Common Input and Output Devices

Common Input and Output Devices are:

• Mouse
• Scanners
• Touch Screens
• Light Pens
• Printers
• Plotters

Mouse

A mouse is an input device that is used to point and select an option on the VDU. A mouse can be classified on the basis of the number of buttons it has, the technology it uses and the kind oi interface it has with the computer.

A mouse may have one, two or three buttons. The function of each button is determined by the program that uses the mouse. A software package may use one, two or all three of them. In its simplest form, a mouse has one button. Moving the mouse on a flat surface produces a corresponding movement of a pointer on the screen. Placing the pointer on an option and clicking the button results in selecting that option.

Two-button Mechanical Mouse

A mouse may be classified as a mechanical mouse or an optical mouse, on the basis of the technology it uses. In a mechanical mouse, the ball that projects through the bottom surface rotates as the mouse is moved along a flat surface. The direction of rotation is detected and relayed to the computer by the switches inside the mouse. Microsoft, IBM and Logitech are some weil-known makers of the mechanical mouse.

An optical mouse uses a light beam instead of a rotating ball to detect movement across a specially patterned mouse pad. MSC Corporation makes the optical mouse that uses LEDs and photo-detectors to trap movement.

Scanners

Another input device that is being increasingly used is the scanner. Often, there are situations when some information (picture or text) is available on paper and is needed on the computer disk for further editing. The simplest way would be to take a photograph of the image directly from the source, and convert it into a form that can be saved on the disk and then printed.

Flat-bed Scanner

A scanner scans an image and transforms the image to ASCII codes that is the code used by a computer to represent the characters you find on your keyboard—letters of the alphabet, numbers, punctuation marks and graphics. These can be edited, manipulated and printed.

Scanners are of two types—the flat-bed type and the roller-feed type. While in a roller-feed scanner, the image is passed over a roller where it is captured, the flat-bed scanner works like a photo-copier. Flat-bed scanners can scan and store images from books without having to remove the page, which is difficult in the case of the roller-feed scanner.

Many of the scanners available now-a-days are capable of not only scanning text and graphics, but also integrating text and graphics files.

Scanners are used to reproduce photographs on the computer screen. They can be employed in training programs using the computer and in desktop publishing. Businesses use scanners for storing documents on the computer.

Touch Screens

Imagine a cardiac surgeon performing an intricate bypass surgery. In between, the surgeon needs to check on the laboratory test results of the patient. Assuming that the information is stored on a computer, the surgeon can access the information through a keyboard, or better still, through a mouse. But how much more convenient it would be if the surgeon could only touch the screen on the option that is required to display the information. A touch screen input interface does exactly this. Touch screens are normally used when information has to be accessed with minimum effort.

Two popular technologies exist for touch screens. In one, the screen is made sensitive to touch and the exact position is detected. In the other, the screen is lined with light emitting devices and photo-detectors. When the user's finger approaches the screen, the light beam is broken and is detected by the photo-detectors.

Touch screens are used in information-providing systems like the one explained above. It is also used in airline and railway reservation counters. The users indicate their current location and the destination by touching the screen (maybe on a map), and all the possible routes with timings and rates are displayed. These interfaces are also used in travel agencies to display the names and addresses of all hotels, restaurants, and other places of interest, at a desired destination. Touch screens are also used in stock exchanges.

Light Pens

A light pen is a pointing device that can be used to select an option by simply pointing at it, or drawing figures directly on the screen.

A light pen has a photo-detector at its tip. This detector can detect changes in brightness of the screen. When the pen is pointed at a particular spot on the screen, it records change in brightness instantly and informs the computer about this. The computer can find out the exact spot with this information. Thus, the computer can identify where you are pointing on the screen.

Light pens are useful for menu-based applications. Instead of moving the mouse around or using a keyboard, the user can select an option by pointing at it.

A light pen is also useful for drawing graphics in CAD. An engineer, architect or a fashion designer can draw directly on the screen with the pen. Using a keyboard and a light pen, the designer can select colors and line thickness, reduce or enlarge drawings, and edit drawings.

Printers

Printers are output devices that print the result of an operation on paper. A printer is capable of producing output that contains printed words. It is also capable of printing straight lines and simple figures like squares, rectangles and circles. Printers are classified based on a number of parameters, like the mechanism used for printing, the speed of printing, the quality of output, the direction of printing, and the kind of interface they have with the computer. The following sections will elaborate on these parameters.

Mechanism

Depending upon the technique used for printing, printers can be classified into two broad categories - impact and non-impact printers.

