USRobotics V.34 - Manual (Page 196)

COURIER HIGH SPEED MODEMS
H-2 Modem Testing
ANALOG LOOPBACK

&T1, &T8
This test checks the operation of the modem's transmitter and
receiver. Data flow is shown in Figure H-1.
Figure H-1   Data Flow During Analog Loopback
There are two analog loopback options. The first, &T1, involves
your typing data that you can verify at your screen.
The second option, &T8, is an internal self-test that does not
involve the keyboard or screen. It isolates the modem from the
computer interface to give you a more specific result.
NOTE: Dual Standard modems must be set to B0 (default) or
tested at
2400 bps or lower to avoid HST asymmetrical
modulation at higher speeds.
&T1
1. If you are testing an HST or Dual Standard modem, set
your terminal or software to 2400 bps.
2. The modem must be in Command mode. If you wish, set
Register S18 as a test timer, as explained earlier.
3. Send the modem the following command:
AT &M0 &T1
The modem disables error control, enters analog loopback
(AL) mode, and sends a CONNECT message. The MR
status light flashes.