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Chapter 8 - Modem Testing
8.6 Synchronous Mode Testing
The following tests must be run with your modem in Synchronous mode (DIP Switch #12 in the Up
(OPEN) position), DIP Switch #9 controls the modem’s Synchronous mode testing function. (Refer to
Chapter 9 for DIP Switch information.) The test procedures for Synchronous mode are different from
those for Asynchronous mode. In Synchronous mode, you cannot access the modem’s AT
commands.
With DIP Switch #9, you can perform either the Digital Loopback Test (remote/automatic) or the
Digital Loopback Test (local/manual) in Synchronous mode, as described in the following sections.
There is also a Local Analog Loopback Test, documented in section 8.7.
8.7 Local Analog Loopback Test (Synchronous Mode)
This test diagnoses the connection between your modem and your computer or terminal. In Local
Analog Loopback Test mode, data entered at the local computer or terminal are sent through the
local modem’s transmit and receive circuits (much like entering an ATU or ATU1 command in
Asynchronous mode). You then compare the test characters (multiple upper case “U” characters in
Figure 8-5) on your monitor with the characters you typed. If the characters don’t match, check your
computer’s COM port setting, then verify your communication software’s configuration.
To initiate the Local Analog Loopback Test, with the modem in Synchronous mode:
1. Enter AT&M1U. This first switches your modem form asynchronous to synchronous mode, and
places it into the Analog Loopback/Originate mode. The modem is now out of the Command
mode and in the pseudo On-Line mode.
2. Once you receive a connect message (if responses are enabled), enter 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.
3. For a more complete test, you should also test the modem in Answer mode. To do this, Type the
Escape Sequence (+++AT<CR> or <BREAK>AT<CR>) which brings your modem into
Command mode, while still maintaining the connection. Then type AT&M1U1 and hit ENTER to
place the modem in Analog Loopback mode, in the Answer mode. Then repeat step 2.
Computer or Terminal Local MultiModem
Digital Analog
UUUUU
UUUUU
Figure 8-5. Synchronous Mode Local Analog Loopback Test