Hexadecimal Digits on the HP-12C

Some time ago I began writing random notes for an article around the HP-12C, but I received an unexpected job (which was very welcome!) and forgot them completely. But while reviewing those notes I’ve thought there is enough stuff for an article.

I have two HP-12C calcs, a 1982 model with the first Spanish edition of the manual and a recent Malaysian unit, also with its corresponding manual. Both books are almost identical but I noticed some differences which might be interesting to readers with an old guide. Originally I intended to describe all changes but I wrote down only a couple of them. Both are in appendix E.

The first printing doesn’t mention the asterisk which appears when batteries are low. Instead it says that numbers won’t appear properly, some parts will become dim. But the asterisk does exist, at least in my calcs. So it’s probably an error in the manual as Wlodek noted in his excellent book "A Guide to HP Handheld Calculators and Computers", 2nd edition, page 47.

The second difference I observed is in the section "Verifying Proper Operation." The first printing says that when the machine will not turn on or does not operate properly the batteries should be removed and reinserted. Then it describes the self-tests (ON and ¸ , +, or ´ ). And that’s all. It does not mention the option of shorting the battery terminals together. And it does not include the following paragraph extracted from the later manual:

For a calculator that does not respond to keystrokes:

1. Press the PMT and ON keys simultaneously and release them.
   This will alter the contents of the X-register, so clear the X-register afterward.

(Readers with Spanish manuals will notice that this last sentence is not translated correctly there)

I tried out various values into the X-register before pressing ON+PMT and noticed that some caused the appearance of weird symbols among the figures viewed when executed the mantissa function (f-PREFIX). For example, if 3 is keyed into the X-register then after pressing ON+PMT and f-PREFIX (and while holding down the enter key) this is what is displayed:

In fact, this "lower-case o" is a cipher. This can be proved by subtracting 10^-6 from the number in the X-register. The result is (viewed with f-PREFIX):

Let’s do it again:	r 0 0 0 0 0 0 0 0 0
One more time:	9 0 0 0 0 0 0 0 0 0

So "r" is the way the HP-12C displays the hexadecimal number A, "-" corresponds to B and "o" to C. What about the symbols for D, E and F? The previous procedure works but starting now with the double of the original number. The following sequence:

3   ON+PMT   ENTER   +   f-PREFIX

will show:	1 E 0 0 0 0 0 0 0 0
Subtracting 10-6:	1 P 0 0 0 0 0 0 0 0
and again:	1 o 0 0 0 0 0 0 0 0

Oddly enough, the symbol for F is a blank: this keystroke sequence

3   ON+PMT   ENTER   ENTER   +   EEX   CHS   6   -   +   f-PREFIX

returns

2   0 0 0 0 0 0 0 0

Now we can build this table:

Why these symbols? Well, with these symbols the HP-12C can form the error messages, including the "Permanent Error" (Pr Error) and also the display shown by MEM. I guess "Error x" is actually FEAACAFxFF!