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CGA Card Issue [black and white where should be color]

K

kiyotewolf

Experienced Member
Joined
Apr 15, 2009
Messages
135
Location
Urbana, IL
Got a CGA card off ebay, and it's only displaying B/W composite graphics on the TV.

[IN 16 COLOR COMPOSITE MODE -- TO A TV]

There are two RCA jacks, one with what appears to be B/W video on purpose, and the other, supposedly a color video jack.

Do you have to set a border color or something, to make the color composite mode stick?

I'm using my TSR which twiddles the color burst flag in the 3D8h port, which enables the color burst mode in DOSBOX, but, with my board, it splits the video into four copies of the same image, and much darker.

Maybe my board has a bug somewhere on it?



~Paul

Here's the code I use to make the CGA color burst flag activate..


DEF SEG = 0
a = PEEK(&H410)
POKE &H410, (a AND &HCF) OR &H20
OUT &H3D8, INP(&H3D8) AND 251
DEF SEG

[update]

The Wikipedia article said that a misaligned trimpot for the color might make it not product any color at all.

Looked, no trim pot.

It's a Rev 4 board, the long very old style ones.

Also, there's a jumper, marked "E1" at the end of the board, and the two header connectors near the front for the light pen, and other [i forget?] connections.

[update 2]

http://stason.org/TULARC/pc/graphics-cards/U-Z/UNIDENTIFIED-CGA-Monochrome-CGA-PC-T.html

This is a schematic shaped diagram of what my card looks like.

[update 3]

I think that little E1 jumper enabled the alternate font, I jumpered it. Did not blow up.

I also used Trixter's Tweakmode CGA test program, VGA2CGA.EXE, and it came up black and white.

Maybe this is truely a monochrome card and I'm out of luck.

[update 4]

http://www.ebay.com/itm/IBM-5170-Co...deo_TV_Cards&hash=item45ffb83459#ht_500wt_949

http://www.ebay.com/itm/IBM-6133807...eo_TV_Cards&hash=item2ebb5a4788#ht_4330wt_932

I'm looking at these two cards on ebay, and I can't seem to see a little trimpot on the boards, which worries me.

I don't wanna shell out more money, if these aren't color CGA cards. [color in composite mode]
 
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The trimmer (it's a actually a variable capacitor, not a potentiometer) is on the motherboard, not the CGA card.

If it was a monochrome card it wouldn't produce TV output at all, probably. It is certainly possible that it's broken, though.

If the card is in 80-column mode, you might need to set the border colour to 6 (brown/dark yellow) to get the TV to recognize the colour burst, depending on the particular TV (the signal is a bit out of spec in 80-column mode). This should not be necessary in 40-column mode.

The "splitting the screen into 4" bit suggests that it's not a actually a CGA card at all (or at least not register compatible) - it sounds like its doubling the clock rate instead of setting the color burst bit. Can you post a photograph of the card, as high resolution as possible?

One other thing, probably a silly question but have you checked that the TV is properly displaying colour signals, that you don't have the saturation turned right down or something like that?
 
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Paul, I have a spare working IBM CGA card with verified color composite output, and I can sell it to you for far less than those outrageous eBay prices. Send me a private message if you're interested.
 
@reenigne

Going to try adding code which sets the border color to 6.

@vwestlife

Sent you a PM.

I had to edit my original post so many times. Duh.

I have ebay and my paypal signed up together. Heaven help me.



~Paul
 
[new updates]

When I use my TSR to trigger the color burst flag, not only does it make the Composite video go wonky, but my Tandy CM-11 monitor's refresh rate changes, and it skews the video, so it's angular, and stuff..

The card does work in color with the TGA monitor, (and it looks so sexy..), and it does give crisp B/W text to any monitor that accepts Composite video, (like my equally sexy green screen monitor), ..

When I watch my *.CGA type movies, which are a data stream of SCREEN 2 data, it shows like it would in a DOS Command com fullscreen on XP.. Watchable, but the lack of color is sometimes forgivable.

Haven't yet tried the COLOR 6 TO BORDER tweak, going to add the code for that, so it does a BIOS call for the border when it goes to play the Composite 16 color artifact graphics, and see if it helps.

I'm using a more modern computer than the CGA card was supposed for, maybe the oscillator lines on the motherboard are putting out a color burst, or otherwise other clock frequency, that the card doesn't like?

Or, maybe a bad chip somewhere on the board.. I dunno.

It's a good little grunt work CGA card, does 95% of what I want it to do, .. so..

Now just waiting on the above mentioned CGA card I was offered to buy, which I did.



~Paul

I'll get the scan of the circuit board soon..
 
The 14.318MHz signal on the ISA bus must be at least close to correct because most if not all CGA cards derive their horizontal and vertical sync frequencies from the same clock signal that the color burst is derived from, so if it was too badly wrong you'd get no picture at all. It's possible it's off just enough for the color burst not to be recognized by your monitor though - the trimmer capacitor on my XT motherboard yields a monochrome signal if it's too far off the correct position.
 
Maybe, maybe not. It's certainly something that I could imagine a board manufacturer leaving out - those boards are probably not used with CGA cards driving colour composite monitors very often. I found this and can't seem to see one on there. There's really only one way to find out for sure though - look for a 14.318MHz crystal on the board, the trimmer (if it's there) should be very close by. If it's not there (and the problem is indeed that the frequency is too far out of range) replacing the CGA card probably won't help unless you can find one which has its own color burst oscillator rather than using the one from the bus - i.e. one designed specifically for this situation. I don't know if such a thing exists though.
 
It was not common even for PC/XT clones to have the trimmer. For example, the original Tandy 1000 (a 1984 design) does not, even though it does provide a color composite video output. But by the time PC clones became popular, color RGB monitors were becoming common and affordable, so color composite output (which many clones didn't even have) was less of a priority.
 
I might have to get my PS/1 from my parents house, PS/1 tower, and see if it behaves better inside that computer.



~Paul
 
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