4-24 Technical Description 8525B/8528 Technical Service Manual
Issue 6 Publication No: 15-02036
Switches on the panel overlay (substrate) form a 4 x 6 matrix. The row and
column connections are taken to the Display PCB. (A table listing the switch
functions and the corresponding connections is given in Section 6.2.3.) In
the Display PCB the row connections are polled by the microprocessor, via
IC2, taking them to 0V one at a time. (Three of the four lines are taken to
0V via open collector drivers V2, V3 and V4.) A switch operation is
detected by the corresponding column connection to the 8-bit I/O port, IC3,
going to 0V. Data indicating the switch closure is encoded and returned, via
the bus, to the microprocessor.
To compensate for the long lines connecting the Control Head to the
transceiver, on entering the board each line is filtered to prevent interference
picked up on the lines from affecting the operation of the head. The Clock
line is also buffered by a small amplifier formed by V8 and V9.
The A rail from the transceiver is used by various circuits on the board and
is also applied to a regulator, IC6, to provide the +5V supply for other
circuits within the board.
An amplifier formed by IC4b and its associated components amplifies the
microphone input to a suitable level for application to the lines. V5 on the
input to the amplifier operates as a clamp in the same way as V7 on the
Audio and IF 1650kHz PCB. This circuit is powered by a separate supply
derived from the A rail by zener diode V6.
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