Saturday, March 9, 2013

Peripheral Devices

Peripheral Devices

Peripheral Devices

A peripheral is a device outside the central processing unit but controlled by it.

The term device is used to refer to any machine, but particularly computer peripherals.

A medium is a material which data is stored on or output to.

Note: It is important to use the terms 'medium' and 'device' carefully.

e.g. 1 A printer is an example of an output device; printer paper is an example of an output medium.

2 A magnetic disc unit is a storage device; the disc is a storage medium.

There are four main categories of peripheral:


These consist of an input and an output device combined in one unit.

Examples of input/output devices

1 A visual display unit.

2 A teletypewriter terminal.


These accept data from outside the computer system and transmit it to the central processing unit.

Examples of input devices

1 The keyboard of a computer terminal.

2 A mouse.

3 An electronic digital weighing scales with a computer interface.

4 A document reader.


These receive data from the central processing unit and change it to a form which can be used outside the computer system.

Examples of output devices

1 A lineprinter.

2 A screen on a computer terminal.

3 A voice synthesizer.

4 A microcomputer-controlled robot which welds cars.


These are used to write to and to read from storage media outside the central processing unit.

Examples of backing storage devices

1 Magnetic tape unit.

2 Magnetic disc unit.


On line means directly connected to a computer and under its control. Thus when a peripheral is in use it is on-line.

Off line means not under the control of a computer's central processing unit.

Examples of on-line and off-line

1 Data is prepared and validated on a disc using a key-to-disc station. This station is on-line to a minicomputer. The disc is then transferred to another disc unit for processing. This second disc unit is on-line to a mainframe computer.

2 When a computer terminal is connected to a computer it is on-tine. When it is disconnected from the computer it is off-line.

3 When a magnetic tape is stored in a rack it is off-line. When it is placed in an on-line tape drive it is on-line.