Netgear XM128 ISDN User Manual download pdf (Page 90)

Reference Guide for the Model XM128 ISDN Digital Modem

10-2 Troubleshooting

The LNK LED light s up for a half second to indicate the success of the power-on self-test. Then

the LNK LED becomes the normal physical layer (layer 1) active indicator.

Loopback Tests

The Model XM128 modem performs several loopback tests as part of its diagnostic capabilities.

ISDN Loopback test (AT&T9)

The AT&T9 command invokes an ISDN loopback test connection. The loopback point is in the

S/T interface chip (Siemens 2086 chip) or the U interface chip (Siemens 2091 chip) just behind the

line transformers. It checks almost every part of the Model XM128 modem and RS-232 cable

except the passive front end of the ISDN S/T or U interface.

During this test, data from the terminal or computer is sent through the DTE interface to the

transmitter of the Model XM128 modem. Data is then packetized to the proper frame format

according to the B channel protocol selected. Finally, the data is looped back to the receiver,

depacketized, and sent through the DTE interface back to the terminal or computer screen. You can

tell if anything is wrong by looking at the screen. The screen shows the data you sent to the

Model XM128 modem.

Loopback with Self-test (AT&T10)

The AT&T10 command invokes an ISDN loopback connection with self-test. The data is

generated by the Model XM128 modem and goes through the same path as the above loopback test

does. The data pattern is printable ASCII characters, so you can see the result on the screen. The

looped back data is compared with the transmitted data. Should an error occur, the LNK LED

starts to flash. Any character sent through the DTE interface to the Model XM128 modem

discontinues the test.

Note:

The LED lights up during the test and blinks if the test fails.

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Netgear XM128 ISDN User Manual Page 90