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5155 CGA not outputting color on composite

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bhtooefr

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Dec 9, 2008
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Newark, OH, USA
Just got a 5155, and noticed something weird when trying to use an external composite monitor (a Dell 2001FP). Text-mode software that's supposed to display in color isn't displaying in color, even after issuing a MODE CO80 manually to be sure. Haven't gotten a chance to run any graphical software (need to get a serial connection set up to copy files off and on), but I wouldn't be surprised to get the same results.

Is CGA's signal too dirty for my Dell 2001FP (which would surprise me, given how the 2001FP can take some downright nasty retrocomputer signals and work well with them), and it's refusing to acknowledge the color burst, or is something else going on that I'm missing? It is worth noting that the system doesn't have an IBM planar, it's got a VIP TXM/II-10 (a turbo XT clone board), although I'll be putting an IBM planar in it for originality's sake.
 
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80-column mode is not in colour on CGA by default, because the signal timing isn't stable enough if colorburst is enabled.
You can manually enable it (which we do in 8088 MPH), but regular software won't do that.
You'll need to test 40-column textmode or graphics mode to see if your colour output works at all.
 
Ah, user error, got it.

Interestingly, I'm getting periodic (2 Hz or so) momentary (maybe 1 or 2 frames) flashes of color burst when in a mode where it's actually activated, but otherwise mono. I think the right colors are there, just not displayed for long. That could be the LCD not liking the signal, or something.

8088 MPH detects the 10 MHz clone motherboard as 4.77 MHz, and runs properly other than the (unrelated to the demo) color problems (meaning it really is in a 4.77 MHz mode). It is an IBM CGA card, though.
 
bhtooefr said:
Interestingly, I'm getting periodic (2 Hz or so) momentary (maybe 1 or 2 frames) flashes of color burst when in a mode where it's actually activated, but otherwise mono. I think the right colors are there, just not displayed for long. That could be the LCD not liking the signal, or something.
Click to expand...

Yes, that sounds like it.
On a real IBM 5150/5155/5160 motherboard, there's an adjustable capacitor to fine-tune the 4.77 MHz clock frequency. Since the CGA card is also clocked off this crystal, it effectively fine-tunes your composite NTSC signal and therefore the colours.
You could check to see if your motherboard has this as well, and try to adjust it.

bhtooefr said:
8088 MPH detects the 10 MHz clone motherboard as 4.77 MHz, and runs properly other than the (unrelated to the demo) color problems (meaning it really is in a 4.77 MHz mode).
Click to expand...

That could well be, most 'turbo' clones start up in 4.77 MHz mode, and need to be switched to the higher speeds via a key combination or a simple commandline tool.
 
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