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.
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.
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.
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.
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.
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