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Chapter 8 - Modem Testing
8.9 Digital Loopback Test (Remote/Automatic) (Synchronous Mode)
This test must be run when you have a data connection with another modem. In this test, data is
passed to the remote modem and is looped back to the local modem (as if an ATU2 command was
issued in Asynchronous test mode), as shown in Figure 8-7. This lets you test the local and remote
modem’s transmit and receive circuits, as well as your computer’s serial COM port and the phone
lines. If the test results in a mismatch of entered/received data (multiple upper case “U” characters in
Figure 8-6), the Local Analog Loopback Test should be performed on both the remote and local
modems. If that test is successful, the problem may be the phone lines.
First make certain that you are set up for Synchronous operation by placing DIP-Switch #12 in the
UP position. To initiate the Digital Loopback Test (remote/automatic), DIP-Switch #9 must be in the
DOWN position, and the Answer/Originate switch (front of the modem) must be toggled to the UP
position. Once you receive an OK message from your modem (if responses are enabled), the local
modem is placed in Digital Loopback mode. To exit the Digital Loopback Test (remote/automatic),
toggle the front panel Answ/Orig switch to the Answer position.
Computer or Terminal Local MultiModem
DIP-Switch #9
UP; Answ/Orig
Switch Toggled to
Answ position
when On-line
Remote MultiModem Computer or Terminal
Digital Analog DigitalAnalog
UUUUU
UUUUU
Figure 8-7. Synchronous Mode Digital Loopback Test (remote/automatic)