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Thread: CGA Snow

  1. #21

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    Quote Originally Posted by SomeGuy View Post
    To me that doesn't look like "snow" snow.
    Now that we can actually see the problem it doesn't look like snow at all, not even a little bit.
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  2. #22
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    When I see things like this, I usually guess that it's increased parasitic capacitance, or parasitic resistance, or both, somewhere. I know that's not helpful, but I've never been in a situation where I thought it was worth trying to narrow it down though. Usually I'd just pitch the offending board and replace it.

    Bad RAM caused this:


  3. #23
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    And a bad connection (effectively high parasitic resistance causing floating levels) caused this:


  4. #24
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    It absolutely does look like snow, as well as random junk being written into CGA RAM (which is what is causing the snow).
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  5. #25

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    So my thought is that a part of some address decoding logic, that would normally keep writes away from the CGA framebuffer addresses, that is defective. The first guess would be the decoder on the CGA board, but you've already said that works normally on another motherboard. The good news is there's not much on an XT motherboard that would potentially be causing this -- DMA, DRAM refresh? I'd have to look at a schematic to do more than take a (possibly barely) educated guess at what. This seems like a Sherlock Holmes quality clue, but it wants interpreting by somebody who's been deeper into the guts of the 5160 in more practical and less theoretical terms than I have.

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