Objective

Input Output Organization

External Devices

I/O operations are accomplished through external devices that provide a means of exchanging data between external environment and computer. An external device attaches to the computer by a link to an I/O module. An external device linked to an I/O module is called peripheral device or peripheral. The figure below shows attachment of external devices through I/O module.

External Devices can be categorized as

1. Human readable: suitable for communicating with computer user. For example - video display terminals and printers.
2. Machine readable: suitable for communicating with equipment. For example - sensor, actuators used in robotics application.
3. Communication: suitable for communicating with remote devices. They may be human readable device such as terminal and machine readable device such as another computer.

Block diagram of external device is described below.

1. The interface to I/O module: The interface to I/O module is in the form of

a) Control Signal – determines the function that the device will perform. E.g. send data to I/O module (READ or INPUT), receive data from I/O module (WRITE or OUTPUT), report status or perform some control function such as position a disk head. b) Data Signal – send or receive the data from I/O module. c) Status Signal – it indicates the status of signal. E.g. READY/NOT READY

1. Control Logic: associated with the device controls on specific operation as directed from I/O module.
2. Transducer: converts the data from electrical to other form of energy during output and from other forms of electrical during input.
3. Buffer: is associated with transducer to temporarily hold data during data transmission from I/O module and external environment. Buffer size of 8 to 16 bits is common.

Assignments