A POS system and method having a POS terminal, at least one input device for providing input data such as product sale information to said POS terminal, a POS server to enable the POS terminal to complete a POS transaction based upon such product sale information, a display device and a printing device further including an input/output control apparatus 12 and at least one function adding server 50 to enable the POS system to perform at least one new functional application without requiring any change in the POS system terminal or server. At least one of connection ports 21b to 23b of an input/output control apparatus 12 is connected to at least one the connection ports 21a to 23a of an existing POS terminal 30 having at least a POS printer 34, a input device 32, or a display 33 connected thereto. The input devices 32 to 34 conventionally connected to the ports 21a to 23a of the POS terminal 30 are connected to the corresponding ports of the I/O control apparatus 12. A function-adding server 50 is also connected to the I/O control apparatus 12. The I/O control apparatus 12 performs an identification operation to determine if input information from the input devices meet specific conditions (identification criteria) and if so additional data is provided by the function-adding server 50 for performing additional functionality.