How hard would it be to make a CGA card?

Discussion about old graphics cards, monitors and video related things.

Re: How hard would it be to make a CGA card?

Postby reenigne » 2018-7-26 @ 09:33

anetanel wrote:I found this card that uses the same chip and has CGA composite output:
Image
(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.
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Re: How hard would it be to make a CGA card?

Postby anetanel » 2018-7-29 @ 13:45

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?
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Re: How hard would it be to make a CGA card?

Postby reenigne » 2018-7-29 @ 16:23

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.

Image
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Re: How hard would it be to make a CGA card?

Postby anetanel » 2018-7-29 @ 19:47

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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Re: How hard would it be to make a CGA card?

Postby root42 » 2018-7-29 @ 20:00

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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Re: How hard would it be to make a CGA card?

Postby anetanel » 2018-7-29 @ 20:18

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! :P

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?
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Re: How hard would it be to make a CGA card?

Postby Jo22 » 2018-7-29 @ 20:47

^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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Re: How hard would it be to make a CGA card?

Postby reenigne » 2018-7-29 @ 21:13

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 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.
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Re: How hard would it be to make a CGA card?

Postby anetanel » 2018-7-29 @ 21:23

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?
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Re: How hard would it be to make a CGA card?

Postby reenigne » 2018-7-29 @ 21:53

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.
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Re: How hard would it be to make a CGA card?

Postby anetanel » 2018-7-30 @ 15:21

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?
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Re: How hard would it be to make a CGA card?

Postby reenigne » 2018-7-30 @ 15:32

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!
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Re: How hard would it be to make a CGA card?

Postby anetanel » 2018-7-30 @ 15:40

Good.
I have to find where I can get the parts locally. Only the shipping from Mouser will cost almost 5 times the cost of the parts :\
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Re: How hard would it be to make a CGA card?

Postby anetanel » 2018-8-01 @ 14:45

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
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Re: How hard would it be to make a CGA card?

Postby reenigne » 2018-8-01 @ 17:02

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 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!
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Re: How hard would it be to make a CGA card?

Postby anetanel » 2018-8-02 @ 08:52

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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2018-08-02 09.06.33.jpg
2018-08-02 08.55.44.jpg
2018-08-02 08.56.25.jpg
2018-08-01 23.01.06.jpg
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Re: How hard would it be to make a CGA card?

Postby spiroyster » 2018-8-02 @ 08:56

Wow! great work!
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Re: How hard would it be to make a CGA card?

Postby root42 » 2018-8-02 @ 10:47

Awesome. You’re done. Just dip it in resin, put it in your case, and you’ll be finished. :D
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Re: How hard would it be to make a CGA card?

Postby Tronix » 2018-8-02 @ 12:19

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
7486.PNG (3.59 KiB) Viewed 151 times


74862.PNG
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Re: How hard would it be to make a CGA card?

Postby anetanel » 2018-8-02 @ 14:25

Tronix wrote:
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.
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