Why the CPU LEDs stay on when the program does not run
The hardware turns the LEDs on at power-up and only the running program turns them off, through display PIA IC18. The CPU first fetches its start address from $FFFE/$FFFF in IC17; if it cannot read IC17, the LEDs stay on. The game code then tests RAM IC13, IC16, IC19 and every PIA, and any failure also leaves them on, whereas Leon's code runs even with bad RAM or PIAs.
Systems
Related tips
- A System 7 CPU board can run any System 3 to 7 game
- Run a System 6/7 CPU board on +5 V only
- Which CPU boards can run which system firmware
- Add two test LEDs to a System 7 CPU board
- System 7 diagnostic display stays lit on a working game
- Run the System 3-6 built-in CPU test (SW1)
- System 7 CPU LEDs are swapped versus System 6
- Late System 7 CPU boards lack LEDs and DIP switches
Source: Repairing Williams System 3 to System 7 Pinball 1977 to 1984, Part Two, Clay Harrell (pinrepair.com)