Re: How does the apple2 RGB card work?

swtownsend said:
Hello,

I have a spare RGB monitor card for the IIe. If you want one to
take apart and study, or just to put in your IIe and use, let me know.
swtownsend(at)hotmail.com

Yes. Yes. Yes.
For both study and use. I'll send you an e-mail.
I've just been figuring out how to use gschem (So much neater than
hand-drawn). I'd have no objections to doing a simple circuit trace and
schematic for those who are interested.
I'm going to be doing the same on a C64 Viatel adaptor I have in the
near future too.

Unlike Mr. Parkoff, unfortunately I don't have things like a CRO, but I
can usually figure out the ins and outs of a circuit fairly quickly. I
am very interested to see the results of his research though, because
like I said, I don't have access to the equipment.

Having a //e platinum can make ascertaining the 'big picture' a little
difficult at times too because of the custom ICs.

Bryan Parkoff said:
Right now, he and I are working together to develop schematic. After
the schematic is developed, he may be able to design aux controller card for
VGA monitor, but it is almost impossible to deal between 15.734K and 30K
horizontial freq. DMA may be needed.

Speaking from a nonspecialised viewpoint, I can't understand why these
may be problematic.
If you are talking about directly pumping data to the monitor, then
yes, it is a problem. However, if all you want to do is perform an RGB
encoded version of what is being displayed via the video out, it is a
little simpler.
CGA and EGA monitors run at ~15Khz horizontal refresh, as does the
video out.
What I am uncertain of is whether the memory reads for the video
display are opaque to the expansion bus.
If not, then listening to the reads performed should be sufficient to
create the basis of a fully mirroring RGB device.
The sync signals are another matter which I am unable to address
currently.

If the card were to cache the writes done to the video area of memory,
it would render the card independent of the bus and allow custom
refresh rates.

Recently I have been considering the merits of interfacing an 8-bit VGA
ISA bus card to other architectures. So far I have not come up with any
reasons why it would not be possible.
There is an upside and downside to doing this:
-Upside-
*Heaps of pretty colours
*A second display
*The use of modern monitors
*Standard set of registers make VGA easily programmable
*VGA cards have their own RAM, thus having near nil impact on system
performance

-Downside-
*Custom written software would be needed
*The adapter would be bulky

.

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