3Com 56K manual Loopback DL testing

Digital Loopback Testing (AT&T3)

Loopback￿￿￿￿￿	(DL)￿￿￿￿￿￿￿￿testing.￿￿￿	Testing the Business Modem using AT&Tn 14-5
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This test can help you locate a problem with a remote device or with the telephone line. The figure below shows the data flow during Digital

1Send the command AT&M0&N3S14.0=0 to the modem to prepare for testing.

This command disables error control, fixes the connection rate at 2400 bps, and makes the Business Modem return to command mode when you type +++.

2Establish a connection with the remote device.

3Send the Business Modem the escape code, +++, to bring it back to Command mode.

4Send AT&T3 to the modem to enter Digital Loopback mode.

5Have the remote user type a short message. It will be looped back by your Business Modem's transmitter for verification on the remote user's screen. You will not see the message or any other data.

6When the remote user has completed the test, send the escape code,

+++, and then AT&T0 to end the test.

Alternatively, you can end the test by sending ATH or ATZ. Be careful, because ATZ resets the modem in addition to ending the test.

In any case, the Business Modem responds OK. If the modem sends an ERROR message, you have issued an invalid command.

7Send AT&M4, unless you used ATZ.