06-10-2012, 04:23 PM
Input/Output Devices & Storage Media
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Input Devices
An input device is a peripheral which accepts data and sends it to the CPU. Data presented
to an input device has to be in the right form for the device, e.g. a bar-code reader will only
read bar-codes. The input device converts the data into the computer's own internal code
before sending it to the CPU.
Keyboards
The most common method of data input device is the QWERTY keyboard (so named
because of the order of the first few characters on the
top row of letters reading from left to right. This is now
the standard layout for keyboards in most countries of
the world. The computer continuously scans the
keyboard and when a key is pressed, a signal is sent to
the CPU using the code for the particular character
represented by the key.
Keyboards are used
• On-line data entry.
Data is input straight into the computer for processing.
• Off-line data entry.
This is usually achieved by methods known as key-to-disk or key-to-tape. As data is
typed using a keyboard, it is recorded on disk or tape. The data is later transferred from
the disk or tape to a computer for processing.
Mouse
A mouse is used to input data in two different ways.
Firstly, as the mouse is moved, a ball underneath it
revolves and this movement is detected by sensors
which relay the data to the CPU. Secondly, buttons on
the mouse act in a similar way to keys on a keyboard. When they are pressed,
the code for the relevant control character is sent to the CPU.
Methods of Reading Marks
Mark Sensing
Small electrical 'brushes' touch the surface of the document or card. When they contact a
pencil mark, a circuit is completed. The marks must be made by a pencil which contains
graphite (HB) as this conducts electricity, e.g. objective test papers.
Optical Mark Reading (OMR)
A beam of light is directed onto the surface of a card or document. The beam is reflected
from the surface to a light sensor. When a mark passes under the beam, less light is
reflected back and so the presence of a mark is registered, e.g. National lottery forms.
Graphics Tablet
A graphics tablet (also known as a digitising pad or digitiser) is a board which can detect the
position of a pen-like stylus on its surface. A design is put on the board and its shape is
traced out with the stylus. The device digitises the analogue signal from the stylus and sends
the data in digital form to the CPU for processing.
Voice Recognition
A microphone is used to detect speech sounds and relays these in analogue form to an
interface. At the interface the signal is converted into digital form and sent to the CPU for
processing. Following programmed instructions, the computer analyses the sound patterns
received from the microphone and compares these with patterns stored in its memory. In this
way, the sound patterns of words can be identified.
Concept Keyboard
This is similar to a touch-sensitive screen except that the touch-sensitive surface is laid over a
board (similar to a graphics tablet in appearance). As in the case of the touch-sensitive
screen, the computer is able to detect the precise point at which the surface of the concept
keyboard is touched. Concept keyboards are particularly valuable for use with young children
who have not learned to read or with people with reading difficulties. For example, a menu
which uses small pictures instead of words can be drawn onto a sheet of paper (overlay) and
the paper laid over the touch-sensitive surface. When one of the drawings is touched on the
paper overlay the computer (with the aid of a program) is able to detect, from its position on
the surface, which picture it is.
Scanner
A scanner can be used to convert a picture into digital form so that it can be processed by a
computer and printed out. For example, a photograph could be scanned
and incorporated into a wordprocessor or desktop publisher document.
Like the light pen and the bar-code reader, a scanner measures the levels
of reflected light and codes these into a digital signal which is processed by
a CPU. There are various designs of scanner, e.g. some are hand-held
and others are flat-bed types. Some scanners produce only black and
white pictures (with colours being represented by shades of grey) while
others faithfully reproduce the original colours.