It was a joint effort by Paul Brogger and Tony Duall. The last thread was here: http://www.hpmuseum.org/cgi-sys/cgiwrap/hpmuseum/archv006.cgi?read=11137.
I was able to duplicate Tonys circuit and add a PIC to help out. I had one talking serial comms at 75 baud with a PIC mpu interfaced to the analog muxs connected to the keyboard lines. It worked for loading programs into the 42S, but I was not able to complete the project and contain it within the calculator. The problems I encountered are:
1. Trying to fit the PIC, cmos mux, and MAX232 chips inside the 42S. With a .032" pc board, the surface mount chips are still too thick to fit without hitting the metal shield on the back plate.
2. Finding a suitable connector for the serial comms to fit inside the case.
3. Capturing the I/R output stream and decoding to redirect to a PC via standard ASCII. The actual decode is easy from a software standpoint, but it is the increase in processing power required leads to significantly decreased battery life. I figured I would want upload ability if I changed or wrote a program in the field and wanted to archive it.
It all came down to anything I did to add external circuitry would tax the weak little 90mah button cells big time. That lead to supplying power from the pc. Sounds easy, but I couldn't find a decent 4 pin connector that would fit inside the case. The super mini-usb connector used on the new digital cameras seemed like a good idea, but I couldn't figure out a way to secure and connect it. The 4 pin 2mm connector on the 48 would work with some case hacking, but I didn't want to bother.
I put the whole thing aside months ago and haven't given it further thought. It seems to me the proper way to do it is to disassemble the ROM and add the input via I/R through an unused input pin on the Saturn processor. The extension code could go in a 28 pin otp eprom and be soldered onto the logic board in the vacant position used by the language rom on the international version 17Bii. Very clean, but requiring a large effort to find the menu handling and key assignments in the rom. The I/R stream handling is probably already done somewhere in Saturn assembly as it exsists in the 48. Christof had started some disassembly, I don't know what progress he made.
As much as I wanted to make this work, I just don't have the time to devote to it. Maybe if I win the lottery...