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IBM/C128 CGA conversation, capture, and VGA display success stories

Trixter

Trixter

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(This is an oft-repeated topic, so I felt there should be a dedicated thread for it.)
Every so often, people see the cheap "CGA2VGA" or "CGA2RGB" arcade adapters on ebay and wonder aloud if that is a good solution for replacing their aging or broken IBM CGA or Commodore 128 monitor. Unfortunately, those adapters either don't work, or produce subpar results (only half the colors displayed, colors transposed, or both). Proper handling of digital TTL CGA requires three things:

  1. TTL RGB to analog RGB conversion
  2. Proper handling of the intensity ("I") pin so that all 16 RGB colors are output
  3. Handling of the color "yellow" so that it is displayed as "dark brown" (which matches what CGA monitors do internally)
While there are rumors of commercial products from the late 1980s and early 1990s that can perform these conversions (Extron produced some, but I have not yet tested the units in my possession), most success stories are from homebrew adapters. These adapters massage CGA TTL into analog RGB, then another device either converts the resulting analog RGB to VGA (where a cheap "CGA2VGA" device IS used) or performs capture directly with a capture card that can deal with 15.7KHz horizontal frequencies. Here are a few success stories that I've seen:

Here are other solutions that should work but thus far have been unverified by me:

  • Another Richard Goedeken's design board, this time revised by John Carlsen: http://www.bit-c128.com/ (I have this in my possession but have not yet verified its functionality)
Please feel free to add to this thread with additional success (or failure!) stories.
 
I've got one of these:

http://www.vintage-computer.com/vcforum/showthread.php?40965-Commodore-128-Video-DAC-Board-Available

That works with the GBS-8220 to convert to VGA...works great. I have not carefully studied to confirm how the yellow/dark brown coloring displays. Only thing missing is a nice case to hold it all...need to do something with mine rather than having the boards laying around.

I seem to recall reading that the bit-c128.com board you reference is basically the same thing as what ajacocks put together in the link above. Maybe a slight tweaking, and also SMD rather than thru-hole.

Prior to that, I actually contacted the folks at Gonbes who confirmed, as we know, that TTL CGA would not work direct into the GBS-8220. However, they did tell me that the GBS-8219 would do the job. Unfortunately, it does not handle the intensity pin...and it is quite expensive.

Thanks,

Wesley
 
I use an original NEC MultiSync which connects to CGA, EGA and VGA cards directly, works perfectly and alleviates the need for three monitors. :)

I've got one left in storage.
 
I have something of a question:
These CGA TTL-to-RGB circuits would also work on low-res EGA. But have you ever seen any that would also support the extra EGA colours for high-res?
I would think that it would not be too difficult to add EGA support to such a circuit, for the people who designed them. But perhaps they never thought of that, because they concentrated only on CGA and/or C128.
 
That C128 adaptor looks very neat. I put together a simple resistor ladder circuit to do the same thing here: http://www.vogons.org/viewtopic.php?f=25&t=40135#p374305
No brown fix though.

Also, an Extron 202xi (not rxi) will do this, they are available cheaply on eBay.
A scandoubler like the GBS units is still required. I was never 100% happy with these though, and ended up buying a CGA monitor eventually.
For Euro people a Philips CM8833 or similar makes an excellent CGA monitor, I prefer mine over my IBM 5153.
 
CGA has more colors than 16 shades of grey???
Click to expand...

(From a HP 100LX user)


Well i have been tempted to get one of these 20 dollar china converters, but if you have an ebay one where they show the input sockets you already know these ain't gonna work.


These 150 Dollar china converter thingies work though. Well more like "150 to 200 Dollar"... Prices vary on these, but i have seen them as low as 150 bucks (If that really counts as "low").

http://www.ebay.com/itm/RGB-CGA-EGA...569807?hash=item2a2ed88b4f:g:2bsAAOxyRHdR386W







And then there is this one guy who sells modern TFTs (Well modern as in "the year 2003 just called and wants its TFT displays back") converted to CGA. I'd be interested in that since it also features lovely jumpers to go to mindblowing green or B&W MONOCHROME...

