4-22 Technical Description 8525B/8528 Technical Service Manual
Issue 6 Publication No: 15-02036
4.8.4 Switch Matrix
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
IC3, by taking them to 0V one at a time. (Three of the four lines are taken to
0V via open collector drivers V4, V5 and V6). A switch operation is
detected by the corresponding column connection to the 8-bit I/O port IC5
going to 0V. Data indicating the switch closure is encoded and returned, via
the bus, to the microprocessor.
4.8.5 Microphone Amplifier and Interface (04-02096)
The board used in the extended control head to compensate for the long
lines connecting the head to the transceiver is the same as the one used for
the same purpose in the 8525B series. (Refer to Para 4.7.4).
4.8.6 8532 Display Head
The Type 8532 Extended Control Head has a smaller frontal area and depth
(180mm w x 62mm h x 45mm d) than the Type 8531, making it more
suitable for mounting in confined spaces. It has all the features of the 8531
except the loud speaker is external and the control panel is back illuminated
for use at night.
Overlays on the front panel of the control head contain sealed tactile
membrane switches. A liquid crystal display (LCD) and light emitting diode
(LED) indicators on a board behind the panel show through the appropriate
part of the overlay when activated. Circuits on the board interface with the
microprocessor in the transceiver to drive the displays and indicators, and to
decode the operations of the front panel switches.
Surface mounted components are used in the display PCB. For removal and
replacement of these components it is essential to use tools specifically
designed for the purpose. Repairs to the board should not be attempted
unless the appropriate tools are available.
A single PCB, Display PCB (Assembly 08-04666), is used in the Extended
Control Head (Refer to circuit Diagram 04-02875).
The LCD, H14, has twenty digits and two enunciators which allow it to
display the channel number and the transmit and receive frequencies of the
currently selected channel. In addition it can display the identification
number of a called or calling selective call station. The LCD is driven by
IC5. The display data is applied to the driver via an I
2
C bus. The drive mode
is 1:4 multiplexed with 1/3 bias. The driver is powered by a separate supply
derived from the A rail by diode D7 and zener diode V7.
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