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video card with intel 8275 having rolling picture

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nimeshasilva

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Hi all.

I recently got a project to replace the old CRT display of a machine with an LCD. the output is monochrome composite signal. the problem is that, when I connect it to the LCD's composite input, the picture keeps on rolling down. the horizontal part is ok though.

and the weird thing is that, the picture is ok for the old CRT, the problem only comes with the LCD. the LCD is also working properly with the other composite sources (for ex: composite out from the TV)

the video card is made with the 8085 processor and the intel 8275 CRT controller.

what might be the problem? can anybody please suggest a solution? I can modify the dot/timing logic if necessary. but no idea where to start.

thanks in advance
 

"Rolling down" means no vertical sync takes place, either sync signal hasn't the right level or vertical frequency isn't supported by the monitor.
 
thanks FvM.
I looked at it using an oscilloscope. the vertical pulses are present and the level is similar to the Hsync pulse.
so it might be a frequency issue as you've mentioned..

Any idea to correct it??

Thanks
 

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