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Thread: replacement for 80286

  1. #11
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    Same h/d as the model 25? Which was designated ide but wasn't "real" ide, as in 16 bit ide. Invented by Compaq if I'm not mistaken.

  2. #12
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    I acquired a 1rst gen 5170 motherboard (512K stacked chips/6 MHz) that worked fine, but I didn't have a 5170 case to put it in. Later I got my hands on a whole 5170, but it's motherboard was damaged by battery leakage, and I could never quite get it working right. This system had the next gen motherboard with the 8 MHz CPU. The crystal is socketed on these boards so I pulled the 8 MHz CPU and crystal out of the damaged board and installed them on the 1rst gen board, then I built up the system around that. I've read that some of the support chips on the 1rst gen motherboard can sometimes be unreliable past 6MHz, but this one works flawlessly at 8MHz.

  3. #13

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    Has anybody ever tried to replace a 80286 with an 80386-SX?
    With kind regards / met vriendelijke groet, Ruud Baltissen

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  4. #14
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    Quote Originally Posted by Ruud View Post
    Has anybody ever tried to replace a 80286 with an 80386-SX?
    Is that even a thing? Even though they both use a 16-bit bus I would assume that there are enough differences that you just can't pop in a 386SX and have it work. There are the likely differences in pin out, communications with the bus and BIOS support to name a few off the top of my head.

  5. #15
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    They sold 286-386sx upgrade boards that plugged into the CPU socket and basically had no glue on them so, yes, the 386sx is directly compatible from an electrical standpoint. (And while there are minor BIOS issues they can be worked around with drivers.) The main problem is Intel never released the SX in a physically compatible form factor; it *only* came in a surface mount flat pack, so you need an adapter board to put it in a 286 socket. To make it harder 286s actually came in several socket types, so you need a board that fits your particular motherboard.
    My Retro-computing YouTube Channel (updates... eventually?): Paleozoic PCs

  6. #16

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    more to the point: why? clock for clock the 386sx is not faster than a 286, so unless you need to run 32bit code it's not gaining you anything

  7. #17
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    Quote Originally Posted by AndyM1981 View Post
    Is that even a thing? Even though they both use a 16-bit bus I would assume that there are enough differences that you just can't pop in a 386SX and have it work. There are the likely differences in pin out, communications with the bus and BIOS support to name a few off the top of my head.
    Not all that much difference. Some of the very early 386SX computers used 286 motherboards with a tiny daughtercard to hold the 386SX. Everex was one that did that in the rush to get a 386SX out the door.

    Note that some of the upgrade boards included an oscillator allowing for a nice clock speed boost.
    Last edited by krebizfan; March 26th, 2020 at 07:12 AM.

  8. #18
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    Quote Originally Posted by maxtherabbit View Post
    more to the point: why? clock for clock the 386sx is not faster than a 286, so unless you need to run 32bit code it's not gaining you anything
    The one reason I can think of for it is if you have a 286 with a lot of extended memory you’d like to be able to use as EMS, or to run multitaskers like Desqview.

    Note of course that *most* 386sx upgrade boards sold commercially did have more than just the CPU on them because they often included some cache memory and a circuit to allow the CPU to run faster than the bus, but a very few “feature only” adapters that ran the CPU at the native speed did exist. It would be sort of amusing to have one of those in an original 6mhz AT.
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  9. #19
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    Very interesting! I had no idea that the 386SX was that close to the 286 electrically. Thanks for the interesting lesson all!

  10. #20
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    It's kind of strange in retrospect that when clone CPUs from AMD and Cyrix became a thing that nobody released a 386sx variant in PGA and PLCC packages to directly plug into 286s. (From a strategic standpoint it makes sense that Intel didn't, because they really wanted to push new computer sales.) Cyrix in particular made internally clock-doubled 386sx clone CPUs that they could have packaged as plug-and-play boardless upgrades.
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