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C Loopback Tests
64
MultiModemZPX User Guide
3. 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.
4. 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.
5. Your modem passes this test if the characters received on your monitor are the
same as the characters entered from your keyboard. If different characters ap-
pear 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 receiv-
ing 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
local 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 mo-
dem and over the phone line to the remote modem, where it is then looped back to
your modem.
Computer or Terminal
Digital Analog
AT&T6
OK
Local MultiModem Remote MultiModem Computer or Terminal
DigitalAnalog
AT&T6
OK
UUUUUUUUUUU
UUU
Figure C-2. Remote digital loopback test.
Test procedure
1. Arrange to have &T6 set on the remote test modem.
2. Using your communication program, set the baud rate to 9600 and go into ter-
minal mode. Type AT and press ENTER; you should get an OK message. Type
AT\N and press ENTER to disable error correction.
3. Dial the remote modem and establish your online connection.
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.