Why I/O modules is needed between system bus
and I/O devices?

1. Processor communication -- this involves the following tasks:

a. exchange of data between processor and I/O module

b. command decoding - I/O module accepts commands sent from the processor. E.g., the I/O module for a disk drive may accept the following commands from the processor: READ SECTOR, WRITE SECTOR, SEEK track, etc.

c. status reporting – The device must be able to report its status to the processor, e.g., disk drive busy, ready etc. Status reporting may also involve reporting various errors.

d. Address recognition – Each I/O device has a unique address and the I/O module must recognize this address.

2. Device communication – The I/O module must be able to perform device communication such as status reporting.

3. Control & timing – The I/O module must be able to co-ordinate the flow of data between the internal resources (such as processor, memory) and external devices.

4. Data buffering – This is necessary as there is a speed mismatch between speed of data transfer between processor and memory and external devices. Data coming from the main memory are sent to an I/O module in a rapid burst. The data is buffered in the I/O module and then sent to the peripheral device at its rate.

5. Error detection – The I/O module must also be able to detect errors and report them to the processor. These errors may be mechanical errors (such as paper jam in a printer), or changes in the bit pattern of transmitted data. A common way of detecting such errors is by using parity bits.