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S-100 was, as I understand, a very popular way to connect various computer equipment together in the past. Also, from what I understand, S-100 closely matches the same signals as the Intel 8080, as does the Z80 which the ZX Spectrum etc have. The only thing I can see is that the S-100 needs some unregulated power lines that have different voltages than the ZX Spectrum provides, but that's not insurmountable.

So to me, it seems extremely obvious to expand a ZX Spectrum or ZX81 with an S-100 backplane, to use disk controllers, modems, RAM expansion and all of that, but it seems it never was done, at least not commercially. Even though both are in a similar kind of budget bracket. So I suppose there are technical reasons for that?

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  • S-100 and ZX-81 are not exactly contemporary. I do know a number of people who have equipped their Zeddies with an ECB-Bus (a European standard similar to S-100) interface, but none that went for S-100
    – tofro
    Commented Sep 29, 2018 at 11:32
  • @tofro - cromemco were still designing and building new S100 systems in 1981, and I believe continued selling them until the late 80s, so S100 was definitely still current during the Sinclair era
    – Jules
    Commented Sep 29, 2018 at 12:47
  • @Jules it was still already nearly 10 years old at the ZX time, and you simply couldn’t get cards at an acceptable price where the Sinclairs dwelled
    – tofro
    Commented Sep 29, 2018 at 13:22
  • I don't think it is technical. It's economic. Expensive card cages paired with cheap consumer/home computers never made economic sense, for any platform.
    – Brian H
    Commented Sep 29, 2018 at 17:05

2 Answers 2

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Can S-100 cards attach to the ZX machines?

That calls for a clear Yes, But :)

S-100 closely matches the same signals as the Intel 8080, as does the Z80 which the ZX Spectrum etc., have.

Adding a basic S-100 bus bridge for the ZX80/81 or Spectrum would in fact be no big deal - only some money to spend for bus board, cage, PS and so on. It would become a bit more troublesome when addressing schemes were considered, as the Sinclairs not only already fill the address-space with RAM, ROM and I/O, but also do not always fully decode such. As a result, the available address space is rather limited, and not compatible with many S-100 boards.

it seems extremely obvious to expand a ZX Spectrum or ZX81 with an S-100 backplane,

Well, while S-100 was still a big thing when the ZX80/81 came, and still around for the Spectrum, it also was mostly a US issue. While the Sinclair mania was more confined to Europe (*1), with Britain being a famous hot spot :)

to use disk controllers, modems, RAM expansion and all of that,

Keep in mind that people who bought a ZX machine weren't exactly buying at the upper end of the market - and each of the above S-100 products was rather expensive, especially when compared to expansions made to fit the Sinclair computers directly (and at the lowest possible cost).

but it seems it never was done, at least not commercially.

Right, to my knowledge there are no products for that. Everything that was available was using the ZX Bus directly as their expansion bus. Just think about all these pluggable Memotech modules for the ZX 81. They were not only less expensive than similar S100 boards, but also didn't need the investment in a bus board and a cage.

Bottom line:

  • Wrong World Region
  • Wrong Market ((Cheap) Home vs. Professional)
  • Too Expensive
  • Non Trivial Configuration
  • Incompatible Resources (Address Ranges)

Especialy the last two would it make hard even for a dedicated hobbyist.

*1 - Sure, there was the TS1000/1500 in the US, but they never reached sufficient popularity to make anything worthwhile.

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    Indeed, I/O address conflicts are likely to be a serious problem for most S100 boards, as IIRC both the ZX81 and ZX Spectrum ULAs decode and respond to all even-numbered I/O ports, which could lead to serious issues, as many expansion boards will want to use a contiguous block of port numbers.
    – Jules
    Commented Sep 29, 2018 at 13:43
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    I was in the computer hobbyist scene in the early 1980s in the UK, and S-100 was something that was only ever mentioned in US magazines, like Byte. It was simply in a different price bracket.
    – John Dallman
    Commented Sep 29, 2018 at 19:51
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not quite an answer to the question:

In our post-90ies and beginning-of-2000ies ZX practice, we were connecting ISA modems to ZX Spectrums here, used BBSes and FIDO via them (there is software for that, of course). Bus convertor was nothing more complex than /RD /WR etc. signals generator, address decoder and signal buffers.

then the formal yes useless answer:

Using enough creativity you can build virtually any bus to any bus converter. Probably S-100 bus is CPU-cycles-centric and therefore the converted would be not very complex.

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    While it is possible, yes, ironically translation to a PC's bus would probably be simpler than that to an S100. That's because while nominally intended for the same sorts of processors, the S100 did a variety of odd things which quickly fell out of fashion, while the PC's ISA was a more direct pass-through of the concepts of a descendant processor's local bus.
    – Chris Stratton
    Commented Sep 29, 2018 at 23:25

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