A
Apocalypse
Ok so I've replaced the suspicious I/O chip on my dead Mega Play board thanks to @rewrite who provided the chip:
https://www.arcade-projects.com/for...-o-chip-wanted-present-in-some-megadri.8361/&
Unfortunately this changed nothing, still a weird fixed voltage on one of the data pins of the Megadrive area Z80.
However continuing probing revealed the /WR signal of the video RAM for the system VDP (BIOS) was floating. It's coming directly from the nearby 315-5330 custom chip, which it seems you can find in some Master System consoles, but again must be rare since none of my consoles has that chip.
And again I'm not even sure the chip is faulty but I see no reason why the /WR signal of a RAM wouldn't be either high, low or pulsing.
[EDIT]
Of course VRAM has been tested good out of circuit, so not it dragging /WR signal where it shouldn't be, but for the peace of mind I installed a known working one to rule that out of the equation. Still it works fine on an other board.
[EDIT 2]
System Z80 never acknowledge the first VDP interrupt which would be consistent with a VRAM access problem I think.
https://www.arcade-projects.com/for...-o-chip-wanted-present-in-some-megadri.8361/&
Unfortunately this changed nothing, still a weird fixed voltage on one of the data pins of the Megadrive area Z80.
However continuing probing revealed the /WR signal of the video RAM for the system VDP (BIOS) was floating. It's coming directly from the nearby 315-5330 custom chip, which it seems you can find in some Master System consoles, but again must be rare since none of my consoles has that chip.
And again I'm not even sure the chip is faulty but I see no reason why the /WR signal of a RAM wouldn't be either high, low or pulsing.
[EDIT]
Of course VRAM has been tested good out of circuit, so not it dragging /WR signal where it shouldn't be, but for the peace of mind I installed a known working one to rule that out of the equation. Still it works fine on an other board.
[EDIT 2]
System Z80 never acknowledge the first VDP interrupt which would be consistent with a VRAM access problem I think.
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