Multi-Tech Systems MT5600ZDX - Manual (Page 63)

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Owner’s Manual
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)