HELL YEAH MONOCHROME (Note: This sentence meant to be typed in all caps and bold...Oh yeah here we are again...)


http://www.ebay.com/itm/AMDEK-722-C...6d6676b&pid=100010&rk=2&rkt=9&sd=200334916402


I'd be tempted to get one just to see the internals. Well but shipping to Japan or Germany (where i usually hang out) might be even more expensive...


I'd be interested in feedback for these Richard boards and other homemade stuff. Not so really interested in C-128 as I always used mine on a TV Set (Works fine. At least for the German C-128D...I swear!), but for CGA these converters would be great.
 
We in Europe have for years had the luxury of RGB on SCART. Most TVs after 2000 support 60Hz as well, and some of these, typically the upper-range CRT ones, are optimal for retrocomputing. SCART is unfortunately on it's way out, and there are no good option for HDMI.

Most retrocomputer-to-SCART adapters relies solely on a resistor (typically 220 Ohm) on every signal to get "analog" video. If you want intensity bit with dark yellow you'll need a resistor network instead. On the other hand, if you want brown instead of dark yellow the best way is problably to throw in some logic before the resistor network. You'll problably need a logic gate in there anyways to get the requiered CSync.
 
I have the Andoer GBS8200 connected to the DAC from here: http://www.bit-c128.com/ running on a 5150 with the ibm cga card. It is looking ok but not perfect. I have a few questions and then I'll post photos.
1) What do I do with the intensity pin? It is currently unused.
2) What would be a good color test including the brown that has been mentioned?
3) Is there a good game or software to demo it?

Larry G
 
retrogear said:
1) What do I do with the intensity pin? It is currently unused.
Click to expand...

It should be connected to the DAC. C128 has the same RGBI TTL interface as CGA does, so you should connect intensity as well, to pin 6.
Pinout is here: http://www.allpinouts.org/index.php/CGA

retrogear said:
2) What would be a good color test including the brown that has been mentioned?
Click to expand...

CGACAL seems to be a good one for that: http://www.oldskool.org/pc/cgacal

retrogear said:
3) Is there a good game or software to demo it?
Click to expand...

I like to use GP-01 myself, it contains brown: https://scalibq.wordpress.com/2015/08/09/fixing-genesis-projects-gp-01/
 
Thanks for the tips. I don't see Intensity listed on the input to the DAC so no place to attach pin 6.
I've emailed Syncopated Systems. I received no documentation with either device, typical in today's online culture. Couldn't find much online either.
Finally got the GBS-8200 menu switched from Chinese to English. I've got it displaying fairly well with IBM 5150 CGA.
The white text looks ok but I'm getting some slight fringing and bleeding on the colors in the high frequency transitions.
Once I know all signal lines are accounted for I'll go after the video with my oscilloscope and see if I can determine where the distortion is coming in.
This stuff is all just bread boarded so I suppose noise could get in that way too.

Larry G
 
Hi,

today i builded this circuit: http://www.insentricity.com/filedb/7/1/7417.png (http://www.insentricity.com/a.cl/219/cgargbi-to-analog-for-the-commodore-128)

01a0229d64174fcaa0eec94deb702863.JPG


I tested it with GBS-8200 VGA video converter and Taiwanise CGA clon:

e7e80ab3af434509a45b9ada0e959824.JPG


All together:

0e2baad03f7c4001893f1c5069aa18b5.JPG


CGAPAL ( http://www.oldskool.org/pc/cgacal ):

e41d753586a749aa9fb0ff1dbbf6f199.JPG


CheckIt video test:

603c7d4ad96345f8ad4dd6c39eecebb8.JPG


So, quality is acceptable in my opinion ;)
 
Scali said:
Yea, image quality is good. Just needs a small tweak to get brown :)
Click to expand...

Ooops! I make mistake when soldering, i connect A0 and A1 from 74LS138 to GND, but it need be connected to VCC according original circuit. I will correct the soon and make new image of CGACAL :p
 
> I don't see Intensity listed on the input to the DAC so no place to attach pin 6.

Forget what I said about intensity. Yes, Scali is right. It is handled internally by the DAC.
I'm thinking ahead to my Zenith Z100 which has no intensity pin. I read somewhere to tie it high on the input to DAC.
More to come ...

PS - Tronix, thanks for your links. Now I see why C128 DAC has a jumper to combine H and V sync.

Larry
 
What about adding a switch to A0 or A1? Then you can easily swap between brown and dark yellow :)
 
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