New facts on HP-42S


Hello all,

In the process of developing an emulator for the HP42S, I made some discoveries on the HP42S internals that could also be useful for the people that want to expand their machine. I found that it could be possible to add a second ROM inside the HP42S that can be accessed by the hidden XFCN function. I will never open my machine to test it, but I know that some of you did it to upgrade the RAM. You will find the complete description in the article forum.

Best regards.



Thanks for your very interesting article! I'm new to the 42S but have spent a lot of time with the 41. Can you point me to more details about the 42S Memory Browser. I didn't realize this was possible. Also, how do you obtain the INPRT program for the 48. Now I'll have to go get a 48 to try everything out.



INPRT for the 48 is available at is a good place to check for anything related to 28, 48, 49, 38, 39, and 40 calculators.

Regards, James


Thanks for the excellent article. One question: is it possible to access the internal clock and use it for time/date functions, like in the HP-28S? A clock/timer would be an excellent complement for the 42S.


I wrote a quick user code program that can be used as a clock/timer for the 42s. It keeps well enough time for timing most things, about a second or 2 +/- in an hours time.


I bought my HP 42S in 1991. I read in an issue of Chip Chapter newsletter that HP had plans for an expandable version of the HP 42S, but when when the HP 48 series calculators were selling, the idea was put on the shelf. It would be nice if an HP 42S emulation were available for the HP 48GX (there are HP 41 emulators on A big wish of mine would be for HP to introduce a next generation expandable graphing calculator to succeed the HP 48GX and have HP 42S emulation built in.

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