Multi-Tech Systems MT3334SMI Local Analog Loopback Test (V.54 Loop 3)

ModemModule Developer’s Guide

MT3334SMI56

Introduction

Each time you turn on your modem, it performs an automatic self-test to ensure proper operation.

Your modem also has three diagnostic tests: local analog loopback, remote digital loopback, and

local digital loopback. These ITU-T V.54 loopback tests isolate telephone circuit and transmission

problems.

In a loopback test, data from your computer loops through the circuits of your modem and/or a

remote modem before it appears on your monitor. When the loop has been completed, the data on

your PC’s monitor should match the original data.

The local analog loopback test allows you to verify that the modem’s transmitter and receiver circuits

are functioning properly.

The local digital loopback allows you to verify that the local computer or terminal, the two modems,

and the transmission line between them are functioning properly.

The remote digital loopback test allows you to verify that the remote computer or terminal, the remote

modem, the serial ports, the telephone line, and the local modem are functioning properly.

Note: All loopback tests operate at all speeds except 300 bps.

Local Analog Loopback Test (V.54 Loop 3)

In this test, data from your computer or terminal is sent to your modem’s transmitter, converted into

analog form, looped back to the modem’s receiver, converted into digital form, and then sent to your

monitor for verification. No connection to the phone line is required.

Figure 6-1. Local analog loopback test.

1. Connect the modem to your computer. Using your communication program, set the desired baud

rate and go into terminal mode.

2. Type AT&T1 and press ENTER. This places your modem in analog loopback mode in the

originate mode. A CONNECT message should appear on your display. The modem is now out of

command mode and in a pseudo-online mode.

3. Note that the CD LED is on. If you are set for 14,400 bps or higher, a 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

entered.

5. To exit the test, type the escape sequence +++AT and press ENTER. This puts the modem in

online command mode. Then type either AT&T or ATH to return to command mode.