Reading switches is the easiest part of this process. The switches are organized into banks of 8, and each bank is energized by a different switch strobe. The lines U10A:PA0-PA4 are the strobes and the return values are read on U10B:PB0-PB7. A…

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The M6800 implementation of solenoids uses the 120Hz ISR to control the solenoids. They did this for several reasons: Time out the duration that solenoids are kept on. According to the Bally Theory of Operation document most solenoids need to be kept…

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Compared to the displays, writing to the lamps is fairly easy. Well, that's not exactly true. It's fairly easy once you understand how they're using the MC14514 chip, which actually took me a little while to figure out. My first attempts weren't…

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The games I've worked on have five 6-digit displays. There are four player displays and one for credits/ball-in-play. These displays only show one digit at a time, but they flip so quickly between them that it looks like all six are on…

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PIA Chips – 6820

The 6820 PIA (Peripheral Interface Adapter) chips are how the M6800 talks to the outside world. They're used to either latch data to send to the lamps, display, or solenoids, or they're used to grab input data and hold it for the…

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Overview of MPU

The Stern MPU-100 is identical to the Bally AS-2518-17 board. At least I think it is. I've worked on both, I can't find any difference, and I've read that plugging the ROMs from one into the other works just fine in a…

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