Loopback Tests - ZyXEL Communications U-336S User Manual

failure was due to a temporary power-on reset problem and will not
affect modem operation.

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. The
following Bit Error Rate Meter example shows 11120 Kbits sent,
22 bits in error. When the number exceeds 99999 Kbits, it will reset
itself back to zero.
130 Diagnostics & Troubleshooting