How the switch matrix reads a switch
The matrix runs on +12 V. The CPU pulls one column low at a time through the ULN2803 at U20; a closed switch pulls its row low, and an LM339 comparator on that row reports it. A switch counts as closed when its row and column are both low together.
Systems
Related tips
- How the Williams switch matrix is scanned
- WPC switch matrix basics
- Switch matrix chips on the CPU board
- Checking the switch matrix with a logic probe
- Reading WPC switch numbers and the switch test display
- Narrowing a switch matrix fault to the CPU board, wiring or opto boards
- Using the switch edge test
- A whole switch column dead or all closed
Source: Repairing Williams/Bally WPC Pinball Games from 1990 to 1999, Part One, Clay Harrell (pinrepair.com)