the remote modem. Your modem

passes this test if the data received on the remote monitor is the same as the data entered

from the remote keyboard.

6. To exit the test, type the escape sequence +++AT and press ENTER. This puts the modem

in online command mode. The modem should respond with an OK message. If you wish to

stay on line with the remote modem for normal data transmission, type AT&T and press

ENTER to exit the test, then type ATO and press ENTER to return on line. If you wish to

terminate the call, type ATH and press ENTER to hang up.

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Multitech MT4X56USB Guida Utente Pagina 63