HP-28C

With the HP-28C, Hewlett-Packard completely revamped the scientific calculator. This model did symbolic math, used named variables, understood many data types and was the first full RPL calculator. (The HP-18C used System RPL internally but didn't make RPL available to the calculator user.) It was extremely powerful with a wealth of functions and many features inherited from the HP-18C including infrared printing which freed the user from cable clutter, labeled soft keys, menus and the dedicated alphanumeric keyboard which made creating complex programs and expressions more convenient.