Page 34 - WilliamsMidway WPC 91 Schematics 16-9289

CPU Board Operations
The CPU Board performs two ma in operations , logic control and switch control.
Microp~r (U4)
WPC uses a 68B09E microprocessor to control and process data . Using an oscilloscop e,the address
and data lines should be square waves , at least 4 vol ts peak- to-peak
The processor runs on a 2Mhz clock supplied by pins 81 and 82 of the ASIC. Pins 34 and 35 of the
processor should be square waves, at least 5V peak-to-peak , Reset (pin 37), IRQ (pin 3), and
RIW (pin32) should also be at least 4V peak-to-peak during normal operation.
ROM(U6)
WPC uses one, 1M or 2M ROM to contain the game program . The processor looks to the ROM for
instructions when handling data. Using an oscilloscope the address and data lines should have
4V peak-to-peak square waves.
RAM (U8)
WPC uses a 2064 CMOS RAM which stores game specific information such as, scores , credits ,
custom adjustment settings, etc., in temporary memory. There is a battery back-up circuit for this
chip. Pins 26 and 28 are connected to VCC and the cathode s of D1 and D2 , which are part of the
battery circuit. When the game is On, Pins 26 and 28 should have +5V peak . When the gam e is
Off, pins 26 and 28 should have at least +4V peak . Ifthe voltage on pins 26 and 28 drops below +4V,
memory reset occurs.
ASIC (U9)
ASIC stands for Application Specific Integrated Circuit. This chip handles address decoding
(except widgets) , system timing, provides a real clock time, and system sequencing. Using an
oscilloscope, the address and data lines should be at least 4V peak-to-peak . The othe r pins on this
chip should have either a solid high or a solid low pulse, nothing should be floating. Although this
chip is not game specific ,it is specific to WPC.
The ASIC provides two clocks, a real time clock and a system timing clock. The real time clock ,
which inputs to pin 70 of the ASIC, operates on a 32 Khz crystal. The system timing clock ,which
inputs to pin 55 and 56 of the ASIC , operates on a 8Mhz crystal.
The blanking circuit is monitored by the ASIC . If the microprocessor is not functio ning properly ,
blanking is asserted and no output function is allowed. During the time blanking is active the
reset line toggles to try to bring the game back up. Blanking is active during power-up until the
microprocessor is running , and has reset the latches to the normal operating mode. This is to
prevent coils or motors from energizing when the game is turned On . Once the microproce ssor
has reset the latches blanking becomes +5V level.
The diagnostic LEDs are control1ed by the ASIC. During normal operation, D19 (blanking) is Off,
D20 (diagnostics) is flashing, and D21 (power) is On. If the ASIC detects a logic circuit failure D20
flashes an error code: One flash = U6 , two flashes = U8 , three flashes = U9.
Miscellaneous Buffers and Latches (UI, U2, us, U5, U7, U12, U21)
These chips are used as a temporary memory storage for the microprocessor and for interfacing to
the 'outside' world. The address and data lines should be at least 4V peak-to-peak. Any address or
data lines that is not pulsing should have a solid high or a solid low ,nothing floating.
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