The computer will be of no use if it is not communicating with the external world. Thus, a computer must have a system to receive information from out world and must be able to communicate results to external world. Thus a computer consist of a I/O system. This system include two basic components, one is the I/O device and another called I/O module, which not only connects an I/O device with the system bus, but play a very crucial role in between. A device which is connected to an I/O module of computer is called a peripheral.