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Thread: VESA Local Bus Graphics Cards on PC XT?

  1. #1

    Default VESA Local Bus Graphics Cards on PC XT?

    Was wondering if a VESA card will work on 8 bit pc xt? For example, Trident TGUI9440, ATI Mach 32?

  2. #2

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    Short story: nope. not even a little bit.

    Long story: it might be theoretically possible, but not without a whooooooole lot of custom hardware you'd have to design and fabricate, and software you'd have to write.

  3. #3

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    For a bit more context, PCs starting with the original IBM PC have had a "bus" technology that was used to connect expansion cards to the motherboard.

    https://en.wikipedia.org/wiki/Bus_(computing)

    The bus used by the IBM PC XT is the ISA bus. https://en.wikipedia.org/wiki/Indust...d_Architecture

    The bus defines a ton of parameters about how an expansion card connects...everything from the physical dimensions of the card edge connector and how many pins it has to the mechanism for communication. These parameters are completely different for VLB cards and it really would take a massive engineering effort to somehow adapt a VLB card into a PC XT.

    https://en.wikipedia.org/wiki/VESA_Local_Bus

  4. #4

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    You guys are ignoring the crucial fact that all VLB cards also have an ISA edge connector on them. The real question then becomes whether any of these cards are capable of communicating with the host system through the ISA slot only, when the VLB extension is left unconnected.

  5. #5

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    Quote Originally Posted by bakemono View Post
    You guys are ignoring the crucial fact that all VLB cards also have an ISA edge connector on them. The real question then becomes whether any of these cards are capable of communicating with the host system through the ISA slot only, when the VLB extension is left unconnected.
    If I had to bet money on it, I'd still say no, but I do think this was the spirit of the OP's question. Similarly to how some 16bit ISA cards work with only the 8bit edge connected

  6. #6

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    Quote Originally Posted by bakemono View Post
    You guys are ignoring the crucial fact that all VLB cards also have an ISA edge connector on them. The real question then becomes whether any of these cards are capable of communicating with the host system through the ISA slot only, when the VLB extension is left unconnected.
    I actually didn't realize that! I was answering the question assuming a low level of knowledge of the issues but in fact iit looks like the question kind of went over my head with a whoosh

    Given how few 16bit ISA cards actually really work in the XT reliably based on Modem7's testing my guess is still that this almost certainly doesn't work but it's a more interesting question than I realized.

    I can see the reasoning behind why some VGA card makers would have wanted to provide a 1 size fits all card that supported both 8bit and 16bit ISA machines.

    It's much harder to imagine manufacturers going to the trouble of creating VLB cards and then making them backwards compatible even with 16bit ISA, let alone 8bit ISA.

    Very interesting question though!

  7. #7

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    The answer is still no.

    The only thing you'll find wired on the ISA part of the card edge connector is power.

  8. #8

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    Quote Originally Posted by bear View Post
    The answer is still no.

    The only thing you'll find wired on the ISA part of the card edge connector is power.
    Thanks to everybody's answer but I don't think the ISA portion contributes only to the power supply.
    s-l1600.jpg
    s-l16001.jpg

  9. #9
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    Default

    If I had to hazard a guess based solely on how the traces run I'd say the BIOS ROM is attached to the 8-bit ISA stub. (For performance the card probably relies on motherboard BIOS shadowing to copy it to faster RAM.) I see the data lines going to a buffer to the right of the chip and to the left is a chip who's printing I can't read but could be the address decoding for it. Feel free to poke around with a continuity tester and see if the 8-bit data lines go anywhere else; I see them running off to what looks like VIAs that terminate in empty pads for something surface mount near the back of the card, but I don't see them going towards the main "video portion" of the circuitry. Granted, all bets are off if that's more than a 2 layer PCB.

    Edit: This might be proof that the video portion can't be driven via ISA: Pin B11 is "MEMW", and if I'm counting correctly on the picture of the back of the card that's not populated. "MEMR" is, which would make sense if it's only capable of *reading* the BIOS ROM through the legacy connector.
    Last edited by Eudimorphodon; March 11th, 2020 at 10:44 AM.
    My Retro-computing YouTube Channel (updates... eventually?): Paleozoic PCs

  10. #10
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    Default

    it's theoretically possible for card that has separate VGA chip from GUI accelerator. but why bother? 16 bit VGA cards are (somewhat) plentiful.

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