RGB-VGA Converter -- buy or build?

From: Richard Erlacher <edick_at_idcomm.com>
Date: Wed Feb 7 14:39:40 2001

I'll bet you do have an RGB monitor, unless it's monochrome or
composite-input.

Before I can answer your questions, I have to ask a few of my own. First of
all, what, EXACTLY, do you mean by "RGB monitor?" The only popular video
cards I've seen made after about 1982 all had R, G, and B outputs, along
with a couple of sync signals. The PC "color" board had compisite outputs
as well, but R, G, and B were there. RGB is not a standard. In fact, it's
not even a slightly adequate description of a monitor, since the only ones
other than composite-input types that are out there in quantity are
monochrome. Let's face it, if you want color to appear on your screen,
you've got to send its constituents into the monitor somehow.

The signals that need to flow into the typical monitor are red, green, blue,
H-sync, and V-sync. There are several ways of getting them there, ranging
from the 3 BNC's commonly used in RS343 (with composite-sync on green)
applications, to the wierd hookups used by workstation manufacturers afraid
you might hook up the wrong devices and break something.

How hard it is to build an appropriate adapter depends on what the monitor
expects to see as its driving signals, both in terms of frequency, and in
terms of polarity. If you have a fixed frequency monitor made for something
other than your application, I advise you to BEG someone to take it away.
Either that or just toss it, because at 50 cents an hour, you probably can't
make it work with a year's pay. A nice new, beautiful and bright monitor in
the 10" range costs <$300. You'll spend that much on aspirin, Malox, and
prune juice, and maybe on whisky as well.

If you feed each sync output from your video adapter through an XOR gate
with a switch to ground attached to a pullup to Vcc at one input and the
sync at the other, that will handily switch the sync polarity easily enough.
If your frequency is wrong, plan to expend about 10 man-years rewriting the
ROM on your video adapter to produce the right frequency. You can probably
program the sync polarity that way as well.

If you're wanting to impose sync on green, that's pretty straightforward and
I can provide you with a schematic that uses a 5-transistor array to do the
entire job very well. It won't fix frequency problems, though.

Let me know what sort of monitor and adapter you're trying to mate up and
perhaps I can help you.

Dick

----- Original Message -----
From: "Rich Beaudry" <r_beaudry_at_hotmail.com>
To: <classiccmp_at_classiccmp.org>
Sent: Wednesday, February 07, 2001 12:04 PM
Subject: RGB-VGA Converter -- buy or build?


> Hello....
>
> I have a video card that outputs RGB, but I have no RGB monitor... Not
> knowing anything about video standards, I ask the following :-)
>
> - How hard would it be to build an RGB-to-VGA converter to use my
computer's
> VGA monitor (well, actually capable of 1024x768 non-interlaced)? Does
> anyone have schematics, or documentation to guide me along?
>
> - Would it be easier to convert RGB to composite video? I have several
> devices that can take composite input....
>
> - Has anyone bought a reasonably cheap (sub-$100 US) RGB-to-VGA converter
> unit, and been happy with it? If so, where did you get it from?
>
> Thanks!
>
> Rich B.
>
> P.S. I subscribe to the digest, so I cannot reply directly....
>
>
Received on Wed Feb 07 2001 - 14:39:40 GMT