Simrad AP12H Service Manual page 17

SIMRAD
Issue 1.2 18/07/03
5 CIRCUIT DESCRIPTIONS
SCP12 PCB Assembly
Introduction. The SCP12 is the heart of the Powerpilot and all other units are linked to it. It
decodes the information from the heading sender and calculates the heading, acts upon user
commands sent from the control unit, reads the rudder position from the linear feedback unit and
controls the drive unit. All system parameters are stored in Non Volatile Memory (NVM) within the
unit. The circuit diagram for the PCB is given in Drawing No. E02879.
Supply and Regulation. The SCP12 is designed to work from a 12 V source. Protection against
incorrect polarity is provided by F1, D1 and D2. D1 protects the low current internal circuitry and
D2 causes F1 to blow should this occur. Capacitor C3 is used as a reservoir to hold up the supply
voltage and reduce any supply dips. F2 is a 400mA re-setable fuse which provides protection for
the internal circuitry and other units supplied from the SCP12. Protection against over-voltage
spikes is provided Zener Diode and Regulator REG1 provides a 5V regulated supply from which
the internal circuitry is powered.
Microprocessor and Reset. Integrated Circuit IC1 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 IC6 either controls or receives data
from the remainder of the circuitry as follows:
NVM IC10. 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 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 TR12 which is controlled from P2.5 of the microprocessor.
Feedback. The feedback signal from the SLF12 is fed via Ic9d to a dual slope Analogue to Digital
Converter formed by IC5a, IC8a and their associated components. IC9c, which is controlled by
port P2.4, is used to reset the ADC before a reading is taken. Circuit calibration, to reduce errors
introduced by component tolerances, is initiated by port P2.3 of the microprocessor which controls
IC9b. The output of the comparator IC5a is connected to the microprocessor port P1.1. Accurate
timing of the conversion is maintained by the microprocessor which provides precision analogue to
digital conversion of the feedback signal.
Motor Drive. Drive signals are generated by the microprocessor and output from ports P1.6 and
P1.7, the lines being HIGH for no drive and switching to LOW to provide drive. These outputs
control TR3 and TR10 which invert the signals and also provide the current required to switch
transistors TR1, TR2, TR8 and TR9. These transistors control the drive MOSFETs TR4 – TR7
which are connected in an "H Bridge" configuration. Zener Diodes ZD2 – ZD5 limit the gate source
voltage to a safe level on each of the MOSFETs.
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