The original HP-15C from 1982 was my first programmable #RPN #calculator. I bought mine in 1985. By then, I was an upper-level #EE student, with a few years of #programming experience in a handful of languages from different paradigms, including assembly. Yet, I found keystroke programming on the HP-15C to be a challenge.
This little machine could store only 500 keystrokes in its memory, and the instructions were shown as numeric codes. It was rather like programming on the ET-3400 or the KIM-1—it was a right mess. Still, the HP-15C was the best engineer's pocket computer I could afford then, and I thoroughly enjoyed learning to use it well.
Not long after, when I entered the industry, EEs were all using MATLAB on Sun workstations, and our RPN calculators mostly sat on the desk. That was the beginning of the end for calculators. Yet, to this day, the HP-15C remains my all-time favourite RPN calculator.
PS—Those who are curious about programming the various HP RPN calculators may read my article on the subject.
https://amenzwa.github.io/stem/ComputingHistory/HowRPNCalculatorsWork/