Layout of arithmetic keys on early calculators


We all know the layout of the HP-35 of 1972:

Shortly after it, TI launched its SR-50:

While the order of the arithmetic keys on the SR-50 follows a simple logic, I can't comprehend the order on the HP-35. OTOH, HP kept it until 1981, so there must have been some reason. Anyone of the experts here got an idea?

BTW, both pictures show quite well the bad contrast of รท vs. + though that's another story.



Maybe HP was trying to capture the left-handed market.


Repetita iuvant!



Edited: 19 Nov 2013, 4:39 p.m.


Repetitio est mater studiorum ;-)

Mille grazie - I will need some time to read (and hopefully understand) that thread.



That's an interesting question, I don't know why on the hp-35 and the following hp calculators the order of the arithmetic operators from bottom to top was /,*,+,- while on the previous and first HP calculator, the hp 9100, it was the same as on the SR-50: +, -, *, / (but on the left side).

It seems there was a decision to have a different order between the desktops and the handheld calculators, but I can't imagine any reason behind it. It may also be that both types of calculators were designed by two different groups within hp who didn't care about consistency at that level of detail.


The story on the HP35, as I heard it in 1972 or 1973 from Barney Oliver himself (then a VP at HP) was that a readily-alterable keyboard connected to prototype HP35 functionality was made available to HP engineers who could configure it however they wanted (i.e. what key positions and what functions they used). The final HP35 keyboard was then more-or-less a vote by actual HP internal users as to what they wanted/used the most.

I used the '35 for a long time from December 1972 onward and never felt constrained or (dis)affected by the keyboard. My personal preference would be to put the + key above the - key though (with + - at the top, rather than multiply and divide).


I guess I never paid that much attention to the layout of the arithmetic keys until reading this thread and the one referenced. But I guess I am most used of the current layout as when I pick up something like one of my 67s it does not feel right. I noticed that the layout on the 97 is the same as the current layout, which must have been fun for people switching between the 67 and 97. My first calculator a Commodore PR-100 has the keys in a square with X / above + -.


I've always thought the primary math keys on the 35 keyboard made sense. Put your middle finger on the home (5) key. Closest reaches for right hand index finger are + and *, the more frequent operations. Then - and / are nearest their inverses. Enter is bigger to bring it into an easy reach for the right hand index finger.


And then there's the HP-10 with divide and multiply horizontally above the digits and the subtract and add vertically to the right, but it's an "adding machine" and not a "calculator."


Addition and multiplication are more frequently used than subtraction and division, so they're kept close together to reduce necessary finger movement.


Back in 2000, at Richard Nelson's request, I put together a little presentation for the HP Calculator Conference that year (in Los Angeles) which compared and contrasted HP calc numeric keypads. The short paper, along with a chart categorizing all models up to that point (including the Xpander, which was supposedly imminent) is located
here and a pdf of the 14 slides showing pictures of all the keypads is here

My two cents,


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