C Loopback Tests
Test Procedure
1.Arrange to have &T4 set on the remote test modem. This command enables the remote modem to respond to an &T6 request for a remote digital loopback test from the local 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. Type AT+MS=9,1,9600,9600 and press Enter to set the local modem to V.32 mode at 9600 bps.
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.
AT&T3 | UUUUUUUUUUU |
OK | UUU |
Digital Analog | Analog Digital | ||||
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Computer or Terminal Local MultiModem | Remote MultiModem Computer or Terminal |
Figure
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