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 handheld 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 handheld 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.

The HP-28C was the last HP model introduced with the suffix "C" in its model designation – a practice which HP had started with the HP-25C back in 1976. The "C" had distinguished those models as having continuous memory. However, by 1988 that capability had become so common on calculators that it was no longer a feature of distinction, as it was an assumed characteristic of all serious scientific and business calculators. So beginning with the HP-28S, HP-17B, HP-19B, and HP-27S, the feature suffix "C" was replaced with a class suffix which was more meaningful in the market: "S" for Scientific, "B" for Business, and later (in 1993) "G" for Graphic.

...
Wikipedia