Modems - Computer Busses - page 254

AT &Q0 <Enter>
AT S18=0 &T6 <Enter>
CONNECT 9600
Help the bridge is on fire <Enter>
+++
OK
AT &T0
OK

Transmit Receive

Computer

Receive Transmit

Phone
line

Local
modem

Remote
modem

Figure 15.9 Remote digital loopback test
15.8.4 Remote digital loopback with self-test (&T7)

The remote digital loopback with self-test checks the local computer to modem connection,
the local modem, the telephone line and the remote modem. The remote modem performs a
loopback at the connection from the remote modem to its attached computer. The local mo-
dem sends a test message and checks the received messages for errors. On completion of the
test, the local modem transmits the number of errors. Figure 15.10 illustrates the test setup.
An example session is:

AT &Q0 <Enter>
AT S18=0 &T7 <Enter>
+++
AT &T0
004
OK

or with a test of 60 seconds then the user does not have to send the break sequence:

AT &Q0 <Enter>
AT S18=60 &T7 <Enter>
004
OK

Test message Transmit Receive

Error checker Receive Transmit

Phone
line

Local
modem

Remote
modem

Figure 15.10 Remote digital loopback test with self-test