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User Guide
the same as the data received on your monitor. If different data is
appearing on your monitor, your modem is probably causing the
problem, although 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.
Digital Loopback Test/V.54 Loop 2 (Local/Manual)
The Digital Loopback Test is an on-line test that loops data sent from one
modem across the phone line to another modem, then back to the first
modem. See Figure 4-2. There are two ways to put a modem into Digital
Loopback mode:
1. Locally or Manually, described here in section 4.2.
2. Remotely or Automatically, see section 4.3.
Note: If you wish to abort any test in progress, enter AT&T0<CR>.
All loopback tests will operate at all speeds except 300 bps. Disable er-
ror correction (AT\N0&W0<CR>) before engaging in loopback tests. In
this test the local modem is placed in Digital Loopback mode. Data is
entered and transmitted from the remote modem (which is not in digital
loopback mode), sent across the phone line to the local modem and
looped back to the remote modem.
The test procedure is as follows:
1. Go into Terminal mode. Type AT and hit ENTER; you should get an OK
message.
2. Dial the remote modem by entering the Dial command and the phone
number, to establish On-line mode.
3. Type the Escape Sequence (+++) which brings your modem into
Command mode, while still maintaining the pseudo On-line mode with
the remote modem.
4. Type AT&T3 from the local PC and hit ENTER. Once you receive an OK
message from your modem (if responses are enabled), the local modem
is placed in Digital Loopback mode.
5. Data is typed from the remote keyboard. For this test, typing multiple
upper case "U" characters is a good way to send an alternating test
pattern of ones and zeros. The data received by the local modem will
enter its analog receiver, be converted to digital data, be reconverted
into analog, and then looped through its transmitter back to the remote