What is an I/O interface? Explain its functions. Why is an interface required between CPU and I/O devices?

I/O interface

Functions of an I/O interface

• Device selection: it decodes the address on the address bus and selects the correct device.
• Data buffering: it holds data in a register so that the fast CPU and the slow device can each work at their own speed.
• Data conversion: it changes the format of the data, for example parallel to serial and serial to parallel.
• Signal conversion: it matches the voltage levels and the timing of the device to those of the CPU.
• Control and timing: it gives commands to the device, and makes the read, write and handshake signals in the right order.
• Status reporting: it tells the CPU if the device is ready, busy or in error.
• Interrupt generation: it sends an interrupt request when the device needs service.
• Error detection: it can check for parity or framing errors in the data.

 CPU buses          I/O interface           Device
 Data    <==>   [data buffer      ]  <==>   data
 Address  ==>   [address decoder  ]
 Control <==>   [control / status ]  <==>   signals
 Interrupt <==  [interrupt logic  ]

Why an interface is needed

The CPU and the I/O devices are very different, so a direct connection is not possible.

Devices also differ from each other in the form of data and the operations they accept. Without an interface, the CPU would have to know the details of each device and would waste time waiting for slow ones. An interface gives the CPU one simple and uniform way to deal with every device. It also protects the CPU from the electrical and timing problems of the device.