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RGB Monitor on MCGA output

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First post, by wrexroad

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I have an IBM PS/2 Model 30 with MCGA output. I also have an Amiga 1084 monitor with a 9-pin connector that says it will accept digital or analog RGB signals. My understanding is that if I had a real CGA card, I could drive the monitor's RGB input with it. Is that also the case with the 15-pin MCGA video output? I've done some looking around, but all I can find is information about driving a VGA display with a CGA output. I've seen adapters like this (https://www.amazon.com/gp/product/B0000511D5) but I suspect the solution isn't that simple.

Reply 1 of 23, by darry

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wrexroad wrote on 2021-02-22, 22:06:

I have an IBM PS/2 Model 30 with MCGA output. I also have an Amiga 1084 monitor with a 9-pin connector that says it will accept digital or analog RGB signals. My understanding is that if I had a real CGA card, I could drive the monitor's RGB input with it. Is that also the case with the 15-pin MCGA video output? I've done some looking around, but all I can find is information about driving a VGA display with a CGA output. I've seen adapters like this (https://www.amazon.com/gp/product/B0000511D5) but I suspect the solution isn't that simple.

AFAIK, that will not work easily, because, again AFAIK, MCGA analogue RGB output is 31KHz (320x200 is line-doubled by the card), like VGA and the 1084 can only accept 15KHz . A workaround that would likely work on VGA cards is VGATV ( http://mirrors.arcadecontrols.com/VGATV/pwp.n … bo.pt/pscoelho/ ) which, coincidentally, was being discussed in this thread Do you want an EGA monitor? .I have no idea if it would work on an MCGA card, though .

Reply 3 of 23, by Benedikt

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I believe that I read somewhere that the MCGA hardware does indeed support 15kHz analog RGB screens, but that that support had been removed from the documentation and that the respective settings have been described as "reserved", instead.
The only explanation that I can come up with is that the engineers wanted to make the machine compatible with existing screens while the marketing department wanted to sell new screens.

So technically, what you want to do could indeed be trivial, but you still have to figure out how to do it.
More specifically, you would have to verify that the machine really does output a 15kHz signal in the "reserved" mode.

Reply 4 of 23, by rmay635703

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Classic 9pin 15khz Targa style RGB+Sync is not compatible with VGA unless you drive it through a specific Averkey VGA-TV adapter that has RGB analog or SCART capabilities.

These screens generally were
Broadcast TV monitors
Amiga / Commodore
and multiple standard ready for CGA/TTL/Analog/RCA/RGBS

Reply 6 of 23, by wrexroad

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Grzyb wrote on 2021-02-22, 23:37:

Well, I am going to have to read that thread several times to understand everything in it... but what I do understand is in this post Re: 8-bit guy gets ultra-rare NOS IBM (prototype?) from Computer Reset then proceeds to obliterate it (which quotes another forum's thread). Looks like I might just be able to make a cable and have it work.

Reply 7 of 23, by rmay635703

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You likely have another issue ,

Combined sync

Most RGB+S did not support separate sync like vga in analog mode.

You would need to check the pinout to know for sure.

Like I said an averkey or a multitude of other vga-> tv adapters would solve your issue without much thought converting your signal to interlaced video

Reply 8 of 23, by maxtherabbit

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no one wants some garbage downscaler converting to interlaced, they want native 15kHz progressive scan

separating or combining sync is trivial with a $5 used extron box from ebay

Reply 10 of 23, by Benedikt

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wrexroad wrote on 2021-02-23, 18:44:

Looks like the 1084 has separate vertical and horizontal sync according this this site: https://www.epanorama.net/circuits/vga2tv/rgb_pinouts.html

Unless you have a 1084 that has a SCART port as its only analog input. But even then you don't need a lot of components to combine HSYNC and VSYNC.

Reply 12 of 23, by Benedikt

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Benedikt wrote on 2021-02-23, 20:39:

Unless you have a 1084 that has a SCART port as its only analog input. But even then you don't need a lot of components to combine HSYNC and VSYNC.

I meant to say: "as its only analog RGB input".

