[Coco] CoCo2 lower graphics

[L. Curtis Boyle]

I did have something similar to this way back with my D board Coco. It had 32K RAM, but using the stacked chip method (16 chips vs the normal 8). Any graphics mode access past the 16K mark produced garbage , anything before was fine (due to the SAM/VDG and how it tries to access the RAM for video).
I don't know if your system is like that, but maybe try swapping the RAM's around and see if the problem changes when it shows up?

Sent from my iPhone

> On Jun 18, 2016, at 4:08 PM, Arthur Flexser <flexser at fiu.edu> wrote:
> 
> Very strange.  This happens both with some Rompack cartridges and some
> programs loaded from disk?  Consistently with the same programs?  Top half
> of the screen looks completely normal?  When you get this, does the program
> otherwise continue to run normally, rather than crashing the CoCo?  If the
> machine crashes, I'd tend to suspect the 6809.
> 
> Art
> 
> 
> 
> On Sat, Jun 18, 2016 at 5:08 PM, Michael Pittsley <mikep at twcny.rr.com>
> wrote:
> 
>> I'm not really sure how to respond to these messages on here.  I hope I
>> have
>> it right.  Anyhow.  Running screen0,1 goto 10 doesn't do much.  From the TV
>> I have the CoCo connected to, it presents an orange line that runs up and
>> down the screen until I press break.   I attempted to attach screen shots;
>> however they proved to be too large to post on here.
>> 
>> 
>> 
>> The CoCo that is having the issue has the XC80652P VDG chip (This chip is
>> said to have been pre-release parts).  I reseated this chip and there was
>> no
>> difference.  I also replaced the VDG chip with a MC6847P chip from another
>> CoCo.  Either these two chips are not compatible or the chip from the spare
>> parts computer is also broken.  Either way replacing the chip with the one
>> I
>> had didn't resolve my issue.
>> 
>> 
>> 
>> Any suggestions will be appreciated.
>> 
>> 
>> 
>> _____________________________________________________
>> 
>> "Michael Pittsley" wrote:
>> 
>>> I have a COCO 2 that when I load certain software or cartridges the
>> 
>>> bottom half the screen is garbled. [...] The garbled graphics is just
>> 
>>> on some graphical games / cartridges.  If I load Tom Mix's Donkey King
>> 
>>> everything works as expected.  Try to run a frogger game or Bridge
>> 
>>> Tutor, I cannot see the bottom half of the screen other than gibberish
>> graphics.
>> 
>> 
>> 
>> Might be worth a photo of the garbled screen.  The games otherwise seem to
>> be running as expected?  Weird failure mode!
>> 
>> 
>> 
>> What happens if you type:
>> 
>> 
>> 
>> 10 SCREEN0,1
>> 
>> 20 GOTO20
>> 
>> RUN
>> 
>> 
>> 
>> ?
>> 
>> 
>> 
>> ..ciaran
>> 
>> 
>> 
>> 
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