Type

Used graphing calculators

HP's RPL calculators, which combine RPN with a programming language, symbolic math and graphs. The HP-28S of 1988 folds open like a book, with the letters on the left in alphabetical order, and does algebra symbolically. The HP 48 series replaced it, and the G models of 1993 added input forms and 3D plotting. The 48G has 32 KB and no card slots, and the 48GX has 128 KB and two slots for RAM and ROM cards. The 48s have a built-in self-test that checks the screen and every key, which is the easiest way to judge a used one. On a 28S, look at the battery cover first, since its notches crack.

3 calculators we cover.