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Thread: IBM 5155 graphics issue.. Any clue how to solve?

  1. #1

    Default IBM 5155 graphics issue.. Any clue how to solve?

    http://i.imgur.com/qkKRh68.jpg

    (referring to the dashes on the screen)
    This only happens when a lot of motion is occurring on the screen - telnet star wars in this example.
    Also happens when I play a PAC-MAN clone or similar.

    Is this fixable, and if so, how?

  2. #2
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    As IBM supplied it, the 5155 contains an IBM CGA card. The IBM CGA card suffers from something known as 'CGA snow', and your photo appears to show CGA snow.

    From http://www.seasip.info/VintagePC/cga.html: "The CGA uses single-ported RAM, which means that if the host PC is reading or writing screen memory, the CRT controller can't. This manifests itself as "snow" - little white rectangles (like extra cursors) briefly appear at what look like random places on the screen. Many CGA clones use dual-ported RAM and don't suffer from "snow."

    A YouTube video showing CGA snow is at https://www.youtube.com/watch?v=_GGw_HvmK7g

    A different CGA card is the answer, one that does not suffer from CGA snow, and one that has the Berg connector on it for the 5155's internal CRT unit (see [here]). Someone on these forums may be able to point you to specific cards that meet those criteria.

  3. #3
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    To be more specific, this snow-issue only manifests itself in 80-column textmode.
    Here is my 5160 machine with a real IBM CGA card running a demo that uses 80x50 and 80x100 tweaked textmodes: https://youtu.be/XxAdJpyZ0HM
    You can clearly see the snow whenever the CPU accesses the videoram (eg with the scroller you can see that it only acceses it at the top of the screen, to update the scroller, at the bottom it just waits for the next frame).
    This is the same demo on a clone with an ATi Small Wonder card: https://youtu.be/2O_-sb0purg
    It has no snow, because I believe it uses SRAM instead of DRAM (which is another solution to double-ported RAM).

    In 40-column textmode or any of the graphics modes, you should not see this. So if those work fine, then everything is normal.

  4. #4
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    It's worth noting that the newest card that has the Portable display connector would be, I believe, the ATI EGA Wonder 800 (the original 1987 version). That would give you CGA modes (in 64 shades of gray on composite, though, not in color, apparently - and half the fun of CGA is being able to run color composite modes), as well as interlaced MDA text, EGA, Hercules, and 132 column text modes. There's other suitable cards as well that are CGA instead of EGA, of course, but that's as new as you can go.

    Of course, anything with a composite output and support for 8-bit ISA could be adapted.

    Personally, I'd just deal with the CGA snow, keep it original.

  5. #5

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    Yeah, after doing some further research myself I determined that it was in fact CGA "snow". I was just concerned that it may be a failing capacitor of some sort. I didn't quite understand why a monochrome computer shipped with a CGA adapter, but I also discovered it has an external monitor out, for those times when you're not "portable".

  6. #6
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    I can confirm the ATI EGA Wonder 800 works correctly in this configuration. "Julian" in the Chicago Classic Computer Club outfitted his 5155 with it and demonstrated it at VCF 8 (2013). Ran Commander Keen as an EGA test, which demonstrated 16 shades of of amber in 320x200x16 mode.
    Offering a bounty for:
    - Documentation and original distribution disks for: Panasonic Sr. Partner, Corona PPC-400, Zenith Z-160 series
    - Music Construction Set, IBM Music Feature edition (has red sticker on front stating IBM Music Feature)
    - Any very old/ugly IBM joystick (such as the Franklin JS-123)

  7. #7
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    "Snow" is caused by IBM designing a cheap memory implementation and the program writing directly to the adapter's memory rather than by using the BIOS - but who does that! IIRC, it can be worked around by only writing to the screen memory during fly-back.
    Last edited by Beerhunter; May 24th, 2015 at 08:56 AM.

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