CGA with VGA

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

CGA with VGA

Postby lorecast162 » 2019-6-10 @ 22:18

Hello, I have a 8088 PC XT clone with both a MDA and CGA card.
I use this computer with the CGA card's composite output but it kinda looks like not good so I want to go the proper way.
Here where I live it's hard and expensive to get old CGA compatible CRTs so I cannot do it that way, for now at least.
I think VGA is the best step, I already have a really good looking Trinitron CRT with VGA I can use this with and if that breaks I have tons of LCDs with it.
Unfortunately though the "easiest" approach also requires a lot of money, which is the MCE2VGA by Serdashop, which is all nice and kind of ideal but way out of my league.
I thought about making my own adapter then but info I found is conflicting or old and without the needed resources anymore (e.g. the schematics) so here I am asking you this: how do I do this? Can you give me instructions and schematics on how to do this? I need it to be a cost-effective way if possible and I think even a half-assed adapter thingy would still be light years ahead of composite.
Thanks
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Re: CGA with VGA

Postby rmay635703 » 2019-6-11 @ 02:15

Your best bet is one of the older Sony LCD screens, if it has vga, rca and everything else there is a good chance it will sync up to cga with a simple adapter

Real CRTs unless they are quite old will need you to adjust the internal sync,v and h pot dials on the pcb (if it has them). Even then you may need an extron box to slide the image into the correct position
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Re: CGA with VGA

Postby lorecast162 » 2019-6-11 @ 06:32

rmay635703 wrote:Your best bet is one of the older Sony LCD screens, if it has vga, rca and everything else there is a good chance it will sync up to cga with a simple adapter

How do I make this simple adapter you speak about?
Also, I have a NEC multisync, that should work too, right? I know it has quite good compatibility with different sync frequencies.
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Re: CGA with VGA

Postby Jo22 » 2019-6-11 @ 14:39

Here are some threads about CGA to RGB (SCART) schematics. Depending on the VGA monitor, these can be adapted.
Keep in mind that these adapters can't do Composite CGA. If you like Composite CGA, a CVBS-VGA converter (Composite to VGA) might be needed.
Image quality is hit and miss, though. Some converters like this do try to "improve" the picture. Also, you may still have the 244p/288p vs 488i/576i problem.

CGA->RGB converters...
viewtopic.php?p=684545#p684545
viewtopic.php?p=578495#p578495
viewtopic.php?p=549562#p549562
viewtopic.php?p=447574#p447574
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Re: CGA with VGA

Postby lorecast162 » 2019-6-11 @ 15:06

Jo22 wrote:Here are some threads about CGA to RGB (SCART) schematics. Depending on the VGA monitor, these can be adapted.
Keep in mind that these adapters can't do Composite CGA. If you like Composite CGA, a CVBS-VGA converter (Composite to VGA) might be needed.
Image quality is hit and miss, though. Some converters like this do try to "improve" the picture. Also, you may still have the 244p/288p vs 488i/576i problem.

CGA->RGB converters...
viewtopic.php?p=684545#p684545
viewtopic.php?p=578495#p578495
viewtopic.php?p=549562#p549562
viewtopic.php?p=447574#p447574


I already saw the SCART converters actually, but I still need to find the BS170 and hoped that since VGA is already HVSync I could go straight to VGA but that is not possible I guess. Thanks
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Re: CGA with VGA

Postby Jo22 » 2019-6-11 @ 19:17

lorecast162 wrote:I already saw the SCART converters actually, but I still need to find the BS170 and hoped that since
VGA is already HVSync I could go straight to VGA but that is not possible I guess. Thanks

I see. How about the 2N7000 instead ? :)
https://en.wikipedia.org/wiki/2N7000
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Re: CGA with VGA

Postby lorecast162 » 2019-6-11 @ 21:20

I don't have that either, is there any difference between the two?
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Re: CGA with VGA

Postby SirNickity » 2019-6-20 @ 18:48

One of the handful of irons in my fire at the moment... Here's what I have so far - untested as of yet. Maybe it helps inspire some ideas?

The concept is to take the TTL RGB+I from CGA / Tandy and derive dark / light 16-color (with brown) analog RGB at 0.7V for VGA. I have not gotten around to studying the H and V sync signal properties. That is, I don't know yet if there's any difference between CGA and VGA. It's on my to-do list for maybe this weekend to finish refurbing my new Tandy, and then start looking at it on a scope.

