There are several reasons why an I/O device or peripheral device is not directly connected to the system bus. Some of them are as follows -

• There are a wide variety of peripherals with various methods of operation. It would be impractical to include the necessary logic within the processor to control several devices.


• The data transfer rate of peripherals is often much slower than that of the memory or processor. Thus, it is impractical to use the high-speed system bus to communicate directly with a peripheral.


• Peripherals often use different data formats and word lengths than the computer to which they are attached.

Thus, an I/O module is required.