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From:	"Matthew Smith" <matt AT the-good-stuff DOT freeserve DOT co DOT uk>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: TSR to run 60Hz CGA at 50Hz
Date:	Mon, 11 Feb 2002 07:38:38 -0000
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"noLCDpanels" <noLCDpanels AT anywhere DOT com> wrote in message
news:3C66FEB7 DOT 396324D0 AT anywhere DOT com...
> > No this post hasn't been lost for 10 years.  I have an 8-bit CGA
with
> > composite out that I wish to use for debugging with RHIDE/DJGPP.
I
> > have fitted a b/w modulator to the card but the signal is 60Hz and
my
> > UK TV can't handle this.  Does anyone have a TSR or something
which
> > will set the output to 50Hz and keep it there ?
>
> Is that a *real* CGA display adapter?  If so, you might be out of
luck.
> The display-adapters of this era (early 1980s) had a fixed set of
> clock circuits, to produce all dot clocks.  If the card happens to
> support a 50Hz scanrate, you might be able to set it through the CGA
> BIOS.  If not, you'd probably have to desolder one of the CGA
board's
> clock generators, and replace it with a different value (X * 5/6)

The dot clock is fixed (well, on this card it is switchable to MDA
mode too) but the horizontal and and vertical sync are generated by
the 6845 chip according to the values set in the registers. These are
very like the VGA registers but at a different port address.   The dot
clock for 60Hz video is very similar to 50Hz video, as the horizontal
rates are 15360Hz and 15625Hz.  This meagrly causes the picture to
shifted off-centre and is not a serious problem.   I can probably
write a simple TSR to switch the mode myself, but I'm looking for a
robust TSR which can survive all the mode switching that Windows,
RHIDE, and a DJGPP/Allegro program will do.  I suspect the best answer
is to find an MDA monitor from somewhere, or get an eprom programmer
and fix the CGA bios, but I have a TV sitting here doing nothing
already.

I actually have a dual-head Matrox G400 installed too.  I bought that
specifically for dual-screen debugging but was disappointed to find
the drivers do not emulate CGA/MDA on the second output.  Matrox are
not interested in this dwindling market :(

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