is it possible to "merge" an HP-42s into an HP-97 ?


I believe the HP-42s is one of the most versatile and powerful calculators by HP, and that the HP-97 has the best “layout” in terms of keys and size as a desk calculator, so I am wondering if it would be possible to combine these 2 jewels into one calculator.

My idea is to make this as simple as possible, which to me means physically installing an HP-42s inside and HP-97 and connect them (though a microprocessor maybe?), and map the keys on the HP-97 to get the Hp-42s results, on a modified double line LCD screen. For ex, when you press the [e^x] key (3rd row, 3rd column on the HP-97 keyboard), internally you are really “pressing” f^ [LN] (1st row, 5th column on the HP-42s).

I understand that there are many challenges like how to account for all those functions on the 42s not present on the HP-97 (for ex [Catalog]), but there are also some functions on the HP-97 not present on the HP-42s that could be remapped (like [Write data], or [Merge] ). The idea would be to make the Printer and Magnetic Card Reader operationally as well.

I am sure that someone with the right training, experience and tools would be able to do it (and I suspect there are many skillful enthusiasts in this forum).

I am ready to offer my HP-42s and my HP-97 as “guinea pigs” for this project, and if it is successful, the brave person who makes this transformation can get either my (2nd) HP-97 or HP-42s, or some monetary reward to be determined.



Sorry. Beyond my abilities, but maybe I can find some help.

Hey guys!

He wants a "Desktop", "Modified", "WP-34S".

When can you start?


Give us the hardware!



If you are serious, I can send you the hardware (if by that you mean the HP-97 and the HP-42s). Just say the word.


Not the way you are thinking. Just butchering two vintage calcs will not produce anything useful. I was thinking of a neat desktop platform with a modern processor and decent display so that we can start porting our firmware to it. A desktop version of Eric's design with a big rechargeable battery and USB for desktop connection and charging would be fine. Adding a printer wouldn't hurt either. :-)


This sounds like a job for Dr. Frankenstein.


I often thought that the 97 should be a good platform for refitting with a modern CPU.


Wouldn't it be better to get a small single PCB computer, put that in the 97 case along with a power supply and run a version of free 42?

- Pauli


Interesting. What "small single PCB computer" could do the work ?

And who would be able to do it ?


Take a look at this article I wrote. I added a somewhat modern but very limited microcomputer to a 97 and programmed it to take control of the keyboard when needed and do some simeple stuff.

It's not really what you're asking about because it's totally dependent on the existing processor in the 97, but it will at least explain the keyboard scanning. It would be easy to rip out the guts of a 97 and replace it with a fast processor that scans the keyboard and runs the display. (I've done exactly this on a broken HP-21 making it into a simple integer calculator.) Printing would probably not be that hard either. Porting all the 42s functionality over and having it work on an extremely limited display is another story.


I would not want to keep the original display, but rather replace it with a DL1416B (or the HP equivalent) alphanumeric LED display. The size matches the original display.
The tough parts really are the printer and the card reader. Are they sufficiently documented?


Marais, I agree that part of the improvement should be to install a new LCD multiline screen. The HP-42s has 2 lines, but 4 wouldn't hurt.


It looks as though Atmel has an eval/dev board for the ARM7 processor used in the retargetable calculators. The board's pricey at $100 but provides breakout access to the processor's I/O for a prototype. That board could take modified WP 34S code that's suitably altered to scan the HP 97 keyboard and drive the LED display. Once the code is written, creating a low-cost circuit board with just the needed parts is an easier proposition.


I wouldn't bother with something custom. Find a microATX or small device -- you get a full fledged PC in a small form factor. Install a basic operating system -- DOS, Linux or even Windows.

Hook up the keyboard -- I'd use an IPAC or equivalent to connect the switches to the keyboard port on the PC.

The only remaining thing is the screen, no ideas here but I'm sure something is available.

Finally modify the free42 software to handle the keyboard mapping and display and set it to start on boot.

The end result wouldn't be battery operated but given the size of the 97's case, this doesn't seem like much of a problem.

- Pauli


I wouldn't bother with the AT91SAM7L which is fine for a power constrained device such as the 30b or 12C but wouldn't be suitable for a desktop machine. We are constantly working on saving space here and there to be able to enrich WP 34S. On a platform with more power and memory this would be much less of a chore.


Pehaps this

Raspberry PI

Model A approx $25.00.

The concept is to run Linux on-board so a Free 42 native application might be possible. I guess a GPIO hack for keyboard and display is also feasible. Not much documentation to date, but tantalizing block diagrams. I confess this is not in my area of expertise so better minds than mine could discredit this hypothesis.



An initial run of 10,000 will probably vanish. They aren't taking pre-orders.


It's an interesting product and the price is certainly good, but I don't think it will make a good calculator. From 6 AA cells, the battery life will be more like that of a cell phone than a calculator.


I was thinking "Desktop". No need for portability.


I would not want to chop up an HP42s.

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