Reply 13 of 23, by rmay635703

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maxtherabbit wrote on 2021-02-23, 15:09:

no one wants some garbage downscaler converting to interlaced, they want native 15kHz progressive scan

separating or combining sync is trivial with a $5 used extron box from ebay

Your thinking of RCA composite not
RGBS component Targa signalling

My Futura 100 portrait system sends a native 512x480 60i
65k color to a 1992 era Magnavox RGBS monitor (a more modern 1080)

The picture is crystal clear albeit interlaced
These screens were designed for hi resolution interlaced video and no scaling is required to send 640x480 to RGBS, the device just drops every other field leaving an untouched clean albeit interlaced image which is what these screens were designed to do anyway.

If we were to build one of these very simple devices he could have protection from blowing up his screen when his video accidentally switches to an unsupported mode.

Starting on a vga monitor , switching to 15khz signaling then switching screens is going to kill any enjoyment of using the screen.

And I wouldn’t trust a home brew rom hack to keep the MCGA adapter in 15khz

If he isn’t worried about desyncing the monitor
His other option would be to use an extron and try to see if the screen has internal trim pots that go far enough to view 320x200 MCGA natively and use dos commands to keep it in 40 column mode on boot.

Reply 15 of 23, by wrexroad

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It works! Sort of... The image started out monochrome, then the blue started to saturate more and more. You can see some lines on the left edge of the screen that appeared as the blue started to come in, then as the blue got stronger the lines grew to the entire length of the screen, then the image begins to deteriorate, so something is really not happy. But the monitor does sync up (using separate vertical and horizontal sync signals).

For anyone that decided to do this, its important to note that you will probably have to get a VGA connector and put it on a 9-pin cable, not the other way around. On the few VGA cables I have laying around pin 11 was internally grounded, which is a monitor ID pin that needs to be left disconnected.

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Reply 16 of 23, by rmay635703

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wrexroad wrote on 2021-03-08, 21:49:

It works! Sort of... The image started out monochrome, then the blue started to saturate more and more. You can see some lines on the left edge of the screen that appeared as the blue started to come in, then as the blue got stronger the lines grew to the entire length of the screen, then the image begins to deteriorate, so something is really not happy. But the monitor does sync up (using separate vertical and horizontal sync signals).

For anyone that decided to do this, its important to note that you will probably have to get a VGA connector and put it on a 9-pin cable, not the other way around. On the few VGA cables I have laying around pin 11 was internally grounded, which is a monitor ID pin that needs to be left disconnected.

Looks like what happens when vhold is fine but your signal is to fast and the image spreads way out.

Need to get the clock rate down to 15khz

Reply 17 of 23, by Benedikt

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rmay635703 wrote on 2021-03-08, 22:07:

Looks like what happens when vhold is fine but your signal is to fast and the image spreads way out.

Need to get the clock rate down to 15khz

I don't know about that. The number of text lines on-screen looks plausible the way it is.

Reply 18 of 23, by wrexroad

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Yeah, I will clarify that the monochrome image that is on the screen looks 100% correct with regard to its size and location on the screen. The color information seems to be where things are falling apart, as it starts monochrome then the blue saturation increases with time. There is also the noise on the left hand side of the screen. I don't think that is the image spreading horizontally because it happens on the entire vertical side of the screen, even where there is nothing else displayed.

I noticed that I had the switch set to digital RGB input, so that might be what is wrong. When I switched it to analog I lost the picture, but I think that is because the analog side wants the composite sync signal.

Reply 19 of 23, by Benedikt

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wrexroad wrote on 2021-03-09, 16:36:

I noticed that I had the switch set to digital RGB input, so that might be what is wrong. When I switched it to analog I lost the picture, but I think that is because the analog side wants the composite sync signal.

That kind of explains the screen's weird behavior. Even the peak voltage of the analog video lines will not yield a stable logic "1".
Regarding composite sync, an XOR gate should solve that problem. The 74LS86 looks like a good choice. It has four XOR gates, but the single-chip solution is still simpler than discrete transistors.