Trying to understand Spice ROM/RAM Layout

[Michael de Estrada]

I have been restoring the HP Spice series calcs for some time now, and have come to the realization that I do not understand how the RAM/ROM is arranged. All Spice series calcs have one large IC chip and 1 or 2 or 3 small ICs. Yesterday, I was working on my HP-34C with non-soldered keyboard and accidentally knocked 2 of the 3 small ICs out of their holder. I took a 50-50 guess as to which one went where. When I reassembled the calc, it seemed to work perfectly in both run and program mode, and passed the self-test, so I assumed that I had guessed correctly. However, when I switched it off and then on, I got a Pr Error and memory had been erased. At first I thought is was a bad connection problem, but when that did not work, I switched the two ICs, and to my amazement the calc worked properly and the Pr Error problem disappeared. In the past when I got the small ICs out of order the calc did not work properly and gave the self-test Error 9, indicating a self-test failure. So, what exacly are the ICs? Is the large IC a ROM and the small IC(s) just RAM? If so, how are the small ICs arranged? I'm sure someone out there knows all this.

TIA - Michael

[Eric Smith]

In the E series, all three eight-pin DIP locations are interchangeable.

In the C series, the eight-pin DIP location nearest the display is wired for battery backup, so if you get the wrong chip in that location, the calculator won't have continuous memory, and may also have reduced battery life.

[Michael de Estrada]

Thanks, Eric. So, if I understand correctly, the small 8-pin DIP ICs are all simply RAM, and in the case of the C suffix models, only the top IC is CMOS. Also, since I've found that the keyboards are interchangeable among the E and C series models, then the wiring of the top 8-pin DIP location only for battery power when the calc is switched off must be a function of the power circuit board, which I know are different between the E and C series. I am still perplexed with an earlier experience with an HP 38E, which did not work when the order of the 3 8-pin DIPs was changed.

[Eric Smith]

The 8-pin chips contain varying mixes of ROM and RAM. I don't think any of them are RAM-only, but some are ROM-only and some are combined.

I don't have any idea why reordering the chips in a 38E wouuldn't work, unless one was backwards (rotated 180 degrees), which is obviously a no-no. All three 8-bit positions are wired entirely in parallel in the E series.

Edited: 17 June 2011, 8:00 p.m.

[Michael de Estrada]

The chips were definitely not rotated, but I cannot replicate the situation since the calc has long since been parted out for use in the repair of other calcs. Anyways, it's just an academic question now, based on your explanation. Thanks, again.

- Michael