The DM42 and calculator musings

I’ve had my Swiss Micros DM42 for about a year now and I thought I’d write some thoughts about it. The Swiss Micros DM42 is a semi-faithful clone of the HP-42S, an RPN calculator HP sold from 1987-1995.

This will be a long and rambling post… you have been warned. It is more of a log of a journey than a review.

So first, why the heck do I even need a calculator? How does someone my age end up using a calculator when iPhones, Wolfram|Alpha, and Excel exist? And why would I spend a decent amount of cash on a physical calculator based on a 30-year old design and not buy a much cheaper and more capable modern design?


I’m a physicist by training. I’ve worked as a plasma physicist, a (mechanical) design engineer, and now as something in between1. I went to high school in the mid ‘90’s and got a TI-81 (I think?) at some point in early high school. By the time I got to college, it was mostly useless, particularly in physics courses. I think it served primarily to hold books open to the right pages. I remember one of my class mates had a weird-ass HP graphing calculator that I could never figure out how to work. That was all the exposure I’d had to HP calculators (and RPN) for the first 30 years of my life.

Fast forward to the late 2000’s. I’ve been to graduate school, never having bought another calculator, and done a post doc as well. All of my math is in python or Mathematica, or honestly, by hand. With the arrival of smart phones, I find myself using my phone as a calculator periodically. I think I read a recommendation on Daring Fireball or And now it’s all this for PCalc and I purchase it for my phone. I slowly start using it more and realize it’s nice not to have to fire up a program for a quick calculation.

Sometime in 2009 there was a release note for PCalc where ‘Optional HP48-style RPN behavior’ was added. I didn’t know what this was so I tried it out. “RPN is weird,” I think. But I start to see the usefulness of the stack and stick with it for a while, then I made the move permanent.2 I also become enamored with the ability to easily access to multiple memories.

A few years later, I start a new job as a (mechanical) design engineer. I’m now disconnected from all of my old tools due to corporate policy. I still mostly am using my phone as a calculator, far more than is reasonable. I finally break down and order some calculators. At this point, I dig RPN and that’s what I want to buy. I do some research and see that HP still sells the HP 35s and the HP 50g, along with some business calculators. I also download a bunch of RPN calculator apps on my phone and find the wonderful Free42, an HP-42S clone. “Man, I wish I could buy an HP-42S still.”

The 50g is a large graphing calculator and is completely overkill for what I need. It’s also complicated in it’s use. HP has also released its next generation calculator at this point, the HP Prime, but it’s also complicated, had more limited RPN at the time, and has a rechargeable battery that doesn’t last ‘forever’. For complicated stuff, I’d rather go to python or something. I want a smallish calculator that I can bang out some quick calculations on, not have it go to sleep on me, and only need its battery replaced once a year.3

So the 35s it is. The 35s is decent but has some annoyances, mostly with its display, the inability to backup to a computer, and partially with its programming limitations. It’s still better than the other options. The multiple memories (storage registers in HP lingo) are killer, as are some of the easy unit conversions and universal constants accessed with just a few button presses. I also become enamored with the easy linear fits to data and HP Solve. This isn’t a review about the 35s but if you are looking to spend < $50 on a programmable RPN calculator, it isn’t too bad.

Free42 and the DM42

As I mentioned, I also found Free42 for my phone around this time and was blown away about 1) how powerful and relatively easy to use an almost 30 year old calculator is and 2) how much time was put into Free42 to make it as good and faithful as it is. The HP-42S (and Free42 by extension) seems to exist in that region of complex enough to be powerful but not so complex to be complicated to use.

During my research of the prior weeks, I also heard about a small company named Swiss Micros who makes some clones of a few of the classic HP calculators. It was rumored they were working on a larger screen HP-42S-like clone as well. And holy shit, they were going to use Free42 as the code base. This was really cool because Free42 is a faithful recreation of the HP-42S, which means you can run any program written for it on Free42. In addition, the author of Free42 put a lot of work into the numerical implementation, giving the calculator 34(!) digits of precision in its calculations. Total overkill, but cool nonetheless.

Over the next year, I saw updates on the calculator and started to think this thing will actually get made. Mid year (I think), hardware starts to show up in a beta test program. I decide to wait it out and just purchase one of the first run units.

