HP21 scientific RPN calculator
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The HP-21 was a scientific calculator produced
by Hewlett-Packard between 1975 and 1978. It was designed as a
replacement for the HP-35 and was the first from so called Second
Generation Handheld Calculators.
As with most HP calculators then and now the HP-21 used RPN entry logic with a four-level stack. It also had a single user-accessible memory register. A shift key provided access to functions whose legends were printed on the fronts of the tall trapezoidal keys.
To enhance practical usability display was modified to show both X and Y register. Also I have eliminated mechanical DEG/RAD switch and moved the function under DSP button.
HP21 has few big enough buttons to be used comfortably on mobile devices.
As with most HP calculators then and now the HP-21 used RPN entry logic with a four-level stack. It also had a single user-accessible memory register. A shift key provided access to functions whose legends were printed on the fronts of the tall trapezoidal keys.
To enhance practical usability display was modified to show both X and Y register. Also I have eliminated mechanical DEG/RAD switch and moved the function under DSP button.
HP21 has few big enough buttons to be used comfortably on mobile devices.
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