With the test EPROM back in, the CPU runs but no PIA output changes.

Program runs but the PIAs never respond

Cause: A data line is shorted or the data buffer at 5E has failed.

Checks

• Measure CPU data pins 26 to 33: each should switch and read 2-3 V.
• Check the same lines after buffer 5E.
• Bend up the buffer's output pin: a signal on the lifted pin means a short further on; no signal means the buffer is bad.

Fix

• Replace the buffer, or cut the line in sections to find the chip causing the short.

Systems

• Data East CPU Version 1/2
• Data East CPU Version 3
• Data East/Sega CPU Version 3b

Related tips

• Test program does not run at all
• Checking PIA outputs with Leon's test EPROM
• CPU EPROM jumpers J4 and J5
• Two PIA exceptions when using the PIA test EPROM
• One PIA output does not toggle under the test EPROM
• CPU board PIA positions and jobs
• CPU jumpers J4 and J5 set the EPROM size at 5C
• Data East CPU diagnostic test EPROM

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