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4-bit digital CGA Output from GERT666 (or other GPIO connection)

Posted: Tue Nov 13, 2018 2:47 pm
by imaginaryeast
I have a monitor which accepts 4-bit TTL CGA Only. Does anyone know if there is a way to directly output 4-bit TTL from the Raspberry Pi?

I know that there exists VGA-CGA converters, but it would be quite difficult to implement in the intended setup. I am looking for a way to output CGA directly from the Pi.

Thanks,

Re: CGA Output from GERT666 (or other GPIO connection)

Posted: Tue Nov 13, 2018 5:00 pm
by drgeoff
First problem is that RPis do not produce TTL signals. Connecting a RPI GPIO directly to a true TTL input is liable to permanently damage the RPi.

Re: CGA Output from GERT666 (or other GPIO connection)

Posted: Wed Nov 21, 2018 3:13 am
by DarkElvenAngel
Anything is possible,

A level shifter is needed to take the 3.3v logic of the pi to 5v then you have the 4 signals for colour Red Green Blue Intensity, then the sync signals Vsync, Hsync

You'd need to set the resolution to a maximum of 640x200

You most likely won't get true CGA out but 8 or 16(if you can figure out how to drive the Intensity correctly) colour text mode should work.

I wish I still had my CGA monitor I could test that out.

Good luck

Re: CGA Output from GERT666 (or other GPIO connection)

Posted: Tue Nov 27, 2018 3:31 pm
by imaginaryeast
DarkElvenAngel wrote:
Wed Nov 21, 2018 3:13 am
Anything is possible,

A level shifter is needed to take the 3.3v logic of the pi to 5v then you have the 4 signals for colour Red Green Blue Intensity, then the sync signals Vsync, Hsync

You'd need to set the resolution to a maximum of 640x200

You most likely won't get true CGA out but 8 or 16(if you can figure out how to drive the Intensity correctly) colour text mode should work.

I wish I still had my CGA monitor I could test that out.

Good luck
Thanks for the ideas.

So I'll have to build my own circuit to drive the monitor. It'll be basically building a vga to cga adapter into the GPIO header as an output device. I suppose I could buy an adapter and reverse engineer it.

Sorta hoping someone else would have done something like this already. lol

Best,

Re: CGA Output from GERT666 (or other GPIO connection)

Posted: Fri Nov 30, 2018 7:18 pm
by GeekRambo
How is it that people, this week, are looking for the same solution I'm looking for.

I have found the GBS 8100 which converters from VGA to 15k RGBS. I'm not sure if that will work on a CGA monitor, like a IBM 5153, though.

My idea/project is to fill an empty IBM 5150 case with a RPi and any necessary support hardware, to make a "sleeper PC". Not too sure if it's possible to get the 5153 CGA monitor to work though.

Re: CGA Output from GERT666 (or other GPIO connection)

Posted: Mon Dec 03, 2018 6:32 pm
by imaginaryeast
i would be a little suspicious if the output would support a TTL CGA input, but who knows? The problem is those amazon converts don't really offer much in the way of support
the 5153 does not have a composite input either, so far as i can tell? which would mean you can't take advantage of programs that use color artifacting. if that isn't a concern, i would focus on running the monitor in EGA mode, which is 16 color 320x200. in my opinion, this is the optimal mode of the 5153 and looks pretty bad-ass.

here's an article with a wiring diagram for the input cable:

http://worldphaco.com/uploads/INCREASIN ... O_AUTO.pdf

if you also want to play older stuff that uses artifact colors in NTSC mode, you may want to use something else like a commodore monitor; or at least check the wiring diagram more thoroughly to see if the db9 connector can accept a composite input. (maybe i missed it)

i'm using a SONY PVM which does both

Re: CGA Output from GERT666 (or other GPIO connection)

Posted: Fri Dec 07, 2018 2:56 pm
by imaginaryeast
Ok. I'm bumping this because I ran into a roadblock.
None of the VGA to CGA converters I can find out there actually produce RGBI.

Does anyone know of a way to get 4 bit RGBI out of the pi's GPIO header?

If that's impossible, a true VGA to CGA (RGBI) converter?

Re: CGA Output from GERT666 (or other GPIO connection)

Posted: Thu Aug 08, 2019 11:09 pm
by tbilan
I'd like to bump this. Has anyone been able to get RBGI output? I'm trying to get an IBM 5153 CGA monitor working with a Raspberry Pi.

Re: 4-bit digital CGA Output from GERT666 (or other GPIO connection)

Posted: Fri Aug 09, 2019 4:38 pm
by pik33
Some ideas...

Use 4 of 18 bits + sync from VGA666 pins (without connecting a vga666) via a level converter. Set 8-bit framebuffer with such a palette that first 16 colors of it will drive these 4 selected bits as a CGA monitor needs. I don't know if RPi will allow to set a video mode with 7.14 MHz pixel clock :) - you will have to set such a resolution in config.txt

Or maybe get a microcontroller, drive a monitor using it and connect the microcontroller to the RPi via UART.