Issue 1.1 18/07/03 SIMRAD
17
5 CIRCUIT DESCRIPTIONS
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 calculates 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 supplied 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 parameters
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 single line Rx / Tx
data bus. The bus control is formed by IC2 and its associated components. IC2a receives
all data being transmitted over the bus and feeds it to the microprocessor, via the interrupt
port INT1, which then determines 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 90
0
to each other are triggered by rotating magnets mounted on a shaft driven
from the gearbox to produce quadrature style feedback.
Motor Drive. Drive signals are generated by the microprocessor and output from ports P1.6
and P1.7, the lines being LOW for no drive and switching to HIGH to provide drive. These
outputs control IC9 and IC10 used as comparators set at threshold levels of +4V and +1V
produced by the resistor network R43, R44 and R45. The signals are inverted and provide
current to switch the MOSFETS in stages and hence prevent both P an N channel
MOSFETS on the same side of the “H – Bridge” configuration, (TR10 and TR11 or TR12
and TR13) being partially switched on at the same time.
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