• I/O modules are the critical element of the computer system after CPU and memory. All computer systems must have efficient means to receive input and deliver output. Wide variety of peripherals in different amounts of data, at different speeds and in different formats, all are slower than CPU and RAM.
• Input-output module provides a method for transferring information between internal storage and external I/O devices.
• Peripherals connected to a computer need special communication links for interfacing them with the central processing unit.
• The requirements of the communication link are to resolve the differences that exist between the central computer and each peripheral.

The requirements are as follows:

1. Peripherals are electromechanical and electromagnetic devices and their manner of operation is different from the operation of the CPU and memory, which are electronic devices. Therefore, a conversion of signal values may be required.
2. The data transfer rate of peripherals is usually slower than the transfer rate of the CPU, and consequently, a synchronization mechanism may be needed.
3. Data codes and formats in peripherals differ from the word format in the CPU and memory.
4. The operating modes of peripherals are different from each other and each must be controlled so as not to disturb the operation of other peripherals connected to the CPU.

• A typical I/O module is shown in Figure 8:

1. Device Communication - communicate with external devices
2. Data Buffering - Temporarily hold data between being transferred between the I/O module and external devices.