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Weird boot-loop issue with MV1C

ack said:
The original diag bios only has 16KB of code, but the rom is 128KB. The remaining 116KB of space is filled with 7 copies of the bios. Towards the end of each bios is a byte to indicate which one it is. The first bios (the one that should be running) is 0x00, first copy is 0x01, .. 7th copy is 0x07.

So the error
Code: 
BIOS ADDRESS (A13-A15)

EXPECTED 00
ACTUAL   01

Is saying it expected to be running the first bios (0x00) but instead the first copy (0x01) is running. This would likely indicate A13 on the bios (pin 35) is floating or maybe something is forcing it high. I would bet its just a bad connection between the cpu and NeoBiosMasta.
Click to expand...

I'll see if I can dig out my logic probe and see what the state is on the CPU pin. Am I correct that it (CPU pin 35) should trace to pin 6 on the BIOS?
 
Kujako said:
I'll see if I can dig out my logic probe and see what the state is on the CPU pin. Am I correct that it (CPU pin 35) should trace to pin 6 on the BIOS?
Click to expand...
A13 on the bios is pin 35. Something to consider too is the bios eprom starts at A0 while the CPU starts at A1. So you should expect A13 on the bios (pin 35) to connect to A14 on the cpu (pin 45)
 
ack said:
A13 on the bios is pin 35. Something to consider too is the bios eprom starts at A0 while the CPU starts at A1. So you should expect A13 on the bios (pin 35) to connect to A14 on the cpu (pin 45)
Click to expand...
I traced from the CPU to the old BIOS pads and pin 35 (CPU) leads to pin 6 (BIOS) and get continuity. But I do not see that pin connected on the adapter (looks like the pin is not connected to any via or pad). Out of curiosity I added a bodge wire from pin 6 on the old BIOS pads to pin 6 on the new BIOS EPROM, but that only stops it from posting at all...

I checked logic on pin35 (CPU) and it seems like it's pulsing. But fair chance I'm measuring it wrong (dont use the logic probe often enough). I don't see a high or low indicator, but I do think I hear a clicking when in TTL mode.

Of course reading your comment now, it's clear that I was just confusing myself. Will recheck pin 35 on the BIOS against pin 45 on the CPU.

Edit: Pin 45 on the CPU has continuity to pin 35 on the BIOS and it is definitely LOW according to the logic probe (no ambiguity this time).
 
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Do you need some parts boards? I have a handful of mv1c boards in various repair states if you need one.
 
avramce said:
Do you need some parts boards? I have a handful of mv1c boards in various repair states if you need one.
Click to expand...

I have some, just not sure I want to try swapping a CPU. Removing one without damage is on the cusp of my comfort level. Also not sure it would address the problems.
 
@ack retested with the logic probe, and on power on I do see pin 45 (CPU) / pin 35 (BIOS) flash high before going to lo. Could that be setting the BIOS state, or would it need to remain high to switch banks?
 
Kujako said:
@ack retested with the logic probe, and on power on I do see pin 45 (CPU) / pin 35 (BIOS) flash high before going to lo. Could that be setting the BIOS state, or would it need to remain high to switch banks?
Click to expand...
Thats hard to say cause there are other things using the address bus. I was kinda expecting pin 35 on the bios to just be floating.

The bios address test is the first automatic that test is run (ie before bios crc or any ram tests). You might try holding down A+B+C+D when powering to skip automatic tests and go to directly to the menu system. Then run those tests and see if it detects other issues that might help narrow down the problem.
 
ack said:
Thats hard to say cause there are other things using the address bus. I was kinda expecting pin 35 on the bios to just be floating.

The bios address test is the first automatic that test is run (ie before bios crc or any ram tests). You might try holding down A+B+C+D when powering to skip automatic tests and go to directly to the menu system. Then run those tests and see if it detects other issues that might help narrow down the problem.
Click to expand...

Unfortunately, holding down the A+B+C+D buttons on post still halts at the same BIOS error.
 
ack said:
Sounds like the board might be having other issues. Here is a modified version that will go directly to the menu.

https://www.mvs-scans.com/misc/neogeo-diag-sp1-skip-automatic.bin
Click to expand...

Thanks, will burn it to an eprom when I get the chance (during meetings tomorrow or tonight).

Edit: @ack OK, so burned that version of the diagnostics BIOS to an EPROM and tried it... the output looks like this and the pin 35 state fluctuates between hi and lo. I did confirm that the new BIOS EPROM works on another system.
1671574022307.jpeg
 
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