(1978) This pocket scientific calculator performs the four basic arithmetic operations and a multitude of complex functions. IIt contains four data registers organized in a stack. It uses reverse Polish notation, where first the data and then the type of operation are entered.
It is equipped with self-test routines to detect malfunctions and identify which circuit is malfunctioning. It went on sale at a price of $ 60, half the price of its predecessor, the HP21, released in 1975.