[Coco] VDG weirdness

[Zippster]

Hi Phill,

Any schematics?

I don’t know what all is on the board, but is it adequately decoupled?

- Ed


> On Oct 12, 2015, at 8:39 AM, Phill Harvey-Smith <afra at ramoth.org.uk> wrote:
> 
> 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.
> 
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