I'm back to shamelessly leech on the knowledge of this forum with another PCB problem, hoping that someone around here knows enough about Toaplan hardware to recommend where to begin looking.
This is my third Toaplan PCB with an issue (out of six) - what an unfortunate statistic
I've actually had it for over ten years, and never spent much time with it, so I never noticed the issue before. But it might have been there all along, since it's easy to assume it's meant to behave like this until you run into one of the situations where sprites just block obvious things like the scoreboard on the screenshot below.
Basically, it seems every sprite in the game is displayed over the background layer used for the topmost overlay graphics like score characters and the gameover label.
Going by MAME, only a few sprites are supposed to display on top of these layers, such as the ships flying up on the title screen, and this priority is indicated by a few bits in the sprite memory and an similarly for BG tiles. Doubt it's a RAM problem, as only the bytes controlling priority are affected - Everything else in the game looks and plays completely fine.
The PCB looks nice and clean (except from the single scratch which I've confirmed doesn't break any traces), so at least there's nothing obvious at a glance, that visibly could be causing trouble.
EDIT: Actually the game has one other issue I forgot. The Coin A switch is constantly activated, so you can't coin up the game that way. Coin B still works though, so it's not a terrible issue, just annoying. I'm not measuring any connection for the switch when the board is turned off, so not sure what's causing it. And I doubt it's related to the sprite issue, but who knows.
This is my third Toaplan PCB with an issue (out of six) - what an unfortunate statistic
I've actually had it for over ten years, and never spent much time with it, so I never noticed the issue before. But it might have been there all along, since it's easy to assume it's meant to behave like this until you run into one of the situations where sprites just block obvious things like the scoreboard on the screenshot below.Basically, it seems every sprite in the game is displayed over the background layer used for the topmost overlay graphics like score characters and the gameover label.
Going by MAME, only a few sprites are supposed to display on top of these layers, such as the ships flying up on the title screen, and this priority is indicated by a few bits in the sprite memory and an similarly for BG tiles. Doubt it's a RAM problem, as only the bytes controlling priority are affected - Everything else in the game looks and plays completely fine.
The PCB looks nice and clean (except from the single scratch which I've confirmed doesn't break any traces), so at least there's nothing obvious at a glance, that visibly could be causing trouble.
EDIT: Actually the game has one other issue I forgot. The Coin A switch is constantly activated, so you can't coin up the game that way. Coin B still works though, so it's not a terrible issue, just annoying. I'm not measuring any connection for the switch when the board is turned off, so not sure what's causing it. And I doubt it's related to the sprite issue, but who knows.
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