Chapter 6 - Testing and Troubleshooting
6.3
Remote Digital Loopback Test (V.54 Loop 2)
In this test, your modem must be online with another modem, such as another ZDX, that is set up to respond to a request for remote digital loopback. With the ZDX, this ability to respond is controlled by the &T command. AT&T4 enables the response to remote digital loopback; AT&T5 disables the response. The modem defaults to &T5 on power-up, so this must be changed on the remote modem before initiating remote digital loopback.
Initiate remote digital loopback with the ATU2 command, which places the remote modem in digital loopback mode. Data from your computer or terminal are transmitted through your modem and over the phone line to the remote modem, where they are then looped back to your modem. See Figure 6-2.
Figure 6-2. Remote digital loopback test.
6.3.1 Test Procedure
1.
Arrange to have &T4 set on the remote test modem.
2.
Open your communications software and go into terminal mode. Type AT and press ENTER; you should get an OK message. Type AT&E0<CR> to disable error correction.
3.
Dial the remote modem and establish your online connection.
4.
Type the escape sequence (+++AT<CR> or <BREAK>AT<CR>) to bring your modem into command mode while maintaining the connection with the remote modem.
5.
Type ATU2 and press 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 online mode.
6.
Enter data from your keyboard. For this test, typing multiple uppercase U characters is a good way to send an alternating test pattern of binary ones and zeroes. Data received by the remote modem enters its analog receiver, is converted to digital data, is reconverted into analog, and then is transmitted back to your modem. Your modem passes this test if the data received on your monitor is the same as the data entered from your keyboard.
7.
To exit the test, enter an escape sequence (+++AT<CR> or <BREAK>AT<CR>), which returns the modem to 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 ATO and press ENTER. If you wish to terminate the call, type ATH and press ENTER to hang up.
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