How the first-flash ROM test works

The CPU adds up every byte in each fitted ROM, in the order the jumpers define, and expects each total to come to zero; each 512-byte block has a byte set aside to make that happen. A single bad bit in any ROM stops the boot at this point.

Systems

• Bally MPU AS-2518-17
• Bally MPU AS-2518-35
• Stern MPU-100
• Stern MPU-200

Related tips

• After the test EPROM: refit the game ROM and count flashes
• Why the MPU has ROM jumpers
• Masked ROM numbers: keep the set matched
• ROM and EPROM types used on Bally MPUs
• Get the MPU booting before you change any ROM jumpers
• Strip the MPU to three chips to get the first flicker
• 9332 masked ROMs can be replaced by 2532 EPROMs as-is
• Two flashes only: U8 5101 RAM

Source: Repairing Bally Electronic Pinball Games from 1977 to 1985, Part Two, Clay Harrell (pinrepair.com)