HP21 scientific RPN calculator
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About HP21 scientific RPN calculator
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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Android
1.6+
1.6+
Bug
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Android app
App History & Updates
What's Changed
2.7. Y^X complex result bug fixed (Error is shown for COmplex result)
2.6. touch area re-alligned to buttons properly - thank you Eric
2.5. Can enter number by EEX button directly
2.4. To improve usablity - now running in full screen
2.7. Y^X complex result bug fixed (Error is shown for COmplex result)
2.6. touch area re-alligned to buttons properly - thank you Eric
2.5. Can enter number by EEX button directly
2.4. To improve usablity - now running in full screen
Version update HP21 scientific RPN calculator was updated to version 2.7
More downloads HP21 scientific RPN calculator reached 5 000 - 10 000 downloads
Version update HP21 scientific RPN calculator was updated to version 2.4
More downloads HP21 scientific RPN calculator reached 1 000 - 5 000 downloads
What are users saying about HP21 scientific RPN calculator
M
by M####:
by M####:
Excellent emulation, keys work great with good feedback feel. As good as the real thing. Highly recommend if you like RPN.
C
by C####:
by C####:
HP21 the first one.
C
by C####:
by C####:
Superb emulation. Please do the 29C, I'll even pay for it!
C
by C####:
by C####:
Loved the Calculator. Love the App. Fantastic. Bill in Oman.
K
by K####:
by K####:
Excelent
C
by C####:
by C####:
Works just like an old HP 21.
C
by C####:
by C####:
Use this to explore reverse Polish notation or to relive using a second generation calculator.
C
by C####:
by C####:
Reminds me of my old hp25. Simple and works.
C
by C####:
by C####:
The keypad is very responsive, the two levels of the stack that are always visible are very helpful. The implementation of this venerable HP design on Android rocks, because the functionality is basic enough for quick occasional calculations, there is nothing more but an efficient and very handy calculator machine at hand all the time with you. A pure delight.
P
by P####:
by P####:
I love Free42, but sometimes I just want a classic bare bones four-banger plus basic scientific RPN calc. This is a great alternative. And the display of X /and/ Y is both useful and historic: Early HP desk calculators did it. Well done!
E
by E####:
by E####:
Upon startup, it returns to the default three decimal places and cannot turn off keypad vibration
C
by C####:
by C####:
Looks great on both my phone and tablet.
C
by C####:
by C####:
Vibration for haptic feedback annoying & draining power... Uninstalling now...
A
by A####:
by A####:
The keypad is very responsive, the two levels of the stack that are always visible are very helpful. The implementation of this venerable HP design on Android rocks, because the functionality is basic enough for quick occasional calculations, there is nothing more but an efficient and very handy calculator machine at hand all the time with you, no nag screen, no intention to push it on you to force you to pay, awesome. The only thing that needs some work is the high battery consumption.
X
by X####:
by X####:
Starting new entry with . does not copy x into y register. Need to preceed the . with a zero. This is a bug.
by C####:
I used one of these in the 1970s. Great emulation, right down to the red LED and front HP label. Well done.