Re: Red Alert help needed (6502 CPU question too)

Franklin Bowen wrote:

> At 01:35 AM 5/3/2004, Rodger Boots wrote:
>
>> Franklin Bowen wrote:
>>
>>> I posted something similar on RGVAC and got no response so I though
>>> I would try it here.
>>>
>>> The voltages on the board are perfect. The thing I can't figure out
>>> is why the CPU address lines are all low! I have placed my logic
>>> probe on them at power up and I never notice the high light activate
>>> or the tone change. But maybe it happens too quickly for it to be
>>> noticeable? I swapped the CPU with another (neither verified good)
>>> and got the same results. Stable picture of garbage with no
>>> noticeable activity.
>>>
>>> So here is what is happening at the CPU:
>>> /RESET starts low, then stays high
>>> Clock in (pin 37) is 733.2 KHz. Supposed to be 1MHz according to MAME.
>>
>>
>>
>> MAME is wrong, probably part of why MAME says emulation is incorrect.
>>
>> Crystal is 12.5 MHz divided by 16 for the processor.
>>
>>> Clock outs both 733.2 KHz.
>>> A15-A0 are all stuck low.
>>> D7-D0 all pulse.
>>> R/_W is high
>>> SYNC is low
>>> /NMI is high
>>> /IRQ is low (I pulled the driving chip and tied the signal high. No
>>> change)
>>> RDY is low
>>
>>
>>
>> Ignore my previous answer. The problem is still the RDY line being
>> low, but although I can't find it on any datasheet it looks like the
>> RDY line may have an internal pull-up. In any case this circuit
>> violates every rule about using the RDY line.
>>
>> Try putting a 1 K resistor from the RDY line (pin 2) to +5 volts (pin
>> 8). The RDY line should then be high and the game may run then
>> unless the 6502 was bad in the first place.
>>
>> It looks like the line gets switched by the coin switch. This
>> violates all DC and timing specs for the RDY input.
>
>
>
> The crystal on my board is different, 11.7?, I think. I'll verify it
> later.
>
> You were right! I tied RDY high and the board displayed something ...
> and then locked up! I'll see if I can make more progress tonight or
> tomorrow. I have the RA coin switch tied to JAMMA coin 1 so I wonder
> if RA needs the signal inverted. The only reason I can think of tying
> the coin switch to RDY is if they were missing coin drops? Hmmmm ...
> maybe not. The code to catch the coin signal would still need to
> execute and I don't see how pausing the processor would help that.
>
> The /IRQ line driver baffled me. It's default is low and the only
> time it goes high is if the game is running properly and /C070 gets
> strobed! I still don't get that.

/IRQ gets driven low at vertical sync. That triggers the interrupt to
update all the video information. The interrupt routine would then do a
read to C070 which resets /IRQ high and retrieves stored data from IC 8A
(unused?).

/IRQ would normally be high with low pulses if the program were running.

I don't think your coin switch level was a problem, /NMI would have been
low if the coin switch or service switch were activated.

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Received on Mon May 3 12:36:11 2004