The S-100 bus was something of an industry standard in the 1970s, for 8080 and Z80 computers built on a backplane design. It was invented with the Altair 8800, which apparently provided 16 or 18 expansion slots, depending on the model. That sounds like a lot, but you tended to need all the slots you could get; memory, for example, had to be added in the form of cards containing only a few kilobytes each. Presumably S-100 computers from different manufacturers, provided different numbers of expansion slots.

What was the maximum number of cards you could put on an S-100 bus? I mean, presumably there must have been some limit, either in terms of the amount of power they would draw, or just in terms of how long the bus could be. Did some manufacturers, trying to offer very many expansion slots as a selling point, approach the limit, or was it so absurdly large that you would never get near it in practice?


The standard (IEEE 696) is available here

Section 1.1 defines the scope of the standard as

  1. Data exchanged among the devices is digital.

  2. A maximum of 22 devices are interconnected.

  3. The total transmission path length does not exceed 25 inches

  4. The maximum switching rate is 6 MHz.

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    The photo on Wikipedia shows a 21-slot backplane, presumably there is also a 22nd "master", or they simply couldn't fit the last one :) – pipe Dec 25 '19 at 14:44
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    There isn't a one-to-one correspondence between "devices" and "slots". The standards says that one "device" can be on "one or more cards." In fact a 25 inch long backplane could theoretically squeeze in 34 card slots. The standard spacing is 0.75 inches and 33 x 0.75 = 24.75.. – alephzero Dec 25 '19 at 15:16
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    Hmm - I wonder if the 25 inch signal path length includes the tracks on the cards themselves. In which case, the worst case for two 5 inch tall cards would leave 15 inches for the bus, which is 20 x 0.75 inches, which gives 21 card slots :) – alephzero Dec 25 '19 at 20:21
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    22 taps distributed over half a meter sounds like more than what an LS245 will handle, and an F245 would probably give you all kinds of funny signal integrity problems.... – rackandboneman Dec 26 '19 at 0:19

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