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Thread: 486 motherboards with ISA, VLB and PCI

  1. #1

    Default 486 motherboards with ISA, VLB and PCI

    Apparently, according to wikipedia, these exist.
    Havenít been able to find any. Either its PCI/ISA or ISA/VLB.

    Anyone know any particular models to look out for?
    Looking For:
    IBM PS/2 8595 Motherboard
    MCA sound card (will pay big bucks)
    IBM PC Server 720/others

  2. #2
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    Such board configurations are uncommon because of design limitations and it didn't make sense to have both at the same time.

    VLB is basically the 486 bus extended out to a slot, while PCI is its own bus architecture independent of the CPU. But what they have to share in common on these early PCI boards is the 486 bus clock, there are no dividers. Driving both a VLB card and several PCI devices can cause stability issues. The same goes on VLB only boards, driving multiple cards can cause bus glitches, which is why motherboards never had more than 2 or 3 VLB slots.

    Another issue stemming from the PCI and VLB clocks using the 486 FSB clock is that the 486 had several different bus speeds. VLB could tolerate 25-40 MHz fairly well, but PCI expects 33 MHz. While it will generally work at a slower 25 MHz with reduced performance, 40 MHz is alarmingly out of spec and 50 MHz used on the DX50 won't work at all.

    I do know of one board which has both VLB and PCI, the PC-CHIPS M919. I had one years ago, it was really an awful board but it worked decently enough once you worked out the quirks. The cache chips (if present) are usually fake bricks of plastic with leads sticking out of them. They're usually embossed with "WRITE BACK CACHE" on the top in big letters with no part number or manufacturers mark. There is a cache slot which looks like a COAST module slot used on later Pentium systems, but it is not. PC-CHIPS used the same connector, but made their own proprietary cache card and they're pretty rare today. Don't attempt to install a COAST module, you'll blow something up.

    A couple of redeeming features are that it can support an enormous amount of RAM and overclock well. If you can source 64 MB SIMMs, you can have up to 256 MB of RAM in this board. I had an Am5x86-133 running at 160, but I had a VLB video card because the PCI slots were running at 40 MHz.

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