HP-29C Neutral Operations

[brouhaha]

(02-10-2024 11:16 PM)rprosperi Wrote:  My 29C has had a 'brain transplant', now sporting an ACT from an HP-21, but I've found it acts perfectly standard in every other way, so while not really suspecting my machine, I do note it for transparency.

Bob, do you know the 1820-nnnn part numbers of your 29C's original ACT and the replacement? I agree that this shouldn't have anything to do with the calculator behavior deviating from the documentation, but I'm interested in data points on both the original factory usage of the various ACT part numbers, and what is known to work.

According to the HP-97 Service Manual, some ACT versions are incompatible with early HP-97 ROMs (specifically the 1818-0225 ROM/anode driver, vs the replacement 1818-0267), and based on my study, the same is likely true for early HP-67 ROMs (old 1818-0227, replacement 1818-0268). Note that this is unrelated to the known transcendental bugs, which were fixed by replacing the common 67/97 math ROM/RAM chip pair used in both models, original 1818-0226 and 1818-0228 with replacement 1818-0551 and 1818-0550, respectively.

The same issue might have affected the 19C/29C; Despite what might be expected, the 19C/29C microcode is much closer to the 67/97 code than to the 25/25C. The 19C/29C ROM/anode drivers did undergo a change in production: 19C original ROM/anode driver p/n unkown, replacement 1818-0432; 29C original 1818-0378 ROM/anode driver, replacement 1818-0431.

Despite what might be expected, the 19C/29C microcode is much closer to the 67/97 code than to the 25/25C.

By my count there are at least seven different ACT part numbers for the "normal" PMOS ACT, and of course one for the special NMOS ACT of the HP-27. I believe there are at least minor functional differences between some part numbers, possibly beyond the one known to affecting the early 67/97 code. I need to get dumps of the early 67, 97, 19C, and 29C microcode for comparison.