In impact printers, characters are printed by pressing a typeface against an inked ribbon, which makes a mark on the paper. The most commonly-used impact printer is the Dot Matrix Printer (DMP).

In a DMP, an arrangement of tiny hammers or pins strike the ribbon to produce the desired characters. These tiny pins typically print in a matrix of 7 dots across and 9 dots down. The impact of the appropriate pins on paper through the inked ribbon forms letters made up of dots. There are printers which use up to 24 vertical pins to print characters with a higher resolution.

Dot Matrix Printer

DMPs are inexpensive but noisy. They can print both text and graphics. They can print in any language without additional hardware. They can also be made to print in color by changing ribbons. These printers are used to produce internal reports and memos needed by organizations. The other types of impact printers are drum printers, daisy wheel printers and golf ball printers. However, these printers are not as popular as DMPs.

Examples of DMPs are: CENTRONICS-702, HP-2635A, OLIVETTI TC480 and Epson LX-80.

In non-impact printers, there is no contact between the typeface and the paper while printing. An example of a non-impact printer is the laser printer.

Laser printers use a light beam to form images on the paper using toner ink as the medium. The light beam strikes parts of a drum surface to form an image. Those parts of the drum surface, which are exposed to the light beam, become electrically charged. Only these electrically charged areas attract the toner ink particles. These toner particles are then deposited on, and permanently fixed to the paper using heat or pressure.

The other types of non-impact printers are thermal printers that use heat to print characters on paper and ink-jet printers that use jets of ink to print characters on paper.

Laser Printer

Laser printers generate a very high quality output, both text and graphics, and are typically used for publishing. Most organizations use them for business correspondence, newsletters, brochures and presentations. Laser printers are expensive.

Examples of laser printers are: Xerox 4010 from Rank Xerox UK, LASERJet IlISi from Hewlett Packard USA and Lexmark Optra from Lexmark USA.

Speed

The speed of a printer is measured in terms of characters per second (cps), lines per second (Ips) or pages per minute (ppm). The speed of a dot matrix printer is measured in cps. The speed can vary from 200 cps to 540 cps. A line printer prints a line at a time. Its speed can vary from 5 to 50 Ips. A laser printer prints within the range of 4 to 20 ppm.

Quality of Output

Printers can operate in three different modes—draft, Near Letter Quality (NLQ), letter quality.

In the draft-quality mode, a DMP forms a character by arranging dots to assemble it. Although the characters can be distinguished, the output is not as good as that of near letter quality printouts. A laser printer prints in the draft mode by using less toner ink to form the characters.

For printing in letter quality, the DMP prints a character twice, thereby making its appearance darker. A laser printer prints in the letter quality by using more toner ink than it uses while printing in the draft mode.

Direction

Dot matrix printers and ink-jet printers can be unidirectional, bidirectional, or reverse. In a unidirectional printer, printing takes place in one direction only. After printing a line from left to right, the printer head (the component that carries the pins or characters) returns to the left without printing.

A bidirectional printer prints both ways, with logic-seeking capability, i.e. it selects the characters in the reverse direction while printing in reverse. A reverse printer has no such capability, and expects the computer to supply the characters in reverse order.

Interface

Depending upon the number of characters received at a time, printers can be serial or parallel. Serial printers are slower than parallel printers. A serial printer receives one character at a time for printing. A parallel printer receives more number of characters and is faster.

Plotters

Very often it might be required to create high-quality visuals on paper, which cannot be obtained using a printer. For this purpose, a plotter is used. A plotter is an output device that is used to create presentation visuals, charts, graphs, tables and diagrams.

A plotter consists of an arm that moves across the paper on which the diagram or graph needs to be drawn (refer Figure). A pen moves along the arm, and the arm itself moves relative to the paper. A combination of the two thus provides movement along the horizontal and vertical axes.

Flat-bed Plotter

In some plotters, the paper is held stationary while the arm and the pens move over it. This is called a flat-bed plotter. In the other type of plotter, the paper is wrapped around a drum and anchored at both ends. The drum rotates while the pen moves laterally along a fixed rail. This is called a drum plotter.

To draw clear, high-quality diagrams, a plotter needs high-quality pens with special inks of different colors.

A plotter can be connected to a PC through the parallel port. A plotter is more software-dependent than any other peripheral, and needs much more instructions than the printer for producing output.

Plotters are used in applications like Computer Aided Design (CAD) which require high-quality graphics on paper. Many of the plotters now available in the market are desktop models that can be used with PCs. Businesses typically use plotters to present an analysis in visual terms (bar charts, graphs, diagrams, etc.) as well as for engineering drawings.