System 4 CPU board changes and ROM layout

System 4 kept the 6800 and its MC6875 clock chip but improved the reset circuit and moved settings to the coin door switches (the DIP switches remain for System 3 use). Game code is either two 512-byte PROMs at IC21/IC22 or one 2716 at the new IC14 socket, with the flipper ROMs at IC17/IC20 and a sixth socket at IC26. A "System 4-" board has pads but no sockets at IC14 and IC26.

Systems

• Williams System 4

Related tips

• Identifying a System 3 to 7 CPU board
• Which System 3 to 7 games use which flipper ROM system
• A System 7 CPU board can run any System 3 to 7 game
• Which CPU boards can run which system firmware
• Obsolete chips on System 3 to 7 CPU boards
• How a good System 3 to 7 CPU behaves at power-on
• Flipper ROM color codes by system
• CPU DIP switches: used by System 3, ignored from System 4 on

Source: Repairing Williams System 3 to System 7 Pinball 1977 to 1984, Part One, Clay Harrell (pinrepair.com)