RPN-55 SD is a photo-realistic simulation of Hewlett-Packard's classic scientific calculator HP-55.

Released in early 1975, the HP-55 was the lower-cost alternative ($395) to the famous HP-65 ($795). Its main feature was a quartz timer. The HP-55 was discontinued in 1977.

RPN-55 SD is a fully compatible version of the HP-55 with twice the program memory.
By setting a simple switch, you can turn on a number of useful extensions.

"The HP-55 features more pre-programmed functions than any other scientific pocket calculator in the world." (Owner's Handbook, 1975)


Features

• Programmable (99 steps)


• Photo-realistic look-and-feel


• Full compatibility with HP-55


• Easier to use than the original calculator


• Clear Program command


• Clear Range of program steps


• Insert and Delete program steps
  with automatic address adjustments


• Print program listing and description, or mail them as PDF documents


• Subroutines
  nested up to four levels


• "Continuous" (non-volatile) memory


• Display mode Engineering


• Optional thousands separators


• Current angular mode shown, if not DEG


• Unrestricted STO and RCL register arithmetic
  (incuding "double register" ∑+)


• Additional functions, like ABS, INT etc.


• Swap unit conversions with Additional functions, like hyperbolics


• Timer supporting real-time clock modes (12/24), date and weekday


• Key-click and key-glow, both optional


• Swipe left across display to correct wrong number entries


• Swipe right across display to show the stack


• Cut/copy/paste display values and program steps


• Display stack registers and all memory registers (tap display once)


• Run/Stop and single-step programs while viewing registers


• Display and annotate programs


• Save programs, optionally including registers


• Print and share programs


• Battery indicator lights up when the iPhone's battery charge drops below 10%

• Calculator Mode: choose between running RPN-55 in vintage mode or extended mode. The setting is saved and restored along with programs.


• Timer Maximum: Set the timer to 100-hours, to 24-hours, or to 12-hours with AM/PM display. In 12/24 modes, pressing CLx sets the timer to the current time of the day.


• Replace Unit Conversions: Select to use different functions in place of unit conversions. The setting is saved and restored along with programs.


• Thousands Separators: Turn thousands separators on or off..


• Haptics: Set the desired level of haptic feedback on pressing any key.


• Key Glow Effect: if enabled, tapped keys are shortly highlighted. Useful if sound is off.


• Show Prefix Indicators: if enabled, prefix keys like f, g, STO will be marked with a white outline as long as they are active. A prefix key may be tapped again to deactivate it.


• Random Number RAND: determines the behavior of the extended function RAND:
  
  If Off: RAND creates a random number between 0 (inclusive) and 1 (exclusive)
  If On:
          If x ≤ 0, RAND creates an integer random number between 0 (inclusive) and 2³²–1
          If x > 0, RAND creates an integer random number less than x, and less than 2³²
  
  This setting is saved with programs and automatically restored on loading programs.

HP-55 Brochure (in German)
Published in March 1975 by Hewlett-Packard, Switzerland. Download

HP-55 in HP-65 User's News
Hewlett-Packard introducing the HP-55 in the first issue of "HP-65 User's News", mid-1975: Download

• • Added warning when attempting to replace a Sample Program.
• • Visual adjustments to iPhone 16 Pro and Pro Max.

• • Internal optimizations and cleanups
    New sample programs are now automatically loaded when opening the list of programs.


• • Modified the info screen to avoid clipping by the Dynamic Island.
  

• Changed:
• • Both slide switches may now be operated either by moving them with the finger, or by simply tapping them.
• • The haptics level may be seleceted in Settings.
• • Like on a real HP-55, executing SST on step 00 simply ends any user input value. The next SST will execute step 01.

• Changed:
• • Like on a real HP-55, executing SST on step 00 simply ends any user input value. The subsequent SST will execute step 01.

• Bug Fix:
• • Exporting a program would cause a crash

• New:
• • Set the haptics feedback level in the Settings

• New:
• • Change the program listing font with a simple tap on the button F
• • Sample Programs may be hidden in the list of programs
• • Apply a filter to quickly find any program

• New:
• • Improved program "Solve f(x) = 0"

• New:
• • Improved mail subject when mailing a program as PDF
• • Updating the File List invokes visual feedback
• • New program: "TVM - Solve for i"

• Bug Fix:
• • GSB could not be pasted as text in PRGM mode

• New:
• • Print (or mail as PDF) the program or its description.
• • A program running longer than 1 minute will emit a sound when finished (by GTO 00).
• • Improved accuracy in extreme cases of y^x with positive, integer x.
• • Call up "What's New" anytime from Info screen
• • New programs: "Nominal vs. Effective Rate", "Num. Integration - Gaussian"

• Bug Fixes:
• • Display issues when operating the iPhone in "Zoomed" mode
• • RND didn't work correctly in regions using comma as decimal point
• • Programs mailed in calculator view incorrectly showed step 00
• • Double- or triple-tapping the empty program listing area caused a crash

• New:
• • n! now appears as x! in advanced mode to reflect the fact that the function can handle non-integer arguments.
• • Added and updated sample programs.

• Bug Fixes:
• • Pressing a number key while single-stepping a programmed number input would start a new number.
• • Much better handling of situations where the calculator was set to vintage mode and the user loaded a program saved in advanced mode.

• New:
• • Stack view shows localized and formatted value while entering a number

• Bug Fix:
• • A GTO or GSB branch to step 1 would halt program execution

• New:
• • Improved stack display (swipe display right to show):
    - Shows unformatted values of x and Last x
    - Long-press to directly call up the register view
    - Correct digit entries by swiping left
• • Added date and weekday to TIMER mode (press ENTER to show)
• • Decimal point now occupies a full digit position
• • Improved compatibility in Vintage mode with real HP-55
• • Added remaining programs from "HP-55 Statistics Programs"
• • Support for iOS 13: Dark Mode, file sharing
• • iOS 11 or later

• Bug Fixes:
• • Using ∑ with large arguments could cause a crash
• • Automatic address correction would produce values larger than 99 in some cases
• • Programs don't continue at step 01 after step 99 anymore

• New:
• • Improved LNx! function:
    - Higher accuracy
    - Now returns y and z such that x! = y * 10^z (where z is an integer)
• • Added program "Exponential & Log Curve Fit"

• Bug Fix:
• • Sharing could cause a crash

• New:
• • Optionally replace unit conversions with additional functions (in Settings)
• • Added "x-estimate" for a given y to linear regression functions. Note that the key for correlation coefficient (r) has moved one row up.
• • Programs saved in vintage mode are now marked with a "V" in the listing view
• • Added programs "Hyperbolic Functions", "Gamma Function (Toth)", "Solve f(x)=0", "TVM with Unknown Interest" as well as all test statistics programs
  from the book "HP-55 Statistics Programs"
• • Tight program loops with many repetitions will no longer cause the display to stutter

• Bug Fixes:
• • Pressing EEX when a number showing separators was displayed would not remove the separators
• • Touching a main register value would not dismiss the register view
• • The % function showed as 1/x in program listings
• • Deleting a range of steps using g CLP in RUN mode could modify unaffected branch commands

• First public version.

RPN-55 SD