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The Altair Bus

When hobbyists at the Homebrew Computer Club opened the Altair they found it nearly empty. There were a couple of circuit boards plugged into a backboard. One circuit board held the Intel 8080 microprocessor, another held 256 bytes of solid-state memory. But what was really interesting was the backboard. This was the bus, basically a large circuit board with a row of 100-pin slots that other circuit boards could be plugged into, a setup similar to that used by some minicomputers as far back as the mid 1960s.

It’s called a bus because, unlike plain wiring where each signal has its own wire, with a bus many different signals can ride on the same line. This simplifies the physical design, though it does require somewhat more complicated software to sort out the various signals.

The great thing about the Altair bus was that anyone could make their own circuit boards to plug into it, and this is exactly what homebrewers began to do. To Ed Roberts’s chagrin, small companies sprang up with memory and peripheral boards to plug into the Altair bus. By the end of 1975 an entire computer based on the Altair bus had arrived: the IMSAI 8800. By the end of ’76 an entire industry based on the bus existed, hundreds of plug-in boards and a dozen or two computers.

A meeting was called to standardize the bus. Roberts refused to attend. Those who did cleaned up the design and renamed it the S-100 bus, short for “standard 100-pin bus.” Since Roberts hadn’t patented the bus, there was little he could do. He regretted the way the bus got away from him, and the new name never sat right, but the fact that the bus was open and available was probably the best thing about the Altair. The first stage of the personal computer revolution was built on it.

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Related Artifacts & People

image of IMSAI 8080

IMSAI 8080

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image of Ed Roberts (1941-2010)

Ed Roberts (1941-2010)

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Altair: The Ultimate Kit
The Altair was created as a traditional build-it-yourself project, but in kit form it was more than many hobbyists could handle.
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Related Links

The Best of Creative Computing Volume 2
Articles from early computer magazine edited by David Ahl, published 1977.

The Best of Creative Computing Volume 1
Articles from early computer magazine edited by David Ahl, published 1976.

Herb’s S-100 Stuff
Information on the S-100 bus.

MITS and others from the DigiBarn.