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Switches. Microprocessor ports P1.0 - P1.4 are connected to switches S1
- S5 and are normally high. When a switch is pressed the relevant line is
pulled low and the microprocessor actions a specific task.
Bus Circuitry. The Rotary Powerpilot system is interconnected via a single
line Rx / Tx data bus. The bus control is formed by IC3 and its associated
components. IC3a receives all data being transmitted over the bus and
feeds it to the microprocessor, via the interrupt port INT1, which then deter-
mines if the data is appropriate. IC3b takes data from microprocessor port
P3.7 and transmits it over the bus to the other units. D2 provides protection
against incorrect wiring of other units.
5.3 SHS12 PCB Assembly
Introduction
The SHS12 is the compass for the Powerpilot. Using a fluxgate element it detects the
earth’s magnetic field, calculates the sine and cosine values and passes the informa-
tion to the course computer. The circuit diagram for the PCB is given in Drawing No.
E02777.
Supply and Regulation. The SHS12 is designed to work from a 12 V source.
Protection against incorrect polarity connection is provided by D1 with protection
against over-voltage spikes provided by ZD1. IC1 provides a 5V regulated supply
from which the internal circuitry is powered.
Microprocessor and Reset. Integrated Circuit IC2 is an integrated reset generator
for the microprocessor which produces a reset high pulse of approximately 50mS
duration at switch on and whenever a 5v supply failure occurs. The microprocessor
IC4 either controls or receives data from the remainder of the circuitry as follows:
Bus Circuitry. The Rotary Powerpilot system is interconnected via a single
line Rx / Tx data bus. The bus control is formed by IC3 and its associated
components. IC3b takes data from microprocessor port P3.4 and transmits
it over the bus to the other units. D2 provides protection against incorrect
wiring of the unit.
Fluxgate Compass. Two anti-phase signals are provided by the micro-
processor from ports P1.0 and P1.1 These are buffered by TR1 and TR2 to
provide drive to the excitation coil of the fluxgate. A reference voltage of 1.5V
is provided by R2 and R3 decoupled by C4. The 2 coils of the fluxgate, mount-
ed at right angles, provide output signals proportional to the sine and cosine
of the earth’s magnetic field. These are fed via IC7, to remove the reference
element of the signal, to 2 dual slope integrating Analogue to Digital
Converters (ADC)s IC 6 and IC5 plus associated components. The outputs of
the comparators IC5 are fed to the 2 microprocessor interrupt ports INT1 and
INT2. Accurate timing of the conversion time is kept by the microprocessor
that provides precision analogue to digital conversion of both sine and cosine
simultaneously. Calibration of the circuit to reduce any errors introduced by
component tolerances is achieved via a “pulled cal” initiated by port P0.1.
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