The HP-28C and HP-28S were two graphing calculators produced by Hewlett-Packard from 1986 to 1992. The HP-28 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 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, and was the first calculator with a Computer Algebra System[1]. 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.
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.
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.
The plastic of the case around the battery cover is prone to breaking[2], surviving examples of this calculator frequently have tape over the cover to hold it in place (including the image attached to this article).
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