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I have a question which is similar to another question on this site:

When I was a child 386 PCs came up. When buying a PC, you could read in the technical data how many 8- and 16-bit ISA slots were available.

Some manufacturers advertized 386 PCs that also had slots that they called "32-bit ISA slots".

I have seen some photos in adverts:

Such slots had a third connector beneath the 36-pin ISA connector (similar to a Vesa Local Bus connectors in 486 PCs); but the third connector had the same appearance (and pitch) as the 62- and the 36-pin connectors and about 20 pins.

(This video on YouTube is showing a sound card which has a similar connector; however, the boards that I have seen did not have 6 additional pins but about 20 additional pins.)

I couldn't find any information about such slots today and I also did not find any photo of a mainboard having such slots.

  • It was not EISA; EISA slots are not longer (but the connectors are heigher) than ISA slots.
  • It was not VLB; the third connector of a VLB slot looks completely differently than the other two connectors.
  • It was not PCI nor MCA; such slots don't have ISA-compatible connectors.

Were those slots manufacturer-specific or was there some kind of (unofficial) standard?

What additional signals were available in these slots?

It makes no sense to me to sell such boards if there are no cards on the market that can be connected to these slots...

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    Oh, ISA bus. My first read of the title was "Instruction Set Extension", i.e. 32-bit protected mode, and 32-bit operand-size in other modes via prefixes :P
    – Peter Cordes
    Commented May 14, 2021 at 1:11
  • Is it possible this was referring to MCA in an IBM-brand machine ? These cards often had blue plastic ends/supports. IBM blue of course.
    – Criggie
    Commented May 14, 2021 at 13:37
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    @PeterCordes I'll better modify the title.
    – Martin Rosenau
    Commented May 14, 2021 at 13:39
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    @Criggie No. I remember photos from advertisements: That these slots looked like a 16-bit ISA slot with an additional connector. I have no doubt that you could insert a 16-bit ISA card in such a slot. This is not the case for MCA nor for PCI.
    – Martin Rosenau
    Commented May 14, 2021 at 13:53
  • @MartinRosenau EISA could take ISA cards, but their extension was below the ISA part, not in another connector.
    – Raffzahn
    Commented May 14, 2021 at 15:32

3 Answers 3

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Were those slots manufacturer-specific or was there some kind of (unofficial) standard?

Manufacturer specific.

(Well, there was EISA, but I guess it's safe to assume that this question is explicitly not about EISA)

What additional signals were available in these slots?

Most likely D16..31 and A24..31. Plus maybe BE0..3 - and that's where the main issue with a 32-bit ISA extension starts: The (write) access to any combination of 1..4 bytes on any of the 4 'byte' groups a 386 could perform.

Extending 32-bit access to I/O would have been a rather high effort in board logic. So all ISA expansions for 32-bit I know do this only for memory access, which gets handled in a different, new protocol, with all primary control on the new connector.

However, there is a video on YouTube showing a sound card which is obviously intended for such a slot.

By offering just 6 additional signals? Not very likely. I'd rather assume the third 'connector' carries signals like Sound-Out and Mic-In. Adding such a connector would save the installation of 1-3 cables, quite helpful in getting production cost down - not to mention that it removes all need to know which cable goes where. So installation of a sound card would be reduced to simply plugging it into that specific slot.

(So these slots were not intended for memory expansion.)

All 386 extensions to ISA that I know of were first of all meant to host memory expansions. Keep in mind that introduction of the 386 to PCs falls into a short window before (widespread) use of memory modules, so having memory boards was the only way to go.

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  • "By offering 6 additional signals?" - I forgot to mention: There are ISA cards (for example the "mm sc16" sound card) where the part of the board connecting to the 36-pin connector is only ~20 pins wide (so the 36-pin connector is not "filled" completely). I assumed that this was the case for the 6-pin connector, too.
    – Martin Rosenau
    Commented May 13, 2021 at 12:43
  • @MartinRosenau Well known, just without any example of a card like the shown with more than these 6 signals, it's safe to assume there are none. Related to the well known logical issue about black swan.
    – Raffzahn
    Commented May 13, 2021 at 13:52
  • Quite interesting that there were no cards on the market... In this case it is possible that the additional ~20 pins were simply "not connected" and the "32-bit ISA slots" were nothing but an advertising lie.
    – Martin Rosenau
    Commented May 14, 2021 at 6:57
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No, this sound card is definitely not intended for such a slot that expands the ISA slot to 32 bits.

This is a Compaq-specific sound card meant for a Compaq-specific ISA slot extension. There is no more than the six pins for Compaq-specific audio extensions.

Based on a few pictures of the card, it simply routes some audio signals between card and backplane. Most likely it allows for routing the PC speaker beeps to pass on to the sound card, and allows the audio played by the sound card to drive the internal speaker.

The motherboards that have custom ISA extensions for memory boards contain enough address, data and control signals for connecting 32-bit wide memory bus to memory chips.

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The 32-bit extension to ISA was Extended ISA but it doesn’t add any additional connectors, instead adding extra pins to the part of the existing edge connector that is reserved for insulation.

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