[Coco] Semigraphics24 and the coco3fpga

[Robert Gault]

I don't have one of these boards but was recently asked if my 
Semigraphics24 sketching program would work with one. There are two 
parts to this question and I'm happy to say that the S24 mode works 
perfectly; better than could be expected.

With a real Coco3, the only way to get S24 partially working is to 
POKE&HFF9C,10. That will get you graphics but not text. There are 
however problems with color rendition.
With a CMP monitor, the colors will never be exactly correctly. The 
reason, as seen in the SN74LS783/MC6883 data sheet, is that three bits 
are used to select colors. That is not enough to get accurate phase 
shift for CMP or RGB levels with an RGB monitor. Clearly these bit just 
pick a color from an internal table which the MC6883 then sends to video 
out.
We know that the Coco3 GIME does not give an exact match between CMP and 
RGB colors.

Now as it happens, the Coco3 RGB colors are a better match to the S24 
colors than the CMP colors. Also the coco3fpga board mistakenly (a perk) 
displays text on the S24 screen as do the emulators MESS and VCC. So to 
run my program just edit line 10 to insert RGB.
10 RGB:etc.
Apparently it is not necessary to reset $FF9C either but that could be 
added to the Basic loader as well.

Robert