22
Issue 1.0 16/12/99
5 CIRCUIT DESCRIPTIONS
5.1 SRD12 PCB Assembly
Introduction. The SRD12 is the heart of the Rotary Powerpilot and all other units are
linked to the SRD12. It decodes the information from the heading sender and calcu-
lates the heading, acts upon user commands sent from the control unit, detects drive
gearbox clutch engagement, reads the Hall Effect quadrature feedback and controls
the gearbox motor drive. All system parameters are stored in Non Volatile Memory
(NVM) within the unit. The circuit diagram for the PCB is given in Drawing No.
E03318.
Supply and Regulation. The SRD12 is designed to work from a 12 V source.
Protection against incorrect polarity is provided by D1 and D2. D1 protects the high
current internal circuitry and D1 the low. Capacitors C1, C3 and C43 are used as
reservoirs to hold up the supply voltage and reduce any supply dips. F1 is a 400mA
re-setable fuse which provides protection for the internal circuitry and other units sup-
plied from the SRD12. Protection against over-voltage spikes is provided Zener
Diode ZD1 and Transient Voltage Suppresser TVS1 and Regulator REG1 provides a
5V regulated supply from which the internal circuitry is powered.
Microprocessor and Reset. Integrated Circuit IC4 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
IC1 either controls or receives data from the remainder of the circuitry as follows:
NVM IC5. Provides the memory for the unit and stores all system parame-
ters
NMEA Reception. The NMEA reception is opto-isolated by IC3 and fed to
Port P3.0 of the built in microprocessor UART.
BUS Circuitry. The Rotary Powerpilot system is interconnected via a sin-
gle line Rx / Tx data bus. The bus control is formed by IC2 and its associ-
ated components. IC2a 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. IC2b takes data from microprocessor port
P3.4 and transmits it over the bus to the other units. D3 provides protection
against incorrect wiring of other units.
Error Indication. The occurrence and nature of system errors is indicated
by a sequence of flashes on LED1. The LED is switched from TR9 which is
controlled from P2.5 of the microprocessor.
Feedback. Mechanical movement is sensed by 2 Hall Effect devices IC6
and IC7 which output to microprocessor ports P2.0 and P2.1 at logic levels.
The devices which are situated at 900 to each other are triggered by rotat-
ing magnets mounted on a shaft driven from the gearbox to produce quad-
rature style feedback.
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