[Techtoolslist] Another midway 8080 and pod (space invaders deluxe) question

[John Robertson]

On 06/28/2014 11:54 PM, David Shoemaker wrote:
> Have a space invaders deluxe board.
>
>   
>
> Rom tests based on quarterarcade fluke crc's are fine on loop.  Game does
> not run correctly.
>
>   
>
> Ram short fails all over the place.
>
>   
>
> So trying to narrow it down a touch I do a write 0x2000 = 0xff then a read
> 0x2000.
>
>   
>
> Comes back with 0xFF.  Hit loop and it starts to wander all over creation.
>
>   
>
> Thinking I have a bad bit I do some testing.
>
> Write 0x01, read & loop is stable=0x01.
>
> Same with 0x02, 04, 08, 10, 20, etc etc.  In fact everything I tried came
> back with a good read loop EXCEPT 0xFF.  With that value I get random
> garbage.
>
>   
>
> WTF?????
>
>   
>
> Any ideas on why ram would fail this way?
>
>   
>
> Thanks,
>
> David
>
>   

Hi David,

How about the 'ol AA/55h test?

This is similar to my problem with my 8080 pod having trouble resolving 
FF on a number of ROM locations - read them as other characters - 
alternating. Do you get the same read value if you loop the rear for 
that memory address?

John :-#)#
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