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

[Darren A]

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


- Darren