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How hard would it be to make a CGA card?

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Reply 40 of 67, by reenigne

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anetanel wrote:
I found this card that uses the same chip and has CGA composite output: http://oldcomputer.info/pc/board_gfx1/mcgvp.jpg (Source: […]
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I found this card that uses the same chip and has CGA composite output:
mcgvp.jpg
(Source: http://oldcomputer.info/pc/board_gfx1/index.htm)

Can it be used to reverse engineer my card to output CGA composite?

Probably! If you can trace the lines to see where everything goes. It's possible (although difficult) to do this from images (I've done it once before) but much easier if you can use a multimeter's continuity mode.

Reply 41 of 67, by anetanel

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reenigne wrote:

If you're not bothered about the colours being accurate and you have (or find) a CGA card with an RGBI output, it's a pretty simple circuit to go from RGBI to monochrome composite.

Until (if ever) I'll be able to reverse engineer a working card, how can I convert the RGBI output to monochrome composite, as you suggested?

Reply 42 of 67, by reenigne

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anetanel wrote:

Until (if ever) I'll be able to reverse engineer a working card, how can I convert the RGBI output to monochrome composite, as you suggested?

Here's a cheeky DaveCAD I just did of the equivalent of the output stage of an IBM 1501981 ("New style" CGA) with colour burst disabled (as in mode 5). If you change the 750 ohm resistor for a 1.5k then this might even give a reasonable output from 8088 MPH final version in New CGA mode. Sorry for the wobbly lines - hope you can read it. You can probably do it with fewer than 3 chips if you're feelng clever and want to optimise the logic.

rgbi_to_mono_composite_s.jpg

Reply 43 of 67, by anetanel

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Well, I spend the last hours learning enough KiCad to draw this unholy mess 😀
I think that I wont try to design a PCB, as it stretches my ability at the moment.
But I guess it wouldn't be too hard to solder this on a prototype board.

One thing I'm not sure about. Where does the +5v come from?

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Reply 44 of 67, by root42

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anetanel wrote:

One thing I'm not sure about. Where does the +5v come from?

If I should hazard a guess: a power supply? 😀

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Reply 45 of 67, by anetanel

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root42 wrote:
anetanel wrote:

One thing I'm not sure about. Where does the +5v come from?

If I should hazard a guess: a power supply? 😀

Preposterous! 😜

So let's say I use a usb charger which gives +5v. To where should I connect the charger's ground? To the same GND path that the RGBI uses?

Reply 46 of 67, by Jo22

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^VBS.. It's called VBS (CVBS or Composite without colour). 😉

What it stands for isn't completely certain, though. Some say Video, Blanking, Sync, others say Video Baseband Signal.

I prefer the later, since it is the kind of signal VHF/UHF modulators were fed with.


(In German, sometimes abbreviated as BAS and FBAS.)

Edit: Also thanks a lot for the drawings.

Edit:

So let's say I use a usb charger which gives +5v. To where should I connect the charger's ground? To the same GND path that the RGBI uses?

Becareful with that. Mixing different PSUs and grounds might cause unpredictable results, I believe.

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In what to one race is no time at all, another race can rise and fall..." - The Minstrel

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Reply 47 of 67, by reenigne

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anetanel wrote:
Well, I spend the last hours learning enough KiCad to draw this unholy mess :) I think that I wont try to design a PCB, as it st […]
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Well, I spend the last hours learning enough KiCad to draw this unholy mess 😀
I think that I wont try to design a PCB, as it stretches my ability at the moment.
But I guess it wouldn't be too hard to solder this on a prototype board.

One thing I'm not sure about. Where does the +5v come from?

I like to power little external boards like this with a keyboard port passthrough or dongle - plug the board into the RGBI output and the keyboard port, plug the keyboard into the board. No batteries to change, no need for another wall-wart, no grounding issues. If you wanted to be neater you could re-route +5V through pin 7 on the CGA card and then there's only one thing to plug in. Shouldn't be too dangerous since +5V would just be interpreted as TTL high to an MDA or EGA monitor that is plugged into there.

Reply 49 of 67, by reenigne

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anetanel wrote:

I like the idea of using the keyboard for 5v. But again (and sorry if it is obvious..) is it enough to tap into the 5v from the keyboard? no need to have ground too?

Check the schematics of your machine to be sure (or just measure between keyboard ground and CGA ground with a multimeter) but I think all ground signals on a PC are tied together so there's no issue with different levels. I guess you might accidentally create a ground loop, but it'd be a small one and all the signals in question are digital anyway so it shouldn't be a big deal. I've never noticed any problem playing games like that with my XT.

Reply 51 of 67, by reenigne

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anetanel wrote:

I have verified that the Ground on the PSU connectors, on the keyboard connector and on the video card are all connected. That means I'm good to go. right?

Yep!

Reply 53 of 67, by anetanel

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I managed to get all the components locally. And it even wasn't all that expensive 😀
Tonight I'm gonna try and breadboard it and hope for the best.
I'll be happy if you could take a look at my design and see if I missed anything:
https://www.tinkercad.com/things/kbdKZueB1jr

Reply 54 of 67, by reenigne

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anetanel wrote:
I managed to get all the components locally. And it even wasn't all that expensive :) Tonight I'm gonna try and breadboard it an […]
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I managed to get all the components locally. And it even wasn't all that expensive 😀
Tonight I'm gonna try and breadboard it and hope for the best.
I'll be happy if you could take a look at my design and see if I missed anything:
https://www.tinkercad.com/things/kbdKZueB1jr

I think you need to double-check it - one of the legs of the transistor is not connected at the moment. Not sure if there are any other problems!

Reply 55 of 67, by anetanel

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Great Success!!
I could not believe that it worked at first try 😀

Next step is to transfer this mess to a prototype board.
Thanks you very much guys, and reenigne in particular!

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Reply 57 of 67, by root42

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Awesome. You’re done. Just dip it in resin, put it in your case, and you’ll be finished. 😁

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Reply 58 of 67, by Tronix

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anetanel wrote:

Great Success!!

You can reduce the number of IC chips from three to two. Remove 74LS04 inverter IC and make inverter with help free part of 74LS86:

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https://github.com/Tronix286/

Reply 59 of 67, by anetanel

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Tronix wrote:
You can reduce the number of IC chips from three to two. Remove 74LS04 inverter IC and make inverter with help free part of 74LS […]
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anetanel wrote:

Great Success!!

You can reduce the number of IC chips from three to two. Remove 74LS04 inverter IC and make inverter with help free part of 74LS86:

7486.PNG
74862.PNG

I understand. Thanks! 😀
I'll give it a try today.