HP-28 series

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The HP-28C and HP-28S were two graphing calculators produced by Hewlett-Packard from 1987 to 1992. The HP-28 was the first calculator capable of solving equations symbolically, as well as being the first graphing calculator. They were replaced by the HP-48 series of calculators, which grew from the menu-driven RPL programming language interface first introduced in the HP-28 series.

The HP-28 calculators shared a flip-open (book style) 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 137x32 LCD 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. A year later, the more common HP-28S was released with 32k of RAM and a directory system for filing variables, functions, and programs. The HP-28C used a single Saturn processor at 640 kHz whereas the HP-28S used a custom chip containing an improved Saturn processor core at 1 MHz (the HP-48S, the replacement for the 28, used the HP-28S CPU core at 2 MHz).

Among the drawbacks of the HP-28 was the lack of a computer interface. This meant that stored information could only be entered through the keypad and not backed up.

Many users of subsequent models miss the alphabetic keyboard which made program entry a lot simpler. The alphabetic keyboard is, however, difficult to use hand held.