A method and system for performing serial data communication between a main device and an external module connected to the main device. The data communication system and method include a main device, and an external module connected to the main device and communicating data with the main device. The external module transmits its identification information to the main device before the external module and the main device communicate the data between each other, and the main device receives the identification information from the external module, confirms its connection to the external module, and transmits an identification information confirmation signal to the external module.