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In 1974, Intel released the 8080, which basically was the first microprocessor good enough to build a serious computer around. That led to the Altair 8800 the following year, which was the beginning of the personal computing revolution.

What sometimes gets forgotten is that later in 1974, Motorola released the 6800, which while not quite as good as the 8080 (in speed or code density, the latter being perhaps even more important in those days), was the second microprocessor good enough to build a serious computer around, and accordingly likewise someone did the following year: http://www.computinghistory.org.uk/det/16739/SWTPC-6800/

It's basically the same kind of design as the Altair: horizontal box with a backplane that takes expansion cards that are expected to hold most of the actual hardware. It doesn't list separate prices for kit versus assembled, so presumably the listed $395 (from the above link) is for assembled, compared to $621 (from Wikipedia) for the Altair.

Why the price difference?

It's not because MITS was overcharging for the Altair; on the contrary, they were drastically undercharging, to the point where they couldn't expand to meet demand and there was no margin for retailers.

It's not because the inferior CPU was cheaper; Motorola matched Intel's price for their CPU. (https://www.techspot.com/article/884-history-of-the-personal-computer-part-2/ "Roberts was able to secure supply of the 8080 for $75 a processor (Intel's list price was $360, as was Motorola's 6800)...")

The SWTPC does not have the front panel with the switches and blinking lights of the Altair, which would have saved some cost, though that component doesn't feel like it should have been very expensive.

Perhaps more significantly, it uses the SS-50 bus compared to the S-100 bus. That's 50 fewer wires on the backplane, 50 fewer pins on each edge connector. Intuitively, it feels that should cut the cost significantly, though it is said that MITS was able to source 100-pin edge connectors cheap as military surplus: Why did the Altair use 100-pin edge connectors?

I am curious about several things regarding this machine and the 'might have been' branch of personal computing history that it represents, but to ask a specific question here:

Assuming the price difference reflected actual manufacturing cost difference, what accounts for the SWTPC being cheaper to build than the Altair?

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    Mind to add the source for the prices? Also, a first hint might be the case. The SWTPC case was not only optional, but as well comparably sheap. – Raffzahn Nov 12 '20 at 1:27
  • @Raffzahn Done. Okay, so what made the SWTPC case cheaper than the Altair one? – rwallace Nov 12 '20 at 1:39
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    Well, comparing boards between IMSAI and MITS already shows that MITS did charge notable higher prices for similar boards. So I would doubt that the financial problems were rooted in a too low price mark. On the other hand, comparing SWTPC kit prices with IMSAI kits show that they are rather similar priced - also in line with other boards. So the only real hardware difference is the better case used by MITS - we know that all these little parts are quite costly. Still, I think the assumption MITS prices were too low are not substantial. – Raffzahn Nov 12 '20 at 1:50
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    Just look at the sturdy case, plus all the small parts to hold cards and so on. While the SWTPC hat only a thin metal bade and a flimsy cover. In addition the Altair brought a front panel with lots of parts, while the SWTPC used a way cheaper serial interface, relaying on an external terminal – Raffzahn Nov 12 '20 at 1:52
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    It's a backplane, an etched PCB, there are no wires. If at all, then the SWTPC board is more complex with its structure as a main bus and an I/O bus. I'd say the difference is, beside more / different hardware a way higher margin added by MITS - their financial shortcommings were for sure not created by hardware cost. They cranked out way too many machines, boards and versions thereof - not at least two complete incompatible systems with next to no exchangeable parts. A perfect way to waste money on design, production and logistics. IMSAI focused on one machine and undercut MITS by 20-30%. – Raffzahn Nov 12 '20 at 3:31
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It turns out that the price difference was not as large as I thought it was. According to http://www.oocities.org/peterochocki/computers/1980comp/swtp6800.htm the $395 for the SWTPC was in kit form, which means it compares to $430 for the Altair, a rather small price difference. Raffzahn is probably right that the $35 difference is accounted for by the Altair having a sturdier case and a front panel with switches and blinking lights.

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