On to the actual DM42

At the very end of 2017, my DM42 arrived, just in time for me to start at NASA as a non-contractor. I use it almost every day, at the very least for a quick conversion or calculation.

I’ve had the calculator now for a bit less than a year and I’ve not had to replace the battery. The calculator can be powered over USB, and it also gets a speed boost when connected to USB power.

The display

Oh, the screen. The screen is big and sharp. I think it is one of the best 1-color LCD screens I’ve seen in a device. Up to five configurable lines can be displayed at once, giving you the full stack and Last X or Alpha register all at once. There are two font choices, several sizes available (configurable on a line by line basis), and several stack layouts to choose from.


In addition to the niceness of the screen while on, when off, the calculator goes into a super low power mode where the screen can still display an image (like eink). I quickly made up some ‘off screen’ images of pictures I like. The pictures must be 1-bit black and white bitmaps. I ran the selected images through another mini-project of mine that I coded up, an implementation of Atkinson dithering, which reminds me fondly of playing with a classic Mac as a kid.

New features

The DM42 has a few features that the HP-42S and/or Free42 do not have:

  • Free42 added some time and date functions that didn’t exist on the HP-42S. Since the DM42 has an internal clock, these functions came along for the ride.
  • The aforementioned ability to display all four stack levels.
  • Ability to import and export programs, inherited from Free42.
  • Ability to save and restore the full calculator state. This is useful for testing purposes and backing up work. Some have suggested that you could set up ‘workspaces’ that you toggle between, i.e. an engineering setup and a finance setup. I’d rather have my stuff all mixed together.


In general, programming on the DM42 is similar to on the 35s and other more modern RPN calculators, and almost exactly the same as on Free42. You can see the actual key strokes entered as opposed to the 12c/15c style address. You can also see up to 10 lines of code at once which is nice. Lastly, you can import and export programs by mounting of the calculator as a USB drive. They do need to be converted to the raw binary format for the calculator to use them.4

I’ve written (and modified) a bunch of programs for work:

  • A ‘time sheet’ program.
  • A program to take data in a matrix and send it to the summation registers for statistics.
  • I customized some available units and conversions programs for plasma physics and electric propulsion applications.
  • Some quick utility functions for converting voltages into real units for a few of our commonly used pressure gauges.
  • A utility to calculate rate of rise as a quick check on leak rates for our high purity feed systems. This utility is something I can run quickly while performing the 24-hour rate of rise test (and before I have access to the real data).
  • A utility to help set flow rates based on other parameters.
  • A simple dice program.
  • A few statistical distribution programs that I used to learn the solve and integration functions.

The negatives

I only really have one negative impression of the calculator. I’m not a huge fan of the keyboard. It’s not bad, but it’s not great. The keys on my 12c are much better, as are the ones on the 35s.5 They are a bit small and hard, and every once in a while miss a press. They also don’t look anywhere near as nice as the vintage 12c/15c keys. I wonder if we will ever see buttons like that again. (No.)

While I would personally spend extra money to upgrade that aspect of the DM42, it should not be a deterrent.


I’m very impressed and thankful that there are ‘crazy’ people like Swiss Micros and Thomas Okken (author of Free42) who have spent considerable energy scratching an itch.

Swiss Micros DM42 summary

Tim Gray

The Swiss Micros DM42 is a wonderful tool. Highly recommended.

Rating: ★★★★★

  1. A ‘Research AST’ is my title, which is short for AeroSpace Technologist. Technically I think I am classified as an Electrical Engineer. ↩︎

  2. Incidentally, this was around the time I started using Vim full time, so I guess I was into experimenting with obtuse and esoteric entry methods. ↩︎

  3. As a side note, while the HP Prime and its TI counterparts are incredibly powerful, they are targeted at the education market which is slightly different than what a ‘professional’ might want. I rarely need to graph something on the go; that is a task for Mathematica or python for me. In addition, I graph analytical functions ever more rarely; I graph data. Portability and battery life are more important than raw power. Likewise, there is something appealing about keystroke programming for quick and dirty recurring tasks. ↩︎

  4. I recently wrote a python script to encode text programs into the raw binary format that the DM42 expects. ↩︎

  5. The 12c is a newer Platinum model. I like the keys on that more than the 35s. The 50g is a bit worse and mushier than the 35s, but fine in actual use. ↩︎