HP 9820A - Introduction The desktop calculator HP 9820A, introduced in 1972, was the first HP calculator with algebraic entry system. However, the HP9820A uses algebraic entry system only at user lever. Internally, the calculator use RPN (Reverse Polish Notation) system. All entries are translated into RPN. The HP9820A has very similar hardware structure like predecessor HP9810A. The three-line LED display at HP9810A was replaced by one-line 16-character 5x7 dot-matrix LED display. The calculator HP9820A without options has five functions (add, subtract, multiply, divide, and square root) and six memory registers (named A, B, C, X, Y and Z). The calculator has a 16-column thermal dot-matrix printer and a 2"-wide magnetic card reader. The magnetic card reader is improved and in the contrary to HP9810A and HP9100B accepts magnetic cards in both directions. The standard magnetic card which is six inches long can store max. of 45 memory registers on each direction. An expanded magnetic card long 10½" has space for 90 memory registers on each direction. Like the predecessor HP9810A the calculator HP9820A has 4 slots in the rear for interface boards. To the interface board exists an I/O expander for next 9 slots. Price for HP9820A without ROMs was in year 1972 around US$5000.