Service manual

CIRCUIT DESCRIPTION
4-11
November 2001
Part No. 001-7240-001
transmit data signal on pin 2 is selected is routed to the
filter, and when it is high the receive data signal on
pin 1 is routed to the filter.
U142B, U142A, and U143A form a 1800 Hz
low-pass filter which attenuates unwanted frequencies
above the SMARTNET/SmartZone data band. The
output of this filter on U143A, pin 1 is fed to
U144A/B which provide DC restoration when data is
being received. The signal is also fed to gate U141C in
the transmit data circuit to be transmitted when appli-
cable.
The DC restoration circuit formed by U144A/B
and U143B is similar in function to the receive data
circuit described in Section 4.7.1. It converts the data
signal from AC floating at half supply to DC levels of
0 and 5 volts that can be detected by the micropro-
cessor. Diodes D141 and D142 charge and discharge
C148 and C150 to establish a DC reference on pin 6 of
comparator U143B that is the average of the positive
and negative going alternations. Q141 turns on in the
transmit mode which grounds pin 6 and disables this
circuit.
In the transmit mode, gate U141C selects either
the SMARTNET data signal or the LTR/Call Guard
data signal. It is controlled by the same signal that
controls U141B. The output signal on pin 4 is then
applied to potentiometer 1 in U147. Refer to Section
4.7.2 for more information on the rest of this circuit.