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Dear all,
can you explain me the difference between the ROM modules for HP41x series where an "X" is added?...for example I have seen the "aviation" and the "aviation x" module. Where and how can be used?. Seems that there are restrictions to use "X" modules in HP41c calculators.

Thanks in advance.



Those were modules with the same XROM id#, so they were not supposed to be plugged simultaneously in the calculator. That's all, not fancier or more strict than that.

Cheers, AM

To add some detail to Angel's response...

The "Aviation" and "Aviation X" modules are identical, except for their labelling. Due to the limited number of XROM numbers (max of 31), HP determined that some modules would have to use identical XROM numbers. Since modules with identical XROM numbers will interfere with each other if they are installed in an HP-41C at the same time, HP selected a group of modules from differing disciplines for which users interests would not overlap. These modules were all assigned XROM 19. At one point, HP thought that it was necessary to clue the user into the fact that these modules were incompatible with each other by adding an "X" designation to the module name.

These are the HP 'X' modules that will conflict with each other if installed in the HP-41C at the same time.

- Aviation (with or without the 'X')

- Clinical Lab X

- HP-IL Diagnostic

- Securities X

- Strc Anal (with or without the 'X')

So, if you were a nuclear pharmacist, designing your own home, managing your own investments, flying your own airplane, and with an interest in designing HP-IL equipment, you could not have more than one of these modules plugged into your HP-41C at any one time for fear that the program in one module would inadvertently call a program in another module as a subroutine. Disaster would ensue, your patients would die, you'd lose your retirement investments, your house would fall down from poor design, you'd crash your airplane, and after all that, your HP-IL analysis would not work. Bummer.


thanks for that explanation about not using the incompatible modules!


many thanks for your answers! now it is clear.
Only one more doubt: this is only for HP1c...not for CV or CX? I suppose it is for all 41x
Just mention that I own the Aviation, Struc. Analysis...so I will follow your recommendation!

Thanks again

Hi Ignacio,

XROM ID conflicts are unrelated with the 41 version you're plugging the modules at.

C/CV/CX/CY/CL... will all shown the same issues if you insert more than one module with the same XROM nr.

Hope this helps.

Best wishes from the Canary Islands.


PS: Note that althoug the modules mentioned in this thread were the only ones marked with an "X" by HP in order to help avoiding problems, there are many different modules sharing other XROM numbers; therefore it is advisable to check no such conflicts arises in advance.

Ángel has a nice tool for this purpose... ;-)

That would explain last week!