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4. Type the escape sequence +++AT and press ENTER to bring your modem into
online command mode.
5. Type AT&T6 and press ENTER. The local modem responds to this command by
transmitting an unscrambled marking signal, which causes the remote modem to
place itself in digital loopback mode. Then the local modem exits online com-
mand mode and enters data mode.
6. Enter data from your keyboard. For this test, typing multiple uppercase U char-
acters is a good way to send an alternating test pattern of binary ones and zeroes.
Data received by the remote modem enters its analog receiver, is converted to
digital data, is reconverted into analog, and then is transmitted back to your mo-
dem. Your modem passes this test if the data received on your monitor is the
same as the data entered from your keyboard.
7. 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 mes-
sage. If you wish to stay on line with the remote modem for normal data trans-
mission, 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.
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 it-
self 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.
Computer or Terminal
Figure C-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.
AT&T3
OK
Digital
Local MultiModem
Analog
Analog
Remote MultiModem Computer or Terminal
C Loopback Tests
UUUUUUUUU
UUUUU
Digital
93
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