Re: Joust CPU problem

I have seen this kind of problem on other boards and it usually comes down
to a 74ls161 that is not counting all the way before reset or has a stuck
line. Basically your vertical refresh clock. I am guessing (no schems
handy) that the cpu starts to read out video memory to screen and 1/2
through the refresh it looses the most significant bit and starts again from
address 0.

David
----- Original Message -----
From: "Kurt Mahan" <kmahan@xmission.com>
To: <rasterlist@synthcom.com>
Sent: Monday, October 15, 2001 8:30 AM
Subject: Re: RASTER: Joust CPU problem

> Is this a screen that is supposed to have upper islands?
>
> My guess (just a guess) would be that your vid ram address decode
> (chip selects) are hosed. I'd probe around there with a scope and
> see if you see multiple chip selects happening at once..
>
> Kurt
>
> >
> > My joust is now doing this http://www.basementarcade.com/doubejoust.JPG
> > The bottom half of the screen is repeated on the top!! The test screen
> > passes all the rom, cmos and only gives a error on one screen prior to
> > the color bar test that states something like "vertical bar error" or
> > something like that. Any ideas as I have exhausted all the William's
> > tech pages I could hunt down.
> >
> > -=Mark=-
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Received on Mon Oct 15 09:57:38 2001