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CGA->VGA Gauging Interest

T

twistedpneumatic

Experienced Member
Joined
Apr 9, 2017
Messages
380
Location
N Bellmore, LI
I was thinking about making a CGA->VGA adapter that gave the full 16 colors with brown capabilties(instead of dark yellow) for MULTISYNC monitors.(basically an improvement over those 9$ CGA->VGA 8 color cables.) Any thoughts? I just want to know if there is enough interest to go through with making them.
 
This is already a project in progress -- search this board for "retro canada cga mda ega vga" or similar. This has extra smarts to line-double and convert the signal.

For a "straight through" converter that will require a monitor that can do 15.7KHz horizontal, the best I've found is the one made by gglabs.
 
I was talking about a cheap converter that was made of passives for like $5, instead of $55. Yes, Retro Canada's board is a major improvement, I just wanted a project to test my skills and use it for something useful.
 
Can be done with a resistors, but you'll need a logic IC for brownfix. I can't find the circuit online, but they just used a 74 series IC.
I used a CPLD on mine as it gave me better control over each colour and let me generate CSync. But that ruins the simplicity/low cost idea (and the guys on the Commodore forum made and sold a similar product anyway).

Edit: as for interest, if you could make it low cost and the results weren't too bad, I think it'd do OK. The hard part will be getting the colours as accurate as you can. Bonus points if you can fit it in to some kind of DB9 to DB15 connector shell so there's no PCB flapping around or big box!
 
syncs are reversed, so you need a 74ls04 at least.

The brown color can be multiplexed through a 74ls138 and a 74ls245 buffer to select the output: if not brown use the RGBI line, otherwise use the Brown from the output of 74ls138, I think you need to negate it (usin the spare 74ls04 gate).

Plus 500 ohms for each of RGB and 1K resistor for the I component (should be added for each individual)

But you need extra logic for example, the I can only influence the active lines like RGBI 1101 you can't have blue so you will need 3 AND ports so a 74LS08...

The sync lines needs a 47 ohm each otherwise some monitors will not accept them.
 
I was thinking a 2 bit DAC resistor ladder on RGB with a voltage divider to convert the 5V to 0.7V. Then a triple AND gate with 2 not gates to shutoff the intensity bit on green if the input is RGBI-1100 and then you can use the rest of the NOT gates from the 74LS04 to drive the inverted sync. Add a few passives for the monitors to be happy and were good.

I want to do this as a little project I could share with the community, because Retro Canada is clearly the more knowledgeable and skilled person in this equation.
 
@Retro-canada, where to the 49ohm resistors go, to ground or to +5V. Can I run the 74LS chips off pin 9 which is supposedly +5V? Anyway, this is what I threw together, how does it look. I haven't calculated the values for the voltage dividers. It should be like 6.1K to 1K top to bottom respectively.

https://imgur.com/e98daje
 

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