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22
Front Panel Test Switch
The modem front panel has a five-position rotary switch that lets you
select the modem’s operating mode. The five switch positions are:
c DATA
use this position for normal data transmissions.
c AL
(Analog Loopback)
this position prepares the modem for
testing by the local terminal device. The transmitter output of the
modem is disconnected from the output coupling transformer and
connected to the modem receiver input. See Figure 3-1.
c DL
(Digital Loopback)
this position configures the modem to
loop back received data to the transmitter and transmit the data to
the remote end. The RS-232-C interface to the terminal is not
active during this test. See Figure 3-2.
c ST
(Self Test)
this position inhibits the modem from
transmitting and receiving data on the COMM LINE. The RS-232-
C interface to the terminal is not active during this test. See Figure
3-3.
The test pattern (dot pattern) passes through the transmitter, loops
back into the receiver, is demodulated, and is checked for errors. If
errors occur, the
TM
LED turns OFF and remains OFF for a period
of time that depends on the pattern generator baud rate.
In Analog Loopback and Self Test modes, the received analog data
is buffered, amplified by 16 dB (if output level is set to 0 dB), and
routed out on the transmit analog pair.
c TTP
(Transmit Test Pattern)
this setting forces the transmitter
on and transmits data from the test generator to the COMM LINE.
The RS-232-C interface lines (RTS, CTS, and Transmit Data) to
the transmitter are not active. See Figure 3-4.
The
TM
LED goes ON when the receive data is good. If there are
errors, or if the received data does not compare with the
transmitted data, the
TM
LED goes off. The RS-232-C interface
lines to the receiver are active during this mode.