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ModemModule Developer's Guide
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.
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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Figure 6-3. Local digital loopback test
MT3334SMI
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