The I/O interface provides a technique for transferring information between external I/O devices and internal storage. Peripherals linked to a computer require special communication links for interfacing them with CPU. Objective of communication link is to resolve differences which exist between central computer and every peripheral. The key differences are:

• Peripherals are electromechanical and electromagnetic devices as well as their operations are different from operation of CPU and memory which are electronic devices.
• Data transfer rate of peripherals is generally slower than transfer rate of CPU and thus a synchronization mechanism may be required.
• Data codes and formats in peripherals differ from word format in CPU and memory.
• Operating modes of peripherals are dissimilar from one another and each one should be controlled so as not to disturb operation of other peripherals associated to CPU.

To resolve these differences, computer systems include special hardware component between the CPU and peripherals to supervise and synchronize all input and output transfers. These components are called interface units because they interface between the processor bus and the peripheral device.