SR-50: A bumpy road??

[brouhaha]

The HP-35 was to a large extent meant to be an improved replacement for slide rules, meaning that it should ideally have at least five significant digits. Each additional digit cost money (as you yourself poined out), and they chose a ten digit significand (often incorrectly called mantissa), even though they knew there would be limitations on the accuracy of transcendental functions. If they had chosen twelve digits, people would be asking why they didn't choose fourteen. No matter what a product has, there are always people that want more. At some point a decision has to be made and stuck with, and they chose ten, which was an enormous improvement over a slide rule.

Once they chose a ten digit significand (fourteen digit word), it would have cost a LOT to change that for later calculators, so they only changed it twice in HP-engineered calculators up until the 1990s:
* shortened to eight digit significand for HP-01 watch (twelve-digit word)
* lengthened to twelve digit significand (sixteen-digit word) for Saturn (all new models from 1984 forward)

Quote:In any case, why were the accuracy reliabilities and their processes abandoned?

They weren't!

Despite sticking to the ten-digit significand for stored numbers for all calculators through 1983 (except the HP-01), they didn't rest upon their laurels and keep the transcendental limitations of the HP-35. They made numerous improvements that substantially increased the accuracy, with almost every successive generation of calculators. The HP-35 had such limited ROM that the internal constants used in the transcendental had lower precision, and that was upgraded in the HP-45, and upgraded more in the Topcat and late Woodstock series. They also went to using a thirteen digit significand for internal computations, and only rounded down to ten digits for storage and display.

Some of the improvements were described in HP Journal articles by Dennis Harms (The New Accuracy: Making 2³= 8) and William Egbert (Personal Calculator Algorithms, parts I through IV).

Quote:Maybe HP could have made a $395 product with higher accuracy work out.

They did! Just not in 1972.

Given how tremendously successful HP calculators were for many years, I personally wouldn't dream of telling them they chose the wrong number of digits. There is no evidence to support the idea that they would have been more successful or profitable with more digits. Remember, their job was to make money, not to please enthusiasts (except to the extent that doing so improves profitability).

I'm not claiming that HP was perfect, or even that they did the best possible job under the circumstances. Even with the benefit of hindsight, that's impossible to know. I'm just saying that they got a LOT right, and that we armchair quarterbacks even with half a century of additional market knowledge wouldn't necessarily do any better job under the same constraints, and could easily do worse.

If I were going to criticize any aspects of 1970s HP calculator engineering, it would be:
* the critical dependency on the NiCd battery (and good electrical contact with same) to limit the DC voltage in the Woodstock calculators to prevent damaging the MOS chips
* the press-fit assembly of Spice calculators (which they later eliminated, reverting to soldered assembly)