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Truxton Fault

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xodaraP

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Hi guys,

Looking at a Truxton board at the moment and it's not booting. /HALT and /RESET are both high, so the board itself is running but the CPU is waiting for data that isn't arriving. There's no schematics for it, but I'm starting with the usual method of ROMs -> RAM -> logic.

So far:

-CPU has clock
-/RESET and /HALT are high, which would generally mean the board is trying to run code
-No chips are hot (unlikely to be a board short)

Will update as I work through it. If anyone has repaired one of these boards before I'd be interested to know which RAMs are responsible for which parts of the board. There's several different types of RAM dotted around the place and since pretty much everything is connected through the custom ceramic IC, it makes it difficult to work out what's associated with what.
 
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Is the CPU receiving an initial reset from the watchdog to get code running after initialization? Try pulling reset low once to see if that jump-starts it.
 
I'll probe it while I power cycle it and see - I seem to remember doing that last night but I don't remember if it actually did.

Edit: Watchdog kick is definitely there, momentary low before going high. I even connected up a ground probe and forced it low and it sprung back high, so it's trying to boot something.

The only thing that strikes me as a bit odd is the RAM between the CPU and ROM 11 (maincpu code) has a floating pin 1 - the datasheet shows this as NC (not connected) but every other one of these RAMs on the board has this connected either to ground or Vcc - even the blank spots where there would be RAM installed for other games have the pin 1 connected to Vcc
 
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May as well run a jumper wire from Pin 1 to Vcc so it matches the others and see what happens. It shouldn't hurt anything to try it.
 
I checked my other Truxton board (also faulty - fairly bad corrosion on one of the customs sadly) and it doesn't have Pin 1 connected to Vcc either so that must be correct.

I'll start by looking at the RAM and the logic driving the RAM right next to the CPU since that seems most likely to be an issue.
 
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