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
![Sad :(](./images/smilies/icon_e_sad.gif)
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.
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.