A keyboard performs the same basic task
which is that you (user) press a key and
the number that it represents is sent to the
system unit. Note, that it is a number not
letter because, internally, computers do
not understand letters, only numbers. So
each letter on the keyboard is assigned a
The most common keyboard in use today
is the QWERTY keyboard. It gets it name
from the first row of keys as seen in the
Along with the letters of the alphabet, there are number keys, arrow keys, function keys
(F1-F12) and special keys that work differently with depending on the key combinations
used. For example you can often press the Escape (Esc) key to cancel an un-intended
operation, or to clear the screen of a message.
The other most often used input device
is the mouse. Once again they can some
in all sorts of colours and various types
of connectors. Most mice on a personal
computer (PC) have two buttons.
The mouse picture is a newer device
which are generally optically based; the
optical mouse- actually uses a tiny
camera to take thousands of pictures
every second and is able to work on
almost any surface without a mouse pad.
One extra feature on the mouse today,
that is not traditional, is the scroll wheel.
It can be used to scroll up and down a document. It can even operate as a third button.
The most common connector for both mice and keyboards is now USB (Universal Serial
The primary output device is the screen. The computer screen has changed noticeably
from the original plain text monochrome (single colour, eg white, amber or green text on
a black background) to the thin high-resolution LCD models.
When selecting a screen the main factors to be considered are:
1 Its physical size — measured from diagonally opposite corners, eg 15'' or the bigger
17-30'' (note the Imperial measurement — inches)
2 Technology — cathode ray tube (CRT) or liquid crystal display (LCD)
3 Resolution — a measurement of the number of dots (called pixels) that it is capable
LCD and CRT monitors
The most common type of monitor on the market today is the 17- 22 LCD display. It uses
a combination of three colours (red, green and blue) to produce the high quality colour
The resolution is the measure of dots (called pixels) on the screen.
When the output from a computer is required to be a more permanent form than
displaying it on a screen, a printer is required. There are three basic types of printers,
classified by their method of producing a printed page:
3 dot matrix.
Regardless of the technology used to put the printed word or image on the page, the
quality of the output is referred to as its resolution. Print resolution is measure in dots-
per-inch (dpi) and, generally, the higher the dpi the better the quality.
Every computer is fitted with a special connector (called a port) that is designed to work
with printers. The connection is called a printer port and is easily recognised as the only
25-pin female socket. However, most printers today use a USB connector. The USB
connector allows devices to be disconnected and connected while the computer is
operational (hot swapping).
Laser printers are known as non-impact printers and they are much quieter than dot-
matrix printers. They are also generally faster than other types of printers.
A standard monochrome laser printer employs a single toner, whereas colour laser
printers use four toners to print in full colour. Colour laser printers are generally about
five to ten times more expensive than the monochrome type.
Laser printers use a laser beam to create an image on a drum. The light of the laser
changes the electrical charge in the drum wherever it gets hit. Then, the drum is rolled
onto a reservoir of toner (black powder), which is picked up by the drum. Consequently,
the toner is being transferred to the paper by heat and pressure, in the same way a
An inkjet printer is any printer that places extremely small droplets of ink onto paper to
create an image. If you ever look at a piece of paper that has come out of an inkjet
printer, you know that:
The dots are extremely small (usually between 50 and 60 microns in diameter), so
small that they are tinier than the diameter of a human hair (70 microns)!
The dots are positioned very precisely, with resolutions of up to 1440 × 720 dots
per inch (dpi).
The dots can have different colors combined together to create photo-quality
Inkjet printing has two chief benefits over laser printers: lower printer cost and color-
printing capabilities. But while inkjet printers are priced much less than laser printers,
they are actually more expensive to use and maintain. Cartridges need to be changed
more frequently and the special coated paper required to produce high-quality output is
very expensive. At a cost per page level, inkjet printing costs about 10 times more than
Inkjet printing, like laser printing, is a non-impact process. Ink is emitted from nozzles
while they pass over the page. The operation of an inkjet printer is easy to visualise:
liquid ink in various colors being squirted onto paper and other media, like plastic film
and canvas, to build an image. The coloured output is produced by mixing the colours
cyan, magenta, yellow, and black, often referred to as CMYK. The colours are often
contained in two cartridges: a three-colour cartridge containing cyan, magenta and yellow
and one higher-capacity black cartridge