HP-25C Eumulator

[PANAMATIK]

(04-20-2017 06:18 PM)rprosperi Wrote:  No, after power-off/power-on, the 25C comes up in FIX 2 mode.

I suppose the thinking was that if you shut it off you are most likely done with your current problem, and then when going back to the 25C for the next problem, you may (or even likely will) prefer to use different display settings.

Since the 25C was derived from the existing 25 f/w, they no doubt didn't want to have to change the display logic settings to add storage into NVR, so would have come up with some logic like the above to explain why it's not saved.

Correct! The transition from HP-25 to HP-25C by HP was ingenious and made a very fast time to market possible at these competitive days. The firmware remained bit by bit identical, even the "Clear Memory" instruction, which cleared the external RAM chips at initialization, was left in place. There were only two hardware changes made: the two PMOS RAM chips were replaced by two pin compatible CMOS RAMs, and battery plus was connected to a previously unused pin of these RAMs. As the PMOS RAM chips itself (and not the ACT processor) decoded and performed the "Clear Memory" instruction, the CMOS RAMs decode unit was modified and just didn't execute this instruction.

The HP-25 firmware used the two RAMs (each containing 56 bytes) for storing 49 program steps plus 8 registers and the LastX register, 49+9*7=2'56=112 bytes, and placed the stack and display format inside the ACT chips volatile memory, the HP-25C consequently could not save the stack and display format after power off, because only the external CMOS RAMs were made non volatile.

I may allow to mention: The HP-25E since Rev 1.10 newACT replacement (HP-25 repair kit) also saves the stack registers and display format together with up to 111 programs and 88 registers in nonvolatile memory, and keeps their state and data even when batteries are removed.

Bernhard