Simplest way to convert TTL RGBI to Analogue RGB

[cwathen]

I'm trying to repurpose a PAL Analogue RGB monitor to work with my Nimbus PC.

The Nimbus' output is CGA-ish - TTL RGBI with 16 colours but at 50Hz and with a slightly higher maximum resolution of 640x250.

I'm after some advice on the simplest way to convert the TTL level output to something compatible with the ARGB inputs on the monitor. No sync conversion is required - the Nimbus has a little switch to change between separated and composite sync so I can get composite sync directly out of it.

Some way of implementing the intensity bit would be nice, but if not 8 colour RGB would do.

 

[gertk]

Search for Commodore 128 to SCART (= RGB input on European TV sets) circuits

Something like this might work:

http://www.softwolves.pp.se/cbm/maskinvara/scart.en.html

 

[Gary C]

Some CGA cards actually put out NTSC colour video on pin 7 ?

 

[bladamson]

You can do it extremely easily with a resistor bridge, as here: https://www.electroschematics.com/cga-scart-adapter/

But color #6, brown, will not be right. The IBM CGA monitor has extra circuitry that makes it actually brown. There is good info here: http://www.oldskool.org/pc/cgacal It would probably be possible to copy the circuitry out of the CGA monitor schematic that is linked to in that page, and get what you want.

Note that both of these will still only give you 15kHz analog RGB. If you want to feed it into a VGA monitor, you'll still need some kind of scan-doubler (such as the GBS8200).

GGLabs makes a board that does all this, but I've never tried it.

 

[cwathen]

Search for Commodore 128 to SCART (= RGB input on European TV sets) circuits

Something like this might work:

This is intriguing - I might build this to use with my C128; I have an S-Video capable TV which I use with Commodore machines, being able to get switchable 40/80 out of the same display is probably worth the effort to build it!

But color #6, brown, will not be right. The IBM CGA monitor has extra circuitry that makes it actually brown. There is good info here: http://www.oldskool.org/pc/cgacal It would probably be possible to copy the circuitry out of the CGA monitor schematic that is linked to in that page, and get what you want.

Note that both of these will still only give you 15kHz analog RGB. If you want to feed it into a VGA monitor, you'll still need some kind of scan-doubler (such as the GBS8200).

Not a huge problem; the Nimbus spends most of it's time running in 4 colour 640x250 mode and can only display black, white, red and blue anyway. The ARGB monitor I'm trying to repurpose is an old Olivetti monitor designed for the Prodest PC/1 (since my PC/1 is dead). It is SCART ARGB compatible (since the PC/1 was supplied with a simple passive cable to connect to a SCART equipped TV through the same port) and it is high enough res to do legible 80 column text since it used to do that with my PC/1 so should be a good fit for the Nimbus if only I can convert the standards.

Some CGA cards actually put out NTSC colour video on pin 7 ?

Thanks but this definitely won't. The Nimbus had CGA-ish compatible video through a DIN connector to sell with the rebadged Mitsubishi CGA monitor they sold with it, and an internal switch to change it to composite sync to make it backwards compatible with the 8 colour TTL RGB monitor sold with the RM 480Z (which also by extension made it compatible with BBC Micro RGB monitors). I believe the switch was deleted in later models, but in that case composite sync could still be derived from the monochrome composite output.

 

[Trixter]

I would have recommended the gglabs cga2rgb unit, but your 50hz refresh rate makes me hesitate.

 

[Eudimorphodon]

I would have recommended the gglabs cga2rgb unit, but your 50hz refresh rate makes me hesitate.

I don't think there's anything about the gglabs board that will care about refresh rate, it's basically just a buffered resistor-ladder with rudimentary "palette register" implemented inside the GAL. Should be fine unless you ran it at a dot clock too fast for the GAL to handle, which seems unlikely for anything even remotely CGA-ish.

 

[1ST1]

Note that both of these will still only give you 15kHz analog RGB. If you want to feed it into a VGA monitor, you'll still need some kind of scan-doubler (such as the GBS8200).

No, you only need something like a good TFT which can handle 15khz signals on VGA input. There are many, but not any. I recommend using some of the NEC Multisync TFT 1990nxp/sxi and 1970sxi series. Instead of the 19 inch models you can use thw 21 inches, like 2190nxp/sxi, but please avoid the 17 inch models, they don't have these 15kHz video timings. There is also a webiste somewhere which lists many many more TFT displays which can handle 15kHz. If ylou like to inform, then just have a look into the Atari ST and Commodore Amiga scene, many of these guys are using these monitors.

http://15khz.wikidot.com/

For the signal conversion you should have a look here, it's just some resistors and diodes. http://www.vcfed.org/forum/showthread.php?65253-M24SP-DB25-to-DB9-adapter/page3 (that thread is about Olivetti M24, but no matter, also it's graphics card is using CGA signals.)

 

[Trixter]

I don't think there's anything about the gglabs board that will care about refresh rate, it's basically just a buffered resistor-ladder with rudimentary "palette register" implemented inside the GAL. Should be fine unless you ran it at a dot clock too fast for the GAL to handle, which seems unlikely for anything even remotely CGA-ish.

If that's the case, I highly recommend the gglabs board -- the conversion quality is the best of those I've tested, and it gets color #6 correct as well as supporting the intensity pin.