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

[Giovanni Jimenez]

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.

Thanks...Giovanni

[Hal Bitton in Boise]

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

[Andrés C. Rodríguez (Argentina)]

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.

[Michael de Estrada]

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.

[Andrés C. Rodríguez (Argentina)]

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.

[Michael de Estrada]

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