Here's my WIP schematic. Prominent is the green channel decoder, since it is the most complicated. R and B decoders are out of frame below and will use the other two channels of the Analog Devices buffer amp. I'm using generic TI configurable logic ICs. They have three inputs, and depending on how you wire it, can be buffers, inverters, A/B selectors, OR, AND, NOR, NAND, and some of the above with one inverted input. I'm only using buffers, OR, AND, and AND with an inverted input (1N-AND in the labels). Any similar logic IC with sufficient bandwidth would work in its place.

Tandy Adapter - Sch.png


Power conditioning and other details are not yet represented. Also, note that this assumes your display will cope with 15kHz signals, vs. VGA's 31kHz+. This was alluded to above as the 200p problem. Solving that requires a frame-buffer, or at least a line-buffer, to transmit each input line twice at 2x the speed -- and of course, reasonably intelligent timing detection to re-generate the pixel clock from the sync pulses. That's why the current true-VGA converters are kinda expensive.

Here's the PCB layout I'm working on alongside the schematic.

Tandy Adapter - PCB.png


In parallel to this, I'm working on a PS2 to XT converter that will live on the right side of the board. That's mostly code for an AVR micro though. The circuit is trivial.
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Re: CGA with VGA

Postby SSTV2 » 2019-6-20 @ 21:48

@ Jo22, is that an electonic parts catalog for DOS? Looks neat :exclamation:
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Re: CGA with VGA

Postby Jo22 » 2019-6-20 @ 22:13

@SirNickity Thank you for sharing your schematic with us! Looks well done!

SSTV2 wrote:@ Jo22, is that an electonic parts catalog for DOS? Looks neat :exclamation:

Hi! Yes, it is. It runs on any PC, even XTs. I mentioned it a while ago in the thread "Anything else besides games on your retro PC ? [..]"
I also made a quick video of it at https://youtu.be/cMZK64LaUn0?t=247. In the description, there's a link to a free flavor of it (has "adds" but is otherwise working fine).
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Re: CGA with VGA

Postby matze79 » 2019-6-21 @ 13:44

Jo22 wrote:
lorecast162 wrote:I already saw the SCART converters actually, but I still need to find the BS170 and hoped that since
VGA is already HVSync I could go straight to VGA but that is not possible I guess. Thanks

I see. How about the 2N7000 instead ? :)
https://en.wikipedia.org/wiki/2N7000



where can i find this catalogue software ?
https://dosreloaded.de - The German Retro DOS PC Community
https://www.retroianer.de
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Re: CGA with VGA

Postby root42 » 2019-6-21 @ 14:54

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Re: CGA with VGA

Postby SSTV2 » 2019-6-21 @ 20:17

Thank you Jo22 a lot, this software is just leet! I've seen that "Anything else..." thread when it was just created, pics are a must ;). Sorry @ lorecast162 for derailing post.

On topic, I'd suggest you to get a VGA card that can work both in 8bit and 16bit ISA bus (for example Tseng ET3000), you could then hook up any VGA compatible monitor to your XT PC.
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Re: CGA with VGA

Postby rmay635703 » 2019-6-21 @ 20:33

Since cga derives it’s pixel clock of the main board crystal or 14.77mhz could it be modded like mda with a external clock crystal for 31khz scan?

I realize you might have to replace the ram with faster parts but at the end of the day that might be cheaper than some of the “other” solutions
You would then need a resistor to protect the vga circuit and voila 8 color cga, getting 16 would require a bridge like circuit which is getting more complex than I like but not terrible
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Re: CGA with VGA

Postby Jo22 » 2019-6-23 @ 05:06

Yes, that could work. However, some of the CGA games may rely on the old frequecies for their timings and don't go well with this modification.
I once tried to modify an Hercules card's on-board crystal (s. channel) and it worked for text-mode only. Graphics mode was too much for it.
Still nice for getting pure text-mode apps like IC-Database going on a Hercules system (=704KiB+ base mem possible!) with a modern LCD monitor.

Personally, I would try out a simple Composite-VGA converter first, since it supports Composite CGA. It also got has a better ADC than the crappy monitors of today have got.
Well, provided that the PC can do output CGA in colour via that RCA port. Also, I would keep a cheap little TV with RGB SCART for testing around.

It can do the 15KHz natively so that only a simple converter is needed (and it can display Composite much better).
After getting this to work, and gaining experience, I would try the CGA-VGA converter.
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Re: CGA with VGA

Postby root42 » 2019-6-23 @ 07:56

I use this converter for my Atari 2600: https://www.conrad.de/de/p/goobay-av-ko ... 62640.html

It takes Svideo and Composite. Should work with the CGA composite as well.
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