Input/output (I/o) unit:
This unit provides the interface between the computer and the computer peripherals. A computer peripheral is any unit, which is attached to, but is not part of, the computer - e.g. visual display units, teleprinters, etc. A simple computing system may have only one input and one output. In such cases, an analogue-to-digital converter (ADC) may suffice for the input, and a digital-to-analogue converter (DAC) for the output. Alternatively, complex-computing systems can literally service thousands of peripherals.
Figure illustrates a simple I/O unit. The I/O unit can be described as a fan-out (and fan-in) device. The computer's 8-bit bi-directional data bus can be connected to port 1, 2 or 3. The port chosen is dependent upon the address, on the address bus. The system illustrated allows three peripherals to communicate with the computer. Only one peripheral at a time can send data to the computer, or receive data from the computer. However, this is not a problem, because the computer works very much faster than the peripheral, and hence, it appears that the computer services all three peripherals simultaneously.