7. Circuit Description
Refer to Figure 3, Block Diagram and Figures 4 and 5, Circuit Diagram.
7.1 Power Supplies
A nominal 12V supply enters the unit on P1 or P4. A varistor placed across
the supply and 0V IN (P2 and P5) provides protection against power surges
and voltage spikes.
The 12V is routed as follows:
1. Through reverse protection diode D5 to IC14, a 5V regulator, and ASIC
IC4, as signal VHELD
2. Through diode D4 as signal VUNREG which supplies button illumination
LED drive transistors TR5, TR6, negative voltage generator IC16 via
transistor TR12, buzzer BZ1 and ASIC IC4
3. Through diode D7 to electroluminescent panel driver via TR7 as signal
VEL. This supply and the corresponding return 0V EL use special PCB
tracks which start very close to connector pins P1, P2, P4 and P5
because of high currents when the panel is activated
IC14 provides +5V (Vcc) to the circuitry on the PCB.
Reverse diode D1 ensures a floating 0V for the circuitry by isolating 0V IN.
7.2 External Signals
SeaTalk
SeaTalk signals from P3 or P6 are routed direct to the Autohelm ASIC IC4.
Incoming signals are converted by the ASIC from SeaTalk duplex line
format to Received Data (DATA Rx) format for the microprocessor. Outgo-
ing signals are converted from DATA Tx format at CMOS voltage levels to
SeaTalk format at 12V level by the ASIC.
Connector Signals
Pin Nos. Signal
1 & 4 +12V Nominal 12V dc
2 & 5 0V 0V Screen
3 & 6 SeaTalk Irregular Pulse Trains, nominal 12V amplitude
Z211Z211
Z211Z211
Z211
ST80 Masterview
ST80 Masterview Service Manual 83028-1 13
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