Usefulness of Woodstock Cs without their C


Hello all.

Had I known to recharge my 25C and 29C batteries while installed into
either a 21 or 25, the CMOS functionality would have lived on.

But, now that the misstep has been made, on the 29C, the X-Register contents are retained. But, the program memory only retains programs less than 50 steps.

As for the 25C, at least I know that program memory is not retained in the Continuous Memory. I'm not certain about the data registers though.

Can the 25C and/or the 29C be salvaged by replacing the CMOS circuitry? Or, is there more damage than just the CMOS?

If these calcs' CMOS cannot be replaced/repaired, how practical do the 29C and 25C units become?

Edited: 8 Dec 2013, 6:41 p.m.


I'd never heard about that 50 program step boundary. Interesting.
They'll still look great in your RPN calculator shrine though.


I'm sorry if that was unclear. Although, yes, I can enter a full 98-step program, I have noticed that, if, after I have turned off the 29C, the next time I turn it on, only the first few steps will be retained.

I'm glad you posted because, now that I am thinking about it, there are times when I enter a program past the 50 step mark, incorrect keycodes are displayed as I enter longer programs. On the 25C, although battery stength is available after a week on being turned off, I will notice that program memory will be erased.

On another note, in RUN mode, manual/keystroke operations will return proper answers/results for the functions, expressions (i.e. calculator rests correspond to owner's manual examples).

Edited: 8 Dec 2013, 7:38 p.m.


If you plug in the AC charger to a Woodstock calculator, without a good battery load, you are likely to brick the ACT chip(main processor unit). In that case, you won't get any performance out of the calculator and it typically has random numbers that appear in the display when turned on.
This sounds like you may have damaged the CMOS memory chips. The only way you can repair this sort of failure is to find a "donor" HP29C or HP25C with good CMOS memory chips.


As I don't have (or have had in the past) a 25C or 29C, please see these are just comments from very old memories; these may or not help you, and I apologize in advance for any inaccuracy.

The HP 21 had no specific memory chip, and used the internal CPU (ACT chip) registers for its operations. The HP 25 included a 16 register memory chip. Eight of these registers were used for data storage, and named R0-R7. As the registers were 56 bit in size, seven bytes can be stored on each register. Hence, assigning seven registers for program steps, and using one byte for each step, 49 program steps were available.
The remaining register was used for LASTX.

When the 25C was introduced, the memory chip was replaced with a 16 register CMOS chip; so R0-R7, all program steps, and LASTX were preserved when the calculator was turned off. Apart from continuous memory, architecture and operations were the same as in the "non-C" HP-25.

For the 29C, CMOS memory was doubled. But other advances were introduced at the same time; for instance, labels and subroutines. In general, firmware was improved in many aspects, and special care was taken to "swap" X and LASTX contents; so the calculator will preserve X (instead of LASTX) when powered down. Now, 98 program steps were allowed, and a "not-previously-assigned" CMOS register was used to keep the display and angular modes. In this manner, all program steps, the display and angular modes, the X register and some storage registers were preserved when the calculator was off. Subroutines also needed at least some storage to keep the return address pointers.

It's unclear for me how it was that some of the storage registers and the Y, Z, T, and LASTX registers were not preserved; it may had to do with software compatibility restrictions between 29C and 19C, but this is just an unsupported guess.

Looking at each model MoHPC description and programming pages, and also looking for better documented postings (for instance, I would search for posts from Eric Smith and Tony Duell, among others) may bring additional information, and better accuracy, indeed.

Best regards.


I have a suite if routines loaded into my 29c totaling 98 lines. Haven't turned it on in 4 months. Just did after reading this, all 98 lines are there and it works perfectly.

Jim is correct, don't use the charger EVER. You can never be 100% sure that a connection is complete through the batteries due to battery failure or battery contact failure.


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