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Chapter 5 - Testing the Modem
2. Dial the remote modem by entering the Dial command and the phone
number, to establish On-line mode.
3. Type the Escape Sequence (+++AT<CR> or <BREAK>AT<CR>) which
brings your modem into Command mode, while still maintaining the
pseudo On-line mode with the remote modem.
4. Type ATU3 from the local PC and hit ENTER. Once you receive an OK
message from your modem (if responses are enabled), the local
modem is placed in Digital Loopback mode.
5. Data is typed from the remote 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 local modem will
enter its analog receiver, be converted to digital data, be reconverted
into analog, and then looped through its transmitter back to the remote
modem. Your modem passes this test if the data entered from the
remote keyboard is the same as the data received on the remote
monitor.
Figure 5-2. Digital Loopback Test (local / manual)
6. When testing is complete, you may end the test by typing an Escape
Sequence (+++AT<CR> or <BREAK>AT<CR>) to bring your modem
into Command mode. The modem should respond with an OK
message. If you wish to stay On-line with the remote modem for normal
data transmission, type AT0 and hit ENTER. If you wish to terminate
the call, type ATH and hit ENTER to hang up.
Digital Analog Analog
Digital
Computer or Terminal Computer or Terminal
Local Modem Remote Modem
UUUU