[Coco] VDG weirdness

[Phill Harvey-Smith]

Also posted this to dragon archive site, so you may have seen this 
before.....

Hi all,

Now that I have the Acorn Atom Colour board working correctly I thought 
I'd try and adapt it to work on the Dragon and CoCo, as they all use the 
6847.

I have adapted the circuit so that only the video output section is used 
as the RAM is handled on the Dragon motherboard.

However I'm getting a strange problem in pmode 4, namely that it only 
displays the left hand half of the screen for some reason :(

I wrote this little program to verify this :

10 pmode 4,1;screen 1,0:pcls
20 for a=0 to 15
30 poke &h600+a,&haa
30 next a
40 for a=0 to 191
50 poke &h600+(a*32),&haa
60 poke &h600+(a*32)+15,&h55
70 next a
999 goto 999

This draws a line of dots all the way across the screen and down both 
edges, and doesn't have any line breaks, so the SAM at least is feeding 
it 32 bytes / line, but the VDG is only displaying the first 16.

Has anyone seen this sort of behavior before ?

If I take my board out and just use the normal video out I get the 
expected results, that the lines are only on the left half of the 
screen. All the other modes seem to work fine.

Switching to pmode 3 in the first line of the above code plots a blue 
line halfway across the screen, a blue column on the left hand edge and 
a yellow down the middle.

Couple of things I did notice, in pmode 4 DA0 seems to be doing long 
cycles (8 vid clocks), where it should do short cycles. Also the voltage 
levels of GM0, 1, 2 seem to only be 4.4V, I've tried swapping the PIA 
but this remains the same. My board wires these direct from the socket 
to the 6847 by a very short track so I can't account for this.

Cheers.

Phill.