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Intel's Socket 3, used for 486 processors, was a 19×19 pin grid array socket. However, all compatible processors, to my knowledge, used 17×17 PGA packages. What was the point of the extra pins around the perimeter of the socket? Were they even connected to the motherboard at all?

If the pins are actually used by something, what were their functions? All pinout charts I can find online only show the pins of a 17×17 PGA processor.

Cody Gray - on strike
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Hearth
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2 Answers2

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The extra pins were forward-planning, on both Socket 2 and Socket 3. Most of the extra pins are used for power (Vcc) and ground (Vss), which is useful to provide more power to a CPU. The other pins are keys, a new INIT pin (F19), and signals used for enabling and controlling the write-back L1 cache. (See the socket 3 specifications in the 486 family datasheets and the Pentium OverDrive datasheets.)

The only CPUs to use the extra rows of pins are the Pentium OverDrives.

Stephen Kitt
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  • Interestingly, the Pentium OverDrives are listed as capable of working in a Socket 2 board however the competing AMD Am5x86 and Cyrix Cx5x86 do seem to require a minimum of Socket 3, from what I can discern. I'm not 100% clear where the difference arises. – mnem Apr 28 '20 at 17:49
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    @mnem Socket 2 is 5V only. Socket 3 supports 5V and 3.3V (or 3.45V) operation. The processor has a voltage detection pin that is used by some boards to automatically switch the voltage. The Pentium OverDrive has its own regulator, so it does not require a board with a 3.3V regulator. – Michael Karcher Apr 28 '20 at 18:33
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    Technically the 5x86 was supported in Sockets 1 and 2, with replacement VRMs. – Stephen Kitt Apr 28 '20 at 18:41
  • So the outer pin rows were only power and ground? Signals were all located starting from the second row? It must have been a hell to route that on the PCB, especially considering those were through-hole sockets... – dim Apr 29 '20 at 09:12
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    @dim I imagine multi-layer PCBs help a lot there... – Stephen Kitt Apr 29 '20 at 10:20
  • Sure, but those quickly become expensive, especially back then. And there's always this guy from the purchase departement, having much more voice than you towards management, who comes and say "Come on, 6 layers? I'm sure that could be done with 4"... And there you go scratching your head. – dim Apr 29 '20 at 11:25
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    @dim I think the 486 layout largely assumes at least four layers, with a power plane connected to the Vcc pins and a ground plane connected to the Vss pins. Adding more Vcc and Vss pins on a through-hole component doesn’t increase the layering requirements, AFAIK. – Stephen Kitt Apr 29 '20 at 12:09
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Socket 3 did not have more pins than needed. It was designed to support the Pentium OverDrive CPUs that Intel released in the mid-1990s. These CPUs could have up to 237 pins, while the Socket 3 allowed 238 pins.

Brian H
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    So it was more pins than needed at first, but it was future-proofing, and did actually get used by a socket 3 pentium, got it. I didn't know there were any socket 3 pentiums. – Hearth Apr 28 '20 at 14:47
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    It wasn't so much future-proofing as that they already had the upgrade chip on the drawing board when the Socket 2/3 came out, so they were asking purchasers to future-proof for an upcoming product. That product just took longer than expected... long CPU delays aren't a recent thing. – SilverbackNet May 14 '20 at 01:37