Multitech MULTIMODEM MT5600ZDX Owner's Manual page 71

4. Type AT&T6 and hit ENTER. The local modem responds to this
command by transmitting an unscrambled marking signal, which
causes the remote modem to place itself in Digital Loopback
mode. Then the local modem exits Command mode and enters
pseudo On-line mode.
5. Type data from your 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 remote
modem will enter its analog receiver, be converted to digital data,
be reconverted into analog, and then looped through its
transmitter back to the local modem. Your modem passes this test
if the data entered from the local keyboard is the same as the
data received on your monitor.
Figure 4-3. Digital Loopback Test (remote/automatic)
Chapter 4 - Testing Your Modem
71