[Coco] PAL Coco 1/2 schematics/timings

[Ciaran Anscomb]

Mark McDougall wrote:
> Hi,
> 
> Again, apologies if this stuff is readily available but I simply can't find 
> any technical information on the PAL Coco 1/2. :(
> 
> Specifically, I'm looking for timing information - master clock frequencies 
> and clocks for SAM & 6847. I'm unclear as to how they got PAL out of the 
> above-mentioned atm.

I've never seen a PAL CoCo schematic, but here's what I know about
the Dragon:

As the PAL and NTSC line rates are nearly identical, the main clock stays
the same (actually *some* Dragons *slightly* alter this to 14.218MHz,
but not all).  Extra circuitry (including another crystal) is, however,
added to encode the colour information.

PAL has 50 more lines per field, so some simple logic is included that
stops the clock to the VDG at two points during every field and inserts 25
"fake" blank scanlines.

If you read vdg.c in the XRoar source (and get your head around all the
#ifdefs that speed things up for various platforms), you can see this
in action.

Of course, PAL CoCos may well do things in a subtly different way (which
would explain a lot...), but I've no schematic to prove that so if anyone
can come up with something, I'd be just as interested!

..ciaran