How to transform an HP-25 into an HP-25C ?


Is it possible to make a Continuous Memory calculator out of an HP-25 ?

Maybe with the right connections from the battery terminals?

Probably adding one of those button-cell 3V batteries to maintain the tiny current needed to keep the chips' memory ?

I am willing to risk my newly acquired HP-25 if someone tells me What / Where / How to make the connections.



Hi Gionanni

The 25C was built with CMOS (complimentary metal oxide semiconductor) circuitry, which used MOSFET logic, which required very little current to operate. The 25 used TTL (transistor-transistor logic), which draws much more current. I'm afraid the only way to make your HP 25 non-volital is to leave it on all the time.

best regards, Hal


The HP-25 and similar calculators were built with PMOS technology, the HP-27 was made with NMOS. CMOS made their appearance in the HP-25C, followed by HP-29C and so. Not all the HP-25C was made with CMOS technology, only part of the RAM memory is CMOS. That's why only registers R0 thru R7, program steps and (if memory helps) X register are preserved with the calculator off. In the 29C, only part of the registers are non-volatile. The HP-41C is a full CMOS design.

TTL is based in bipolar transistors (as oposed to MOS), and its power requirements are much higher; also the density (devices per chip) was much lower than MOS. It is not a suitable technology for portable devices, in general terms.


FYI - The stack X-register is not preserved in the HP-25C when the calculator is turned off, however, it is preserved in the HP-29C.


If I recall correctly, the HP-25C saves R0-R7 (eight 56 bit registers), 49 program steps (seven 56 bit registers, or 7 x 7 x 1 Byte), and the LAST X register; all of them in a 16 register, 56 bit per register CMOS RAM chip. So the X register is not preserved, as you said.

In the HP-29C, there are 32 non-volatile registers (double than in the 25C). Not completely sure about this, but I think that 98 program steps are saved using 14 registers, 16 data registers are saved, and a backup copy of X is also saved (the "real" X is on the CPU). I think that LAST X is not preserved on the 29C, perhaps the remaining regsiters are used to save the program counter and pending return addresses for GSB instructions.


Correct. Last X is preserved in the HP-25C but is not preserved in the HP-29C.

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