Free42 is a re-implementation of the HP-42S calculator and the HP-82240 printer.
It is a complete rewrite, not using any HP code, and it does not require an HP-42S ROM image.
Free42 is an Open Source project. The executables and source code are released under the terms of the GNU General Public License, version 2.
All third-party code used in Free42 is either in the public domain, or licensed under terms compatible with GPLv2, or used with the authors' permission.
During number entry, if you backspaced over a negative number until only the minus sign remained, that minus would not be parsed correctly, and you could get "Not a Number". For example,  [+/-] [backspace] [ENTER].
Ratings and Reviews
A GREAT APP FOR A GREAT RPN CALCULATOR
I believe the HP 42s was the best scientific calculator ever made. And this APP is a fantastic representation of it. If you appreciate the benefits of RPN logic and a calculator that has all the capabilities appropriate for a hand held device, you will enjoy this APP. I have been using the APP for more than a year. The transition from the calculator to the APP was seamless. While there are some minor differences necessitated by the the smartphone platform, they are essentially inconsequential. And the APP has a few major advantages over the calculator such as not loosing programs when the batteries run down.
And the developer has been great. Miscellaneous enhancements and updates required by iOS revisions are very timely and bug free.
I write very few reviews and only when the product is either very good or very bad. If you are looking for a RPN calculator, do yourself a favor and get this APP.
Wish it would make loud click
I love this app so much that I don’t have to carry my HP-32S with me to work( had it for so many years) but I wish it would make a loud click as mentioned also by another reviewer. You get a feel of the click as you press the key on the real one but it would help a lot to make the click sound louder confirming that your entry was entered. It makes you always verify that you have entered it correctly making the process a little slower. That function was my only complaint since I downloaded this app about 5 years ago.
Developer Response ,
Version 2.5.7 has an option for a louder click.
Double love for this app!
My wife and I are both educated as engineers. She got me hooked on RPN after I stubbornly stuck to TI and AOS through undergrad. Then in grad school I started using her HP and before I knew it I was reprogrammed for RPN. We started buying "his and her" calculators as the physical models wore out. When she was complaining about using her iPhone calculator to balance the check book, I selected this app because we had both owned a 42S along the way and loved them. With this outstanding app, well supported with updates, our favorite calculator never wears out. Five stars and a donation. It's well worth it!
With Family Sharing set up, up to six family members can use this app.