HP-28 series

Hewlett-Packard
Industry	Electronics, Computer hardware
Founded	January 1, 1939; 87 years ago
Products	Graphing calculators (HP-28C, HP-28S)

The HP-28C and HP-28S represent graphing calculators manufactured by Hewlett-Packard during the period from 1986 until 1992. Notably, the HP-28C was the pioneering handheld device capable of solving symbolic equations. These models were eventually replaced by the HP 48 series, which enhanced the menu-driven RPL interface initially debuted in the HP-28 lineup.

History

Two variants were launched: the HP-28C in 1987, equipped with 2 kilobytes of accessible RAM and recognized as the first handheld to incorporate a Computer Algebra System, followed by the HP-28S in 1988, which offered 32 KB RAM along with a directory structure for organizing variables, functions, and programs. The HP-28C operated on a Saturn CPU clocked at 640 kHz, whereas the HP-28S utilized an advanced Saturn core known by the codename Lewis running at 1 MHz.

The HP-28C marked the final HP calculator to use the "C" suffix, originally signifying continuous memory, a feature that became standard by 1988. Later models such as the HP-28S adopted suffixes indicative of their target markets: "S" for Scientific, "B" for Business, and subsequently "G" for Graphic.

Clamshell

Both calculators employed a flip-open or "clamshell" form factor, featuring an alphabetic keyboard positioned on the left side and a scientific keypad on the right.

LCD display

Their screens consisted of a 137×32 pixel LCD dot-matrix display, generally presenting four lines: three dedicated to stack or command input and one reserved for softkey labels.

Limitations

A notable limitation was the lack of a computer connection interface, which meant data backup had to be performed manually through keypad input.

Battery cover issue

The clamshell design, shared with the HP-19B series, suffered from a common defect: strong battery spring tension combined with delicate plastic clips securing the battery cover often caused cracking or breakage during battery changes. Users frequently resorted to fastening covers with rubber bands or adhesive tape. This problem was addressed in subsequent models such as the HP-19BII by relocating the battery compartment beneath the main chassis.

Emulators

RPN 28x Calc – A scientific calculator application for iPad featuring an interface similar to that of the HP-28S.
Halcyon Calc – Calculator software available for iPhone, iPod Touch, and iPad devices.
Emu42 – An open-source Windows emulator supporting Hewlett-Packard Pioneer series calculators (including HP-10B, HP-14B, HP-17B/II, HP-20S, HP-21S, HP-22S, HP-32S/II, HP-27S, HP-42S) as well as clamshell models like the HP-19BII and HP-28S.

External links