A ROM emulator for the 71B

[mfleming]

I thought some may be interested in a lockdown project I've been working the past four months or so. It is a PIC18 microcontroller-based ROM emulator for the 71B that can store multiple ROM images and serve up one image at a given time. In its present form the emulator resembles Diego's Clonix line of modules for the HP-41, except for the current single ROM at a time limitation. Here are a few images of the development setup and current board.



I started this project because I wanted to try some of the available 71B ROMs, including J-F's new Math ROM. The problem of course is that most of the original ROM modules (when you can find them) often cost more than the 71B itself. A quick check of the specs for the PIC convinced me that it might work as a software emulator of a ROM module. It was quite a challenge to meet the timing requirements while at the same time figuring out just how the 71B bus works.

The reason you see rechargeable batteries is because the recommended maximum operating voltage of the PIC controller is 5.5 Volts, with an absolute maximum rating of 6.5 Volts. When used with the higher voltage alkaline batteries, the processor lifetime would likely be shortened. Four Eneloops keep things well within recommended operating conditions.

Power consumption is fairly reasonable, drawing about 3.8 mA when operating. The controller does switch to Sleep mode 2-1/2 minutes after the 71B is turned off, drawing less than a micro Amp of current.

I currently have the ROM images of the Circuit Analysis Pac, J-F's Math2B, Finance Pac, Text Editor, and Ampi Statistics Pac loaded in flash. Plugging and unplugging ROMs is done via a POKE command as shown below. I've run through the many examples in the Math and Finance manuals to make sure all is working correctly.

I believe there is sufficient timing slack to also support a hard configured ROM in addition to a soft ROM. Then the Forth and HP-41 Translator ROMs would also be available. Right now, flash is updated by a PICkit 3 programmer in much the same way as one of Diego's modules.



I'm still something of an HP-71B newbie, but I know there is a large repository of LEX files maintained by Joe Horn. If someone could point me to documentation on making your own ROM image (and some favorite LEX files of your own!), I'd love to add a custom ROM to what I have now.

I must thank Diego Diaz for his Clonix module line for inspiration, J-F Garnier for answering questions on the 71B behavior, and especially Tony "teenix" Nixon for answering questions on PIC assembly programming. This has been quite a fun project and it now gives me plenty of motivation to learn the 71B in greater depth and detail!

~Mark

Edit: Add J-F for well deserved credit.