The 9810A was HP's second generation RPN calculator. It replaced the 9100. Instead of a CRT, the 9810A had a three-line LED display. It was also upgradable with ROM modules to perform additional functions (math functions, peripheral I/O, etc). The 9810A had a built-in card reader and an optional 16-character, internal thermal printer. Unlike the 9100 which had a single expansion slot, the 9810A had four expansion slots in the rear of the machine.