HP-29C ACT compatibility

[PANAMATIK]

HP-21/25 ACTs can repair a defective HP-29C

HP used the so called ACT chip (Arithmetic Control Timing) in all Woodstock calculators HP21/22/25/25C/27/29C and later also in the HP-19C/67/91/92/95C/97. Finally they developed a new Chip for the next generation, the "Spice" calculators HP-31/32/33/34C/37/38, which made the ACT chip obsolete. From the first HP-21 to the last HP-97 this chip evolved and got some internal enhancements in several steps, whereas the details are not known. Basically the later ACTs needed to communicate with the PIK I/O chip and had a few more instructions for printer and magnetic card reader handling.

The HP-29C used the later 1820-1596 or 1820-1741 ACTs, whereas the HP-21 and HP-25 used the earlier 1820-1396 1820-1523.

Although I made a couple of repairs of "Woodstock" calculators by replacing their defective ACT chip with the new ACT chip, I never tried to run an HP-29C calculator with an original HP-25 or HP-21 ACT. Yesterday I made this experiment and put an HP-21 and HP-25 ACT into my HP-29C. I tried nearly every function and tested progamming and the "Continuous Memory". It works with both ACTs without any issues. I assume, that the later enhanced ACTs were used in all models by HP, but only the high end calculators with printer and card reader used the enhanced features. The HP-29C however did not make use of them and therefore the earlier ACTs can be taken as an replacement.

I hope you will not transplant an ACT from a working HP-21 into an HP-29C and throw away the HP-21, which would be a shame. But if for some reason you have an HP-21/22/25 ACT as spare, then you could repair an HP-29C with it.

Here a view to my little box with (mostly defective) "Woodstock" calculator chips, the remains of some "Woodstock" repairs.



Bernhard