Back to Contents Page

Loopback Tests: Multi-Tech MT5634ZBA User's Guide

Introduction • Local Analog Loopback Test (V.54 Loop 3) • Remote Digital Loopback Test (V.54 Loop 2) • Local Digital Loopback Test (V.54 Loop 2)

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 remote digital loopback test allows you to verify that the local computer or terminal, the two modems, and the transmission line between them are functioning properly.

The local 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 1. Local analog loopback test

Test procedure

  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 the modem is 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.
  6. 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)

The remote digital loopback test tests the phone lines and the circuits of both your modem and a remote modem. In this test, your modem must be on line with another modem that is set up to respond to a request for remote digital loopback. (Note that some modems might not support remote digital loopback or might have it disabled.) Data from your computer or terminal is transmitted through your modem and over the phone line to the remote modem, where it is then looped back to your modem.

Figure 2. Remote digital loopback test

Test procedure

  1. Arrange to have &T6 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\N and press ENTER to disable error correction.
  3. Dial the remote modem and establish your online connection.
  4. Type the escape sequence +++AT and press ENTER to bring your modem into online command mode.
  5. Type AT&T6 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 online command mode and enters data 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, type the escape sequence +++AT and press ENTER. This puts the modem in online 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 AT&T and press ENTER to exit the test, then type ATO and press ENTER to return on line. If you wish to terminate the call, type ATH and press ENTER to hang up.

Local Digital Loopback Test (V.54 Loop 2)

The local digital loopback test is identical to the remote digital loopback test with one exception. Instead of using your modem to signal a remote modem to place itself in digital loopback mode, your modem is placed in digital loopback mode while the remote modem is not. Data is entered and transmitted from the remote modem, sent across the phone line to your modem, and looped back to the remote modem.

Figure 3. Local digital loopback test

Test procedure

  1. Open your communications software and go into terminal mode. Type AT and press ENTER; you should get an OK message. Type AT\N and press ENTER to disable error correction.
  2. Dial the remote modem and establish your online connection.
  3. Type the escape sequence +++AT and press ENTER to bring your modem into online command mode.
  4. Type AT&T3 and press ENTER. When you receive an OK message from your modem (if responses are enabled), your modem is in digital loopback mode.
  5. Have someone enter data from the remote 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 data received by your modem enters its analog receiver, is converted to digital data, is reconverted into analog, and then is transmitted back to the remote modem. Your modem passes this test if the data received on the remote monitor is the same as the data entered from the remote keyboard.
  6. To exit the test, type the escape sequence +++AT and press ENTER. This puts the modem in online 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 AT&T and press ENTER to exit the test, then type ATO and press ENTER to return on line. If you wish to terminate the call, type ATH and press ENTER to hang up.

Back to Contents Page