8525B/8528 Technical Service Manual Technical Description 4-15
Publication No: 15-02036 Issue 6
The clock and data lines are also taken to connector P1 for application to I/O
ports in the Selective Calling PCB when this is fitted. Provision is made for
all four lines of the bus to be connected via P3 to further I/O IC’s if required.
4.5.3 External Bus
A second four-line bus is provided for connection to 8-bit I/O IC’s and
16-bit open-drain output IC’s on the Display PCB of the front panel and
Extended Control Head. All four lines are buffered in IC1 to allow for the
longer lines to the Extended Control Head and to protect the
microcontroller. Since the buffers can carry data in only one direction, data
read from the control panels is buffered in IC1 and returned to a separate
input of the microcontroller.
To allow control of the transceiver from both the front panel and the
extended control head 8530/1, the 8-bit I/O ports at these locations must
have different addresses. This is arranged by a link on each Display PCB
which is set for extended (E) or direct (D) control, thus allowing boards of
the same design to be used in both locations.
The interrupt line from the Display PCB is ORed with the internal bus
interrupt by diode D5 and is also taken via two buffers in IC1 to a separate
input of the microcontroller. By this means, the microcontroller can detect
whether an interrupt has come from the internal or external bus.
4.5.4 Reset and Watchdog Circuit
The microcontroller is reset by a +5V level on pin 9. This may be applied in
three ways: by applying power to the unit, by the low voltage detection
circuit or by the watchdog circuit.
When power is switched on, the reset input is held high, thus resetting the
microcontroller ready for initial operation.
If the supply voltage (A rail) falls below 8 volts, IC7 output goes low. This
switches on V3 which holds the microcontroller in the reset condition until
the supply voltage is restored.
Schmitt Trigger IC8a, together with R9 and C4, form an oscillator with a
period of approximately 600ms. However during normal operation, the
microcontroller outputs a positive pulse from pin 14 at regular intervals of
less than 600ms. Each pulse switches on V1 which discharges C4 thus
preventing the oscillator from starting. Should the microcontroller operation
fail, or be affected by abnormal inputs, the pulses cease thus allowing the
oscillator to start. Each negative excursion of the oscillator output will
switch on V3, resetting the microcontroller. The reset will therefore be
applied at regular intervals until normal operation is resumed.
4.5.5 RS-232 Port (ALE Option Only)
On 04-03031 there is shown an RS-232 I/O port. V5 is used for the O/P
driver and level translator. V6 is used for input level translation.
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