HP 29 & 21

[teenix]

(11-13-2022 02:14 AM)zixingcheche Wrote:  please check the pic.
my 29C shows "0" when power on.
if I change the ACT chip from HP21 to this HP29C. does it works? thanks!

The HP-21 ACT is 1820-1396, the HP-29C is 1820-1596 (also with HP-25).
The HP-22 ACT is 1820-1523, with pinouts same as 25, 29C.
HP-27 is NMOS so ACT from it is likely incompatible.

External connection differences are the PWO input and the DATA line (both missing on HP-21)

Bob has mentioned swapping them will work, so these two functions look to be implemented inside the ACT's, with the data bus and reset logic not used for the HP-21.

If the ACT's are all compatible (with exception HP-27) then serial number differences might be just chip design modifications.

cheers

Tony