Troubleshooting mpu board on wpc89 machine.
Board don't boot when connected to machine, leds D19 and D21 are solid lit and D20 is out.

On bench (with external power), all 3 leds D19, D20 and D21 are solid lit.
I wonder how the 3 leds should behave when nothing except power is connected to board (on bench)?
Not sure if the board is booting up or not?

On bench, I logic probed address/data lines everywhere (including the ASIC), following the fantastic Sascha V. layout.
Got this:

- "Address CPU" lines A0-A15: A0 is stuck low and A1-A15 are pulsing.
- "Data CPU" lines D0-D7:  All Data lines (D0-D7) are stuck high!
- J202 Data lines D0-D7: All Data lines are stuck high.
- "Switch jmp/dip" Data lines D0-D7: D0 is stuck high, while D1-D7 are stuck low.

Also, the processor at U4 do get a bit warmer than the other chips.

Not sure if these are expected behavior or not?

Found an uploaded clip showing a board operating on the bench, the 3 leds seems to operate the same way as when the board is in the machine.

This means that my board is not booting up (all 3 leds are lit on mine).

Still not sure if the A0 and D0-D7 should be pulsing (keep in mind that the board is on bench)?

What component should be suspected if they are not pulsing?

Thank u Zaza for making my days so much easier
So I am right asuming that A0 should be pulsing and that I should focus on that first (not D0-D7 that are not pulsing either)?

Could there be some other reason than failed ram/rom/asic for A0 not pulsing, like a broken resistor or similar?

If I understand u correct, i should change the cpu/ram/rom/asic one after the other and hope it fixes the problem, there is no way to troubleshoot more exactly what is causing this?

Just a thought, could it be the reset line? Is it possible to ground the reset pin on cpu to trigger a manual reset (need to verify that it is not my computer power supply that is slow at startup, making the watchdog or similar become active on mpu, hence not booting).

And man, MERRY XMAS!!!

Thanks Zaza you are really helpful!

Would you happen to know if there is a way I can manually reset the board?
I read that external power supply may boot slow, making MPU go to watch dog or similar. Someone recomends to ground the reset pin once the external supply is stable, to reset the board. However it was not for wpc MPUs so I am not sure it can be done on these?