Quoted from oldschoolbob:The PB's are switch returns. There's no switches connected. Could the wave forms be from the PIA calling for data?
Bob
I think you are seeing the U10 PA5-PA7 and CB2 port come back through closed dip switches. The CPU reads the dip switches by sending strobes through those PA ports and then reading back the PB ports just like it does when it scans the switch matrix. The return PB ports are in common to both dip switches and normal switch returns, but different PA ports (and CB2) are used for the strobes. PA0-PA4 is normal switch matrix strobes and PA5-PA7+CB2 are the dip switch strobes.
You will also notice U10 PA ports also handle the lamp driver board and display latching (with the help of u20). So when the dip switch is closed and the PIA is controlling the lamp board and displays those PB will show data like you see. The CPU does not care about those PB ports at this point, so it is just ignored. U10 is very busy in Bally games. Has a hand in lamps, switches, and displays.
If you read the theory of operation PDF it talks more about the interrupt routines, switch matrix, and dip switches.
