What is this socket for on 386 motherboard? I have no manual, no documentation and no resources readily available for this machine (Commodore C386SX-LT).

C386SX-LT Motherboard

Source: Probably oldcrap.org


Cropped close-up of socket and (probably) related circuitry

Source: oldcrap.org

Many additional pictures can be found here.

  • 1
    Cool, but what's the question here? Are you asking if this FPU will fit? Or wich you need? either way you may want to check the manual - or at least provide some more info so other can look it up.
    – Raffzahn
    Commented Jun 3, 2020 at 23:16
  • 1
    Could you post another photo without annotations? There's a label that probably says what goes in the socket but it's obscured by the orange line. Commented Jun 5, 2020 at 12:31

1 Answer 1


Well, it could easy be a socket for a 387 type FPU. Size and number of pins would fit a 387 (or some pin compatible Cyrix FastMath) as PLCC carrier. On the other hand it's rather unusual to place it far from the main CPU, seen in the lower left.

But without more information it's hard to say. Maybe some sharp close up can reveal markings supporting this?

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    Specifically, the label (obscured by the hand-drawn orange line) above the "(MEM-PCB-POST)" label probably says what chip is meant to go in that socket. Commented Jun 8, 2020 at 23:01
  • @AlexHajnal yes, quite plausible, I had the same feeling. There is only one other emphased by a box, which seams to mark the memory expansion add on. That's why I asked for a close up.
    – Raffzahn
    Commented Jun 8, 2020 at 23:31
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    I think the board probably supports two 2MB RAM daughterboards ("2M-1" and "2M-2") each connected via a 2×20 pin header and two 2×2 "(MEM-PCB-POST)" headers. Looking at the back side of the board (see the link I found) it looks like that socket in the question ties in to memory bus. Given that the socket is for a PLCC-68 (which is what the 387-SX uses) it's probably for an FPU. IOW, the obscured label was a red herring. Commented Jun 9, 2020 at 8:28
  • @AlexHajnal Sounds plausible.
    – Raffzahn
    Commented Jun 9, 2020 at 8:28

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