View Full Version : Did the S-100 predate the Altair?

July 28th, 2006, 05:14 PM
Given that the IMSAI was on the drawing board before the Altair was released, did the S-100 slot exist before both? If so, what machine used it?

The IMSAI was the first cmputer I ever programmed, at age 13. We ran Altair BASIC, and wrote code to drive a tabletop plotter. Code was stored via Tarbell interface to a Radio Shack tape.


July 28th, 2006, 05:25 PM
The MITS Altair introduced the S-100 bus. The IMSAI was altered prior to production to conform to the new bus for card-level compatibility since a market had already been established.

July 28th, 2006, 06:50 PM
Whoever designed the S-100 bus should be shot! I was using a "volt ohm meter" one day checking out the "runs" for open circuits on the board. My "too large probe" slipped off of the trace and shorted against the next one. Did you know that the +12v and the ground traces are right next to each other? I sure found that out when one of the traces turned into "copper vapor" in my face. I had to take some 22gauge wire and replace the vapor with something more solid. Good thing it was only a 4 Mhz Z80 in there. The wire run didn't seem to screw up the operation any.

October 8th, 2006, 04:08 PM
A little late, but that would be Ed Roberts, of MITS.

Alot of people referred to it as the Roberts Bus back then.

But, yeah, +12v and GND next to each other is sort of ignorant. Especially with the current those old monster power supplies are capable of!


Terry Yager
October 8th, 2006, 04:47 PM
Some early documentation refer to it as the 'Altair Bus', before it bacame known as the S-100, and long before IEEE-696...


October 8th, 2006, 09:48 PM

I've read Altair Bus, Roberts Bus, MITS Bus, etc...

Eventually, it became an IEEE standard, correct?


October 9th, 2006, 05:21 AM
Eventually, it became an IEEE standard, correct?
Yes. IEEE-696

October 9th, 2006, 06:32 AM
After introduction by Altair, the bus was quickly adopted by various
microcomputer manufacturers. A fellow from Cromemco, Roger Melen
called it S-100 or Standard-100 in 1976. The bus signal definitions closely
follows those of an 8080 cpu system.

October 19th, 2006, 05:55 PM
Having those traces in close proximity is bad enough. Much worse is the fact that +12 and -12 are directly opposite each other on the S100 sockets (pins 2 and 52). It's VERY easy to accidentally let your probe tip slip in between them and blow both supplies. That's the main reason you should never probe an empty S100 socket

Nor should you remove a board from an S100 computer if power is applied. As you pull the board out, the relieved spring tension of the facing contacts can cause them to spring toward each other and temporarily touch. The 12v contacts will weld together if power is present. In addition to the power supply, damage to most daughter boards and the front panel can be the result.

You can read more about this in the archived messages at the Altair Computer Club at

Did you know that the +12v and the ground traces are right next to each other?