HP-28 series

The HP-28C and HP-28S were two graphing calculators produced by Hewlett-Packard from 1986 to 1992. The HP-28C was the first calculator capable of solving equations symbolically. They were replaced by the HP 48 series of calculators, which grew from the menu-driven RPL programming language interface first introduced in these HP-28 series.

The HP-28 calculators shared a flip-open ("clamshell") case. On the left side of the flip, there is an alphabetic keyboard (in alphabetic order). On the right was a typical scientific keyboard layout. The display was a 137×32 LCD dot matrix, usually displaying four lines of information (3 stack/command lines, plus one softkey label line).

Two models were produced, the HP-28C came first in 1987 with two kilobytes of usable RAM, and was the first calculator with a Computer Algebra System. A year later, the more common HP-28S was released with 32 KB of RAM and a directory system for filing variables, functions, and programs. The HP-28C used a Saturn processor running at 640 kHz whereas the HP-28S used a custom chip containing an improved Saturn processor core codenamed Lewis and running at 1 MHz.