The display shows wrong screens or command call errors such as "command 9c" even with matched ROM versions.

Odd DMD screens or "command 9c" errors with matching ROMs

Cause: A damaged PIA at 9B on the CPU board is sending corrupt commands to the display.

Checks

• Check the ribbon cable, then test or swap the 9B PIA.

Fix

• Replace the 6821 at 9B.

Systems

• Data East CPU Version 3
• Data East/Sega CPU Version 3b

Related tips

• Whole switch column dead after working under the playfield
• How a coil or flasher is switched: PIA to TTL to pre-driver to driver
• CPU board PIA positions and jobs
• A coil or flasher comes on and stays on at power-up
• Which logic chips drive which solenoid transistor pairs
• Test EPROM flash codes: which chip failed
• One PIA output does not toggle under the test EPROM
• Lamp column still dead after a new TIP42

Source: Repairing DataEast 1987 (Laser War) to Sega 1995 (Batman Forever), Part Three, Clay Harrell (pinrepair.com)