Multi-Tech Systems MT3334ZDX - Manual (Page 45)

4 - Testing Your Modem
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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)