5 Testing the MT 3334HD8
CommPlete Communications Server 55
3. Note that the CD LED is on. If you’re set for 14,400 bps or higher, the Speed LED should be on. If the
CD LED is not on, there is a defect in your modem.
4. Enter characters 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 zeros. The characters entered should be
displayed on your monitor. The TD and RD LEDs should flash when a character is e ntered.
5. For a more complete test, you should also test the modem in answer mode. To do this, yo u must first
terminate the originate mode by entering an escape sequence (+++AT<CR> or
<BREAK>AT<CR>). An OK should appear on your display. Then type ATU1 and press ENTER to
place the modem in analog loopback in answer mode.
6. Note that the CD LED is on. If you’re set for 14,400 bps or higher, the Speed LED should be on. If the
CD LED is not on, there is a defect in your modem.
7. Enter characters from your keyboard. Type multiple uppe rcase U characters to send an alternating
test pattern of binary ones and zeros. The characters entered should be displayed on your monitor.
The TD and RD LEDs should flash when a character is entered.
8. To exit the test, enter an escape sequence (+++AT<CR> or <BREAK>AT<CR>), which returns the
modem to command mode.
9. Your modem passes this test if the data received on your monitor are the same as the data entered
from your keyboard. If different data appear on your monitor , your modem is probably causing the
problem, though it could also be your computer. If your modem passes this test, but you are
receiving errors while on line, the remote modem or the phone line could be at fault.
Remote Digital Loopback Test (V.54 Loop 2)In this test, your modem must be online with another modem, such as an other 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 3.
Computer or Terminal
UUUUU
UUUUU
Type AT in terminal mode; hit
ENTER; dial remote modem;
establish online mode; enter
escape sequence; type ATU2
and hit ENTER
Local MultiModem Remote MultiModem Computer or Terminal
Digital Analog DigitalAnalog
Figure 3. Remote digital loopback test.
Test procedure