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Though I became a HP fan shortly after, my first calculator was actually a TI-1200, a basic calculator with the 4 operations and % with a very nice design. My father's company had given calculators to their best customers, and some had been returned as 'defective'. All it took was to replace the 9V battery and there I had my first calculator.

So just for the fun of it, and in admiration of the equally zen design of the Commodore 7919 (scientific model with just 19 keys), I endeavored to design a Microchip PIC based board for TI-1200 that would convert it to a scientific RPN model.

So here it is: RPN, trig/log/power functions, metric<->US conversions, 4-level stack with LASTX, inpout through a 'command line' with <- capability, 10 memories, store and recall arithmetic, FIX and SCI modes.

The code is written mostly in C (some inline ASM) using the Microchip MPLAB X IDE. The only drawback is that the floating point routines are binary based and the precision is really not good... If anyone has worked on a similar project, or has recommendation / source code for BCD arithmetic and floating point functions in C or PIC assembly, I am definetely interested.

Pictures of the RPN-1200 vs TI-1200 and of the inside of the RPN-1200 are at:


Thank you,


Very nice. I did something very similar with an HP-25 that had a totally corroded PCB and bad chips. I gutted it and used a single SX-28 chip to scan the keyboard and drive the display. This chip has very pretty limited RAM and ROM (128 bytes/2K bytes), so I only put in long (16 digit) integer functions.

Hi Katie,

Thank you for your feedback.

Packing everything in the HP-25 must have been very hard - there is almost no space!

The RPN-1200 is based on a Microchip PIC 18F2550 with 32k-words of program flash (about half of which is used), and the display is driven by a MAX7219. I tried to use a PIC 16F88 which is a simpler chip but the 4k of flash were not enough for floating point operations...

In any case, what I learned through the project is that making a calculator is not easy...


This is just plain crazy ;-)

Awesome stuff.

You should probably look at the source code for the WP-34S. The calculation routines in this are based on the decnumber C library, which represents numbers in a packed decimal form and allows a range of precisions. The WP-34S firmware takes up about 118 kB when compiled which is a lot more space than you have available, but the WP-34S does much more than your calculator does, so perhaps the decnumber routines could be made to fit?

Nigel (UK)

I forgot to say: your calculator looks absolutely amazing!

Nigel (UK)

I hate to be that guy but I miss the double-wide ENTER key. On a serious note: very cool calculator!



Very cool conversion!

My first scientific calculator was the Commodore SR4148R. It was the opposite of the 7919 and had 48 keys. No shift key needed on that model :)


That is awesome. You did the right thing. You saved a 1200 from itself. :-)


Thank you for your encouraging and funny comments

Thank you for your guidance. Will check decnumber libraries.

The RPN 1200 looks nice - great job with the conversion!

And this even surpasses that Sinclair Scientific!!! Besides, full 10+2 digit display as well as at least 12-digit accuracy. Excellent and stylish too!