Analog circuit calibration error.
Loopback Tests
The Analog Loopback Test, Local Digital Loopback Test, Remote Digital Loopback Test can all be initiated with AT commands from the terminal. Use the AT&T0 command to terminate the test.
Analog Loopback (AT&T1)
This test can check almost every part of the modem and the RS-232 cable except the telephone line outgoing interface. During Analog Loopback testing, data from the terminal or computer is sent through an RS-232 cable into the modem's transmitter and is modulated to an analog signal. Then it is looped back to the receiver, demodulated to digital form and sent through the RS-232 cable back to the terminal or computer's screen. You can tell if anything is wrong by looking at the screen. The screen should show the data you have sent to the modem. This test can only be initiated while the modem is off-line.
Analog Loopback with Self-test (AT&T8)
This test generates data from the modem itself instead of input data from the RS-232 interface. The data will go through the same path as it would with the Analog Loopback Test. In asynchronous mode, the pattern consists of printable ASCII characters. You can see the result on the screen. In the synchronous mode, the pattern is made up of scrambled binary 1 and the Throughput Meter will change to a Bit Error Rate Meter. The left reader displays accumulated bit errors while the right reader displays accumulated bits sent.
This test can only be initiated when the modem is off-line.
Local Digital Loopback Test (AT&T3)
This test will loopback the digital form data demodulated from the receiver to the input of the transmitter. During testing, all data
118 Diagnostics & Troubleshooting
