Introduction

Peripherals , strictly speaking, include secondary memory devices; here we just concentrate on input and output peripherals since secondary memory is dealt with under memories .

The physical parts of the computer, sometimes said to be "the bits you can kick" are known collectively as the hardware . 

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Input Devices

The following devices are named in the Subject Guide: 

• Keyboard
• Mouse
• Touch Screen
• OCR
• MICR
• Scanner (page, mark sense and bar code)
• LCD Panels
• Voice recognition
• Sensors
• Digital camera

Some of the most common peripherals will be described in the following sections but you do not have to restrict yourself to knowing about only these (see Bradley R, Understanding Computer Science, 4th edition, ISBN 0-7487-4046-5, chapters10 and 11, for example).

Keyboard
A keyboard is really a series of switches each of which generates a series of different pulse codes so that the computer knows the one that was pressed. They are very useful for typing in text, with word processors for example.  Ergonomic keyboards provide wrist rests and place the keys at a more comfortable angle for prolonged use; thus preventing injuries (such as repetitive stress injury (RSI) or carpal tunnel syndrome).  

Keyboards, of course, are very useful for entering and editing text and they feature short cut keys (function keys and numeric keypads ) to speed up common operations.

Mouse
The movement of the mouse generates a series of pulses by which the computer can tell the direction of mouse movement. This is often used to move a pointer on a screen, select a menu option or change and create different windows in a Graphical User Environment (GUI). The most common example is the Windows operating system from Microsoft or the Mac OS systems used by Apple Macintosh machines.

Graphics Tablet
A graphics tablet is a kind of drawing board on which a pen is moved. In one kind of tablet the pen holds a magnet and underneath the tablet is a fine grid of wires. Movements of the pen cause disturbances in the electrical pulses in the wires and the co-ordinates of the pen can be detected. It is used, for example, in sophisticated computer art packages and video-editing systems for special effects.

One quite well known tablet manufacturer is Wacom who market the Intuos range of tablets. See http://ap.wacom.co.jp/index.html for more info.

Light Pen
This device emits a light beam and detects the amount of light reflected . Dark areas reflect less, light areas more. A sensor detects the amount of light reflected:

A typical use of this kind of light pen is to read bar codes , eg in department stores and libraries - see picture on left). Supermarkets typically use a laser scanner which is fixed because this speeds up the reading of the barcodes, particularly because many items sold in supermarkets are easily handled and it is then quicker to pass them over a bar code reader than to use a hand-held light pen.

A library card with a bar code:

Optical Mark Recognition (or Mark Sensing)
This uses the techniques described above to detect black marks on white paper . A common application is in examinations where multiple choice papers have several boxes; candidates mark the box corresponding to the correct answer (they hope!) usually with a soft penci l.

The forms are an example of pre-printed stationery , some information is already printed on the form (eg, the questions). Your IB registration form is a good example of this.  The picture below shows part of the form used in multiple choice examination papers (together with some helpful hints on its use).

This example shows the numbers at the top being encoded. A similar but more tedious system can be used to enter text.

Optical Character Recognition
The principle of using reflected light is also applied to scanners , fax machines and optical character recognition ( OCR ). The scanner passes a beam of light over a page and measures the reflected light. This gives a two-dimensional pattern of light and dark, which can be represented by 1's and 0's. 

When you have the characters or drawings converted to a binary pattern then you can send them as a string of electrical pulses to a fax machine or import them into a computer program. With OCR, you can try to match the binary patterns to patterns stored in the computer's memory and each recognized character is then assigned its standard code (ASCII usually).

Magnetic Ink Character Recognition (MICR)
This system is used in the UK and some other countries to identify bank account details on a cheque (or check):

When the filled in cheque is processed, the amount is encoded in magnetic ink in the space at the bottom right.

The "funny-looking" numbers at the bottom of the cheque are encoded in magnetic ink for use with MICR readers at the bank.  Some banking systems prefer MICR because of the increased reading speed and extra security against forgery compared to OCR.  Dirty fingerprints and other marks will also not affect the reading. However MICR is much more expensive than OCR.  When the cheque is received by the bank, the amount has to be encoded in the blank space on the right before the cheque is processed.

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Automatic data entry
The scanning and reading techniques described above can be grouped under the general heading direct data entry methods. Do not confuse this with automatic data entry . This occurs when a sensing device is connected permanently to a computer and the computer receives data from the sensor at intervals. Sensors are used to collect data when processes are automated, such as car washes, automatic washing machines, heating control in buildings, petrol delivery in a modern petrol (gas) station. These processes may be controlled with microprocessor applications rather than general purpose computers. In this case the program will be stored in ROM and very little RAM will be needed.

