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Thread: Power connector on IBM PC 750 (6885) riser card

  1. #1
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    Default Power connector on IBM PC 750 (6885) riser card

    Hi. Does anyone know why the riser cards on IBM PC series computers have a power connector? Unfortunately I've had 3 of these computers, but I don't remember if the power supply was plugged into them by default. I seem to remember that they were never connected, and I just hooked up the power supply to them because it fit... I don't know if this power connector can be found on the smaller IBM PCs, or on the newer ones from the late 90's.
    • The only computer from my childhood that I'm missing is an Olivetti M380-40... Please help me find a solution to this problem
    • Looking for pictures of the following Olivetti computers: M380 W, M380 XP4, M400-60, M480... Complete list: http://pastebin.ca/3629976

  2. #2

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    I don't have the 750 but have the 350/330. The Power connector is a 3.3V one for the PCI bus:
    DSCN1940.jpg

    The image shows one 6-pin connector connected. for some reason the 330 PSU's have two of the 3.3V leads, and only one connector on the riser. In the picture the second one is tucked under the one that's connected.
    Looking for: OMTI SMS Scientific Micro Systems 8610 or 8627 ESDI ISA drive controller, May also be branded Core HC, Please PM me if you want to part with one.

  3. #3
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    That's for the PCI, OK.

    That's strange. On the PC 750 there's a connector on the motherboard for that cable that's unused in your system. Again I don't know what it's for. I guess some circuitry uses 3.3V . You can see on your motherboard that there is a space for it, along with some circuitry but nothing was installed there. Maybe on some versions of the board this was installed, or maybe IBM used the same power supply for all 330/350s regardless of the motherboard inside.
    • The only computer from my childhood that I'm missing is an Olivetti M380-40... Please help me find a solution to this problem
    • Looking for pictures of the following Olivetti computers: M380 W, M380 XP4, M400-60, M480... Complete list: http://pastebin.ca/3629976

  4. #4

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    I would agree with you. I think perhaps the board could have the 3.3V circuitry powered directly by the PSU, or via a regulator. I'm not that familiar with the 750, the 330/350 were 3.3V socket 5 Pentium systems based on the 430FX chipset. I don't know for sure but the motherboard looks very much like it's an Intel OEM one.

    The first time I encountered this connector was on the Intel socket 5 430NX Premier PCI II. This is the socket 5 version of the socket 4 Premier PCI 'Batman' motherboard. However It is also present on the 5V Batman motherboard. The manual for that states that it exists to maintain strict compliance with the PCI spec. So 3.3V has to be there but AFAIK unused by any PCI card until the late '90s. In today's parlance it is a "5V PCI slot".

    Here is what I think happened. I don't have any proof, just a guess. PCI and the Pentium were introduced in '93, with PCI having 3.3V in the spec. Shortly after the Pentium went to 3.3V. If you have 3.3V components it would make sense for the PSU to produce that voltage. However you currently have the AT PSU connector as an established standard. So why not design a board that as an interim can do 3.3V via a regulator, for maximum AT comparability, but could also work without a regulator, and have 3.3V come right from the PSU. Then go full 3.3V and specify a new PSU standard. This line of thinking did exist because they came out with ATX in '95 I believe. But then, the plan kinda went off, as the MMX CPU's used the split voltage, additionally you had the clones super socket 7 with all sorts of voltages.

    Another board I have is the Intel (430FX) Advance/AS. This is an AT board with some integrated video. It was found in Dell's p75T and p90T. In the Dell, instead of the standard AT connector it has an ATX-like connector, that's not pin compatible. I had originally thought it was a DELL being DELL. However It does look like an Intel idea, as non-Dell variants have the solder pads for the connector. If you remove the connector, you can solder in a standard AT connector.

    I *think* that 6-pin 3.3V connector is the same pinout for that in the ATX 'AUX' connector.

    Also Intel, had been adding additional PSU connectors to their motherboards since their first 386 motherboard. They've tried more than once to update the AT standard, and by extension motherboard design in general. You can take this FWIW, but all these little oddities look to me, like the pre-history of the ATX standard. There are some additional modifications Intel made, like with their xPress server platform. This is currently my best guess as to all this, but there is very little to go on really.
    Looking for: OMTI SMS Scientific Micro Systems 8610 or 8627 ESDI ISA drive controller, May also be branded Core HC, Please PM me if you want to part with one.

  5. #5

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    The IBM PS/ValuePoint P60/D has an additional power connector on the MOBO as well.

  6. #6
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    The PC 750 is also a 3.3V socket 5 Pentium system but it has more features, such as onboard audio and IBM's SelectaBus technology so its motherboard is bigger, around the size of an IBM PC, and it is based on the VLSI Wildcat chipset. It also has a lot of custom and programmable chips unlike your board which seems to only have the BIOS.

    Unfortunately these computers, the IBM PC series, are barely documented at all on the internet, you need to look there and here and over there for info. A website with pictures of every boards, models and variants would be awesome.
    • The only computer from my childhood that I'm missing is an Olivetti M380-40... Please help me find a solution to this problem
    • Looking for pictures of the following Olivetti computers: M380 W, M380 XP4, M400-60, M480... Complete list: http://pastebin.ca/3629976

  7. #7
    Join Date
    Feb 2015
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    Québec, Canada
    Posts
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    PS/ValuePoints are weird. IBM created them as a new series of PC-compatibles before they came up with the lower-end PS/1-successor Aptiva line and higher-end PS/2-successor PC line. But they were still being made after IBM launched those two computer lines. They're sort of a strange before and in-between line of computers...
    • The only computer from my childhood that I'm missing is an Olivetti M380-40... Please help me find a solution to this problem
    • Looking for pictures of the following Olivetti computers: M380 W, M380 XP4, M400-60, M480... Complete list: http://pastebin.ca/3629976

  8. #8

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    Indeed. They were discontinued after the Pentium 60. I captured some information on the various models on my blog. The most common models are the various 486 25 and 33 models. Occasionally a 386SLC or 486 DX2 will surface. 486 DX 4 and Pentium models rarely appear.

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