Full Duplex Loop Back Test; Troubleshooting - Bell 5902 Hardware Manual


Close the TST switch.
Alternating MARK and SPACE tones are transmitted continuously. Each tone lasts for
approximately 2.5 seconds. The KEY output is energized to put the radio (if any) into continuous
transmit mode.

This test may also be used to test the modem receiver. During this test the modem is
reconfigured for full duplex operation. The transmitter must be looped back to the receiver.

Select 2 wire operation to loop the signal back internally. Close the 2 WIRE switch and open the
4 WIRE switch. This does not test the line isolation transformers.

Select 4 wire operation to loop the signal back externally. Open the 2 WIRE switch and close
the 4 WIRE switch. Connect the transmit (TX) output to the receive (RX) input with external
wiring. This tests the line isolation transformers.
During this test, the CD LED will show the carrier detect status, without any carrier detect or loss
delays. The RD LED is on, and RxD outputs a MARK, when a MARK tone is received. If the
modem output is looped back to the input, the RD LED and RxD output should be on for
approximately 2.5 seconds then off for 2.5 seconds. Connect a break-out box to the DTE port to
observe the RxD signal.
To exit this test mode, return the TST switch to the open position. The modem will reconfigure itself
according to the configuration switches and will immediately begin operation.
6.2.3

Full Duplex Loop Back Test

This test confirms that the modem receiver, transmitter and network wiring are functioning properly
in a four wire, full duplex, point-to-point system.
To select this test mode:

Apply power to the modem.

Close the FDX switch (select full duplex mode).

Close the TST switch.
Any data which received by the modem on the receive wire pair, is re-transmitted on the transmit
wire pair. Transmit any data from another modem. Observe that the same data is echoed back.
During the test CD and DCD will indicate the carrier detect status, without any carrier loss or carrier
detect delays. Soft carrier turn-off tones will be ignored. The CTS output will be de-asserted to
prevent the DTE from transmitting. The DTE will not receive any of the data which is being echoed.
To exit this test mode, return the TST switch to the open position. The modem will reconfigure itself
according to the configuration switches and immediately begin operation.
6.3

Troubleshooting

The following procedure should be followed to isolate modem or communication system faults.
First, confirm the modem is functioning. Perform the test procedure described in section 6.2.1-Basic
Functional Test. This checks the modem CPU, LEDs, configuration switches, the RCD input and
the KEY output.
Second, confirm that the modem transmitter and receiver are functioning.

If the modem is used in a full duplex application, perform the test procedure described in section
6.2.3-Full Duplex Loop Back Test.
5902 Bell 202 Modem Hardware Manual
October 19, 2007
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