Information Technology Reference
In-Depth Information
If a time-limited test is required then the S18 register is loaded with the number of sec-
onds that the test should last. For example, a test that last 2 minutes will be set up with:
AT S18=120 &T1
Transmit
Loop
back
Computer
Receive
Local
modem
Figure 15.7
Analogue loopback with self-test
15.8.2 Local analog loopback with self-test (&T8)
In the analog loopback test with self-test the modem connects the transmit and receive lines
on its output and then automatically sends a test message which is then automatically re-
ceived, as illustrated in Figure 15.8. The local error checker then counts the number of errors
and displays a value when the test is complete. For example, the following test has found two
errors:
AT &Q0
<Enter>
AT S18=0 &T8
<Enter>
+++
AT &T0
002
OK
Loopback
Test message
Transmit
Error checker
Receive
Local modem
Figure 15.8
Analogue loopback with self-test
15.8.3 Remote digital loopback (&T6)
The remote digital loopback checks the local computer to modem connection, the local mo-
dem, the telephone line and the remote modem. The remote modem performs a loopback at
the connection from the remote modem to its attached computer. Figure 15.9 illustrates the
test set-up. An example session is:
