HP-42S. The HP-42S RPN Scientific is a programmable RPN Scientific hand held calculator introduced by Hewlett-Packard in 1988. It is a popular calculator designed for science and engineering students.

Overview
Perhaps the HP-42S was to be released as a replacement for the aging HP-41 series as it is designed to be compatible with all programs written for the HP-41. Since it lacked expandability, and lacked any real I/O ability, both key features of the HP-41 series, it was marketed as an HP-15C replacement.

The 42S, however, has a much smaller form factor than the 41, and features many more built-in functions, such as a matrix editor, complex number support, an equation solver, user-defined menus, and basic graphing capabilities (the 42S can draw graphs only by programs). Additionally, it features a two-line dot matrix display, which made stack manipulation easier to understand.

Production of the 42S ended in 1995.

Specifications

 Series: Pioneer
 Code Name: Davinci
 Introduction: 1988-10-31
 64 KB of ROM
 8 KB of RAM
 Functions: Over 350
 Expandability: Officially no other than IR printing (32 KB memory upgrade and over-clocking hardware hacks are possible)
 Peripherals: HP 82240A infrared printer

Features
 All basic scientific functions (including hyperbolic functions)
 Statistics (including curve fitting and forecasting)
 Probability (including factorial, random numbers and Gamma function)
 Equation solver (root finder) that can solve for any variable in an equation
 Numerical integration for calculating definite integrals
 Matrix operations (including a matrix editor, dot product, cross product and solver for simultaneous linear equations)
 Complex numbers (including polar coordinates representation)
 Vector functions
 Named variables, registers and binary flags
 Graphic display with graphics functions and adjustable contrast
 Menus with submenus and mode settings (also custom programmable) that use the bottom line of the display to label the top row of keys
 Sound (piezoelectric beeper)
 Base conversion, integer arithmetic and binary and logic manipulation of numbers in binary, octal, decimal and hexadecimal systems
 Catalogs for reviewing and using items stored in memory
 Programmability (keystroke programming with branching, loops, tests and flags)
 The ability to run programs written for the HP-41C series of calculators

Programming

The HP-42S is keystroke-programmable, meaning that it can remember and later execute sequences of keystrokes to solve particular problems of interest to the user. The HP-42S uses a superset of the HP-41CX FOCAL language.

All programs are stored in one continuous program space, and may be created and edited by switching to programming mode, using the  key.
Subroutines are enclosed in /Label (name of subroutine) and /Return ( halts execution unless it is in a subroutine in which case it returns to the caller). Keystrokes (of functions) are enclosed between  and  or .

In a running program, a label causes program execution to branch to the specified label and continue running at that line.Executing a GTO instruction from the keyboard moves the program pointer to the corresponding label. No program lines are executed.

 is used in much the same way with one important difference: after an  instruction has transferred execution to the specified label, the next RTN (return) or END instruction causes the program to branch back to the instruction that immediately follows the XEQ instruction.Programs/Subroutines can also be executed with the help label (Execute) key.
Important difference is that  executes the label and returns to previous subroutine and continues execution from the line following ,whereas  branches to the label specified and doesn't return.

 packs the current subroutines and moves to new program space.nnnn can be used to reach a particular line of program.

Programming examples
A simple program to calculate area of circle

Program instructions like , ,  halt the program execution,which can be continued by pressing .

 is used to view contents of a register,For example R in the above example.

Using Integral and Equation Solver 
It is necessary to write a program or subroutine that evaluates f(x) for the function which needs to be solved or integrated.
Variables used in program should be declared using 

Here is a sample program to solve the equation °F = (9/5×°C) + 32

After, accessing the solver using   ( ), select program .

In a similar way, expressions can be integrated. After selecting the variable of integration, enter the lower limit and then press on , similarly input  (upper limit) and  (accuracy).

Emulators

HP 42s can be accessed through Android Emulators like Emu42 and Free42.
200px|thumb|WP 34s

WP 34s
WP 34S project uses a modern HP financial calculator HP 30b as the starting point to create an advanced scientific RPN calculator. It is a more powerful scientific (non-graphing) calculator compared to the original HP 42s.

Functionality

 Euler's Beta and Riemann's Zeta functions, Bernoulli and Fibonacci numbers, Lambert's W, the error function as well as Chebyshev's, Hermite's, Laguerre's and Legendre's orthogonal polynomials, and testing for primality,
Many statistical distributions and their inverses like Poisson, Binomial, Geometric as well as Cauchy-Lorentz, Exponential, Logistic, Weibull for reliability analysis, Lognormal and Gaussian with arbitrary means and standard deviations,
Programmable sums and products, first and second derivatives

Programmability

RPN keystroke programmable à la HP-41 - up to 925 steps in RAM - numeric and/or alpha labels, thousands of steps burnable into flash banks.
Externally programmable by developers via JTAG interface

See also
 FOCAL character set
 Comparison of HP graphing calculators
 HP calculators
 List of Hewlett-Packard pocket calculators
 NOMAS (support)

References

Further reading
 
 
 
 HP-42S Quick Reference Guide (1 ed.). Corvallis, OR, USA, Dex Smith. October 1988. 00042-92222E.

External links
 SwissMicros DM42
 HP-42S intro on hpcc.org
 HP-42S page on hpmuseum.org
 HP-42S resources on hp42s.com (defunct as of July 2017)
 HP-42S description on rskey.org
 HP-42S description on thimet.de
 Free42 for Android by Thomas Okken, an Open Source project.
 

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Computer-related introductions in 1988