Digital cameras
These are becoming increasingly common, with near photographic quality and prices are now comparable to good quality conventional cameras. They can be used with a monitor for applications like video conferencing or simply to take photographs to display on a web page.

Usually some form of data compression is used when storing and transmitting photographic images to save space and decrease transmission time. A common standard is JPEG (Joint Photographic Experts Group)

Voice recognition
In this technique, the application digitally records speech and attempts to match the digitized patterns to the patterns of known words in memory.  Since there is a wide variety of speech tones, accents and pitches, each individual using such a system has first to "train" the application with a set of standard words.  The accuracy is said to be at about 90% for dictation into a word processing document.

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Output devices

Video Display Unit or Monitor
A VDU is like a TV screen but often has higher resolution, the combination of VDU and Keyboard together is usually referred to as a terminal . a typical mainframe computer will have very many such terminals connected to it.

On a pc - which is essentially terminal with it's own microprocessor and RAM - the VDU is often known as a monitor . VDU's may be monochromatic (single colour) or polychromatic. The definition or resolution of a VDU is a function of the number of picture elements or pixels on the screen itself. Each pixel requires a memory location to store information about it's state (usually it's colour). Therefore polychromatic VDU's require more memory to store information about the state of the screen. 

Very high resolution monitors (Graphical Display Terminals or GDU's) might be used in special applications such as cartography, commercial graphics and design work.

LCD
Liquid Crystal Display such as is found on small computers like Palm Pilots and other PDA's (Personal Digital Assistants).  Because this type of screen doesn't perform well in low light conditions, it is often " backlit ".  However, they consume very small amounts of power compared to conventional displays and are thus useful in battery operated devices.

For laptop computers and also more commonly now for desktops, LCD screens are available. Advantages are low power , lighter weight, flatter , thinner and give out virtually no radiation compared to conventional CRT's.

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Printers
Printers may be classified in a number of different ways:

by amount of text:	character, line or page
by character formation:	matrix vs non-matrix;
by method of printing:	impact vs non-impact;
by colour	colour vs black and white

Dot-matrix printers used to be about the cheapest but have now been overtaken in price/performance by inkjet printers) and are often found on low-volume systems such as pc's. They are character, impact and matrix printers, each character is produced by a set of pins punching an inked ribbon onto the page. The lowest quality are 9-pin printers, more pins (24-pin) means higher quality. Typical printing speeds are 50-200 characters per second.

Daisy wheel printers by contrast work like old-fashioned typewriters, each solid character, held on the end of a spoke, strikes through an inked ribbon onto the paper. The quality is better than that of a dot-matrix printer but the character set is fixed and, as it is a non-matrix printer, graphical output is not possible. This type of printer is now very rare, good quality is now obtainable by other printers (inkjet, laser) at comparable or even lower cost and without the associated noise of the daisy wheel.

A typical Lineprinter is based on a similar concept but there is a solid character for each position across the page and therefore the print-head does not move. Since they print 1 whole line at a time they are very much faster than character printers.

For almost all applications dot-matrix and daisy wheel printers have been superseded by Inkjet and Desktop Laser printers. I nkjet printers hold a cartridge of ink which is sprayed onto the page in small dots, they are matrix printers. The dots form the characters much like a dot-matrix printer. However, because the ink is liquid it spreads a little on the page and makes a much smoother appearance. Colour versions are also available and are more versatile than their dot-matrix equivalents.

Laser printers print a page at a time using a whole-page matrix of dots, they are very similar to photocopiers in action. A laser beam creates very small dots of static charge on the paper and powdered toner (ink) is attracted to the charged areas. The paper is then heated and the toner melts onto the page. The quality of these printers is very high and colour laser printers are now available at reasonable cost.

Plotters
There are two main types: electrostatic and pen plotters. Electrostatic plotters (available in black and white and colour) make an image by burning specially prepared paper with a small spark. Pen-plotters hold a "carousel" of several pens of different colours and actually draw onto the page, giving a higher quality ouput. Both types are able to plot onto large sheets of paper which can be of fixed size ( flatbed plotters) or on a large roll of paper ( drum plotters).

Touch Screen
These screens are used both as input and output devices, the options can be shown with text and or graphics and the position of a press on the screen can be detected.  These screens are often used where general information is being provided (banks, hotels, shopping centres) for users who may not be competent computer users (yes, there are still some of these).

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