Peripheral devices

Peripheral devices can be classified generally as input devices, output devices, or both.

Input Devices

Keyboard. In addition to letters and numbers found on a typewriter, computer keyboards have a series of special keys to control sending information to the CPU. The use of these special keys is defined and controlled by the software. For example, with word processing software, arrow keys move the cursor to any point in a document.

Mouse. A mouse is an input device that operates by controlling the position of the cursor (in the shape of an arrow) on the monitor.

Light pen. Light pens, often used in stores, are able to input a large amount of data quickly by moving a light beam across a barcode. This converts the bar- code into digital data that is usable by the computer. Other types of light pens are also used for computer-aided design (CAD) and pen-based computers; the latter interpret and convert human writing into computer form.

Scanner. A scanner is an input device that acts like a miniature photocopy machine connected to a computer, copying graphic images into the computer and allowing typewritten pages to be entered without retyping. Scanners include both hand-held and desktop models. A scanner works by passing a beam of light across the original page or artwork and sensing the reflected light; it then assembles this information into a data file that describes the images as rows of tiny dots, each one noted for its colour and brightness. That file is then passed on to the computer.

Output Devices

Monitors. Monitors, which look like television sets, quickly display and redisplay the computer's output. They are often called CRTs (cathode ray tubes), VDTs (video display terminals), or simply screens.

The image displayed on the screen is composed of many rows of tiny dots, called pixels(short for picture element). The number and size of pixels determine the resolution (sharpness and clarity) of the display. The more pixels, the higher the resolution.

Each type of monitor, such as monochrome or colour graphics, requires a matching type of display adapter in the system unit.

• Desktop Monitors. CRTs, are the desktop monitors that are built in the same as television sets. They can be monochrome or colour:

• Monochrome monitorsshow one colour, generally white, green, or amber, on a dark background.

• Colour monitors(often called graphics monitors) display text characters and graphic images in colour. They have advanced through various stages: CGA (colour graphics adapter), EGA (enhanced graphics adapter), and VGA (video graphics array).

• Printers. Printers create paper copies, called hardcopies, of information sent from the computer. Four types of printers are in common use:

• Dot matrix printersuse a series of dots to form a character. They are fast and inexpensive, but the output quality can be relatively low.

• Daisy-wheel, orletter-quality, printers create print of the same quality as a typewriter. Letter-quality printers are slower but they produce a higher-quality print.

Ink jet printersspray small droplets of ink to create characters. These printers produce a fine-quality print at an extremely high speed; some print in colour.

Laser printersare quickly taking the place of other printers for most uses. Laser printers produce an exceptionally high-quality print at a very high speed.