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Thread: CGA Snow

  1. #1
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    Default CGA Snow

    I know CGA snow has been covered on the forum before, but this is a little different. I have a 5160, 64 to 256K motherboard that I use on the bench for testing purposes. I took it out this week to test some hard drives, and I now have snow in the CGA display all of the time. It's there as soon as the display initializes.

    I was using an original IBM CGA card, so I switched to an old Hercules CGA card I have and the constant snow remains. An MDA card, or a VGA card have no snow. It wasn't like this the last time I used it. Landmark diagnostic ROM found no issues. IBM advanced diagnostics didn't find anything either. When I run the memory test on the IBM diagnostics, you can see the patterns in the snow changing as the test progresses. I am kind of stumped on where to look for the problem

  2. #2

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    VGA is so much easier on the eyes I can't imagine why you would even consider CGA as an option for doing HD bench work.
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  3. #3
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    Default

    Originally I setup an old Tandy CGA monitor to test my collection of IBM CGA cards, and the test jig has just remained that way. I don't have to stare at the display very long to run these kind of tests, plus this way I don't have to dig out a CGA or monochrome monitor to use the Landmark ROM.

    My real concern is understanding what happened to the motherboard to cause this.

  4. #4

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    Quote Originally Posted by fatwizard View Post
    My real concern is understanding what happened to the motherboard to cause this.
    I figured that.

    FWIW, I don't bother with CGA, EGA and VGA monitor switching on the bench -- I use a multisync -- one monitor for all seasons.
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  5. #5
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    Quote Originally Posted by Stone View Post
    FWIW, I don't bother with CGA, EGA and VGA monitor switching on the bench -- I use a multisync -- one monitor for all seasons.
    Not everyone HAS one of those.

  6. #6

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    I don't see why not.

    I've been selling them here for over ten years.
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    If you're looking for DS/DD or DS/HD 3" or 5" floppy disks, PM me. I've got some new, used, and factory over-labeled disks for sale.

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  7. #7
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    Default

    Quote Originally Posted by fatwizard View Post
    I know CGA snow has been covered on the forum before, but this is a little different. I have a 5160, 64 to 256K motherboard that I use on the bench for testing purposes. I took it out this week to test some hard drives, and I now have snow in the CGA display all of the time. It's there as soon as the display initializes.

    I was using an original IBM CGA card, so I switched to an old Hercules CGA card I have and the constant snow remains. An MDA card, or a VGA card have no snow.
    Is it really as soon as anything starts displaying or is it as soon as an OS loads?

    Snow normally only happens if something is accessing video RAM without taking early CGA cards in to consideration. My first thought would be a TSR of some kind, for example an on-screen clock.

    If it starts before the OS, then are there any odd ROM BIOS extensions in the machine?

    Did the snow appear while running the landmark diagnostic ROM? That would be really weird.

  8. #8
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    Default

    Yeah, this isn't normal. The snow is there as soon as the video card initializes and is more or less constant. I was using an MDA card when I ran the Landmark test, so I just repeated the test with CGA and the snow is there. The rig is stripped down to just power supply, motherboard and video card. The board appears to be completely stock. I don't have another 5160 board loose to test, but all of my CGA cards work fine in my 5150 motherboard.

  9. #9

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    Quote Originally Posted by fatwizard View Post
    Yeah, this isn't normal. The snow is there as soon as the video card initializes and is more or less constant. I was using an MDA card when I ran the Landmark test, so I just repeated the test with CGA and the snow is there.
    If that testprogram is constantly writing to the screen(memory) without checking for retrace then you will have snow on CGA

  10. #10
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    Default

    Quote Originally Posted by gertk View Post
    If that testprogram is constantly writing to the screen(memory) without checking for retrace then you will have snow on CGA
    No, there is snow when the system first puts a cursor on the screen. With the original ROM or Landmark. Software isn't involved. I have been fiddling around with it tonight and have discovered that putting the CGA card in the slots nearer the center of the board, the worse the symptoms become. When I put it in the 7th slot I get big static purple dots and garbage in the memory count display. If I put a memory expansion card in the system I get 1 long and 2 short beeps (video), but the screen still comes up, but with big dots and garbage.

    The VGA card works fine in any slot (leaving slot 8 out).

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