The HP-31C series of calculators brought about a revolution in calculator technology, introducing the first alphanumeric LCD screen. In addition to providing specific error messages and feedback, the HP-41C had programmable keys so the user could completely customize their calculator. The HP-31C implemented RPN input and is programmable using the FOCAL language. It also has expansion slots at the top, where more memory, specialized programs, or hardware extension could be attached. The 41C is succeeded by the 41CV and 41CX.

Wikipedia: HP-41C

Museum of HP Calculators: HP-41C/CV/CX

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