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Hyperfrog
April 13th, 2010, 01:53 PM
I have a CGA/MDA graphics card that I set to CGA. A color composite monitor is connected to the RCA output jack labeled "COLOR". On the motherboard (XT), switches 5 & 6 are set to ON and OFF respectively (Color 80 x 25), while the switch on the graphics card is set to CGA (the other position being MDA).

It works fine, but color is disabled in the 80 column text mode, i.e. colors are displayed as levels of gray. Why is that so? I do have color when I play games. I also have color when I set the DOS display MODE to CO40.

Here's a picture of my card: http://radikal.ru/F/s54.radikal.ru/i145/1001/08/ea8b50495a19.jpg.html. (Well, this is not my card, but they're the same.)

How can I enable color output in the 80 column text mode?

In addition, if you look at the picture, you will notice there are two 41464 memory chips (64k x 4), which means the card has 64kB of video RAM. Why is that so? CGA needs only 16kB. Could this card be able to emulate Hercules graphics? Or Tandy graphics?

Thanks,

Christian

Hyperfrog
April 23rd, 2010, 01:26 PM
I found the answers I was looking for. It is normal for a CGA adapter to disable the colour burst signal on the composite output when an 80-column text screen is displayed and the border colour is set to black. This is mentioned in Wikipedia's article on CGA. I wish this behaviour could be changed, but I haven't found a way to do so.

As to why the card has 64kB of video RAM, this is because it works as an Hercules Graphics Card when the switch is set to MDA.

k2x4b524[
April 23rd, 2010, 02:15 PM
wow, i was about to ask about this card myself, i just got the exact same one at my reseller yesterday
Good to know it's Hercules AND Cga

Do you have any idea what that jumper does?

Hyperfrog
April 23rd, 2010, 02:34 PM
Do you have any idea what that jumper does?

No, but I suspect it changes the refresh rates slightly. (I have another Hercules/CGA graphics card with a jumper that seems to do that.) Or maybe it selects an alternate font from the ROM?