Chris Tofu wrote: The original ones had two or three chips. They all (10C, 11C, 12C, 15C, 16C) had a proprietary "Nut" processor, software-compatible with that of the HP-41C, and an "R2D2" (RAM/ROM/Display Driver). The 15C has twice as much ROM and RAM as the others, so it had two R2D2 chips, the second of which has the display driver disabled and not bonded out. The bus is bit-serial with a two-phase clock, an instruction/address line, a data line, a sync line, and a flag input line. They did several cost reductions over the years, reducing it to a single chip, except for the 15C which still had the second R2D2. They introduced the "12C Platinum", which has a single-chip microcontroller made by GeneralPlus, using a 6502-compatible core. The firmware was completely reimplemented (twice!), and is not derived from the original 12C code. Most recently, they redesigned the 12C and 15C (reintroduced after having been discontinued in 1988) using a Atmel AT91SAM7L128 microcontroller, based on the ARM7DTMI core, with 128KB of flash memory. The firmware uses a simulation of the original Nut processor. If you wanted to repurpose a horizontal-format HP calculator, that's the model to use. There are six pads in the battery compartment that provide access to the CMOS-level UART and "ERASE" signal needed to reprogram the flash memory. However, once you reflash it with your own code, you can't make it a 12C again unless you have gotten an appropriate firmware image from somewhere, and it is NOT possible to extract such an image from that UART port. It is difficult to tell whether a 12C in the blister pack is the new ARM-based variant or old stock of the proprietary & masked-ROM variant. With the calculator out of the package you can easily tell because the ARM-based variant has a larger battery compartment that holds two CR2032 lithium coin cells, while the older models had either a single CR2032 or three A76/LR44 alkaline or silver oxide button cells. There is a lot of activity around repurposing the HP 20b and 30b business calculators, which use similar hardware but in a vertical format. The 20b and 30b actually do have a basic set of scientific functions, but they are buried in menus. There is some very powerful alternate scientific calculator firmware for those models, called "WP-34S". Unfortunately the 20b and 30b are being discontinued. The other model that shares essentially the same ARM7 based hardware architecture is the 10bII+ (also a business calculator), but I haven't yet heard of any serious efforts at repurposing that or the new 12C. Eric [Disclaimer: While I've never been an employee of HP nor provided contract services to them, I did provide them with some assistance in debugging their Nut simulation code used in the new 12C and 15C, and provided them with some high-performance BCD arithmetic algorithms. However, I take no responsibility for bugs in their simulation code, including several known bugs.]