[Coco] MC6847 VGA (FPGA) Project underway...

Darren A mechacoco at gmail.com
Thu May 26 12:18:22 EDT 2016

On Thu, May 26, 2016 at 7:41 AM, Brendan Donahe wrote:

> I had the understanding that SG6 didn't work right on the CoCo, too,
> however, it looks to me like it can on some systems, and I don't want to
> exclude the Dragon or MC-10.  (Also, I could swear that I saw 6 pixels per
> character block in one of Steve Spiller's tests of SG6 on my CoCo 2.  Can
> someone describe clearly what is broken?  Is it, perhaps, a limited number
> of colors, caused by what pins are tied together on the board?  I obviously
> have not spent much time investigating this yet...)

The problem with SG6 is that all 256 byte values are used to produce
graphics characters in that mode. But because the CoCo, Dragon and MC-10
all have the A/S pin hard-wired to data line D7, you can only use values
128-255, eliminating half of the available colors in each color set. Byte
values 0-127 will give you the External Character ROM mode. Since there is
no external character ROM you end up with vertically striped characters
corresponding to the byte's bit pattern.

- Darren

More information about the Coco mailing list