Multitech V.90/K56FLEX MT5600ZDXV - Manual (Page 69)

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User Guide
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)