[Coco] On John Linnville's Color Computer Game Master Cartridge Design

[John W. Linville]

On Wed, Aug 02, 2017 at 12:53:24PM -0600, Darren A wrote:
> On Wed, Aug 2, 2017 at 12:23 PM, RETRO Innovations wrote:
> 
> >
> > And, after 3 nights working (fighting) with this IC, I think people should
> > give more credit to John for his design.  He didn't just wire up this IC to
> > the Coco expansion bus, he performed some magic to successfully map it into
> > the Coco address range.
> >
> > The SN76489 takes 32 4MHz cycles to successfully store a value in a
> > register, which is 8uS, though that might be worst case, as I see 3uS
> > typical.  In any event, this need to hold the data valid for so long makes
> > a simple connection to the Coco bus more complex (essentially, unless one
> > wants to HALT the CPU while the write completes, it appears one needs to
> > store the data in a faster "buffer register" and then hold the bus signals
> > to the IC until the IC signals the write is complete (there is a "READY"
> > pin)).
> >
> >
> 
> I have been going through the same exercise recently.  Before trying to
> write some HDL for a CPLD implementation, I rigged up the following circuit
> on a breadboard for testing:
> 
> https://drive.google.com/file/d/0B_o8KocZC3DSenp4em0xWFloOVk/view?usp=sharing

Does this design work? At first glance, it doesn't seem right to me.

John
-- 
John W. Linville		Someday the world will need a hero, and you
linville at tuxdriver.com			might be all we have.  Be ready.