Input/output Units

By: Priyanka Mehta (Assist. Prof. –

CS(SU))

Input/output Devices

The I/O devices commonly called peripherals

connected to a computer system provide an

efficient mode of communication between the

computer and the external world.

The user can enter programs and data using the

input device on a terminal and can execute the

programs and display the output result using an

output device connected to a terminal.

Input Devices

The objective of an input device is to capture

raw data and convert it into computer-usable

form as quickly and efficiently as possible.

Input devices can be classified according to:

Mode of input (eg: mechanical, audio, visual

etc.)

Nature of input (discrete – key presses or

continuous – mouse movement)

Punched Cards

A punched card is a piece of stiff paper that

contains digital information represented by the

presence or absence of holes in predefined

positions.

Early digital computers used punched cards,

often prepared using keypunch machines , as the

primary medium for input of both computer

programs and data.

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An 80-column punched card

Punched card used in Jacquard loom

Keyboards

A keyboard is the set of typewriter-like keys that enables us toenter data into a computer.

The keys on the computer keyboards are often classified as:Alphanumeric keys – Letters and numbers.Punctuation keys – Comma, period, semicolon and so on.Special keys – Function keys, control keys, arrow keys, caps lockkey and so on.

The output of the key switches are fed into electronic circuitryknown as the keyboard encoder which converts them intobinary-coded values. The values are then fed into the computerwhich interprets the key which was pressed.

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Mouse

A mouse is a pointing device that is most oftenused with a personal computer and which can fitcomfortably in a user’s palm.

A mouse consists of an object held under one ofthe user’s hands, with one or more buttons.

Moving a mouse along a flat surface can movethe on-screen cursor to different items on thescreen. Items can be moved or selected bypressing the mouse buttons.

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Mechanical Mouse: It has a rubber or metallic

ball on its underside that can roll in all direction.

Mechanical sensors within the mouse detects the

direction of the rolling ball.

Optical Mouse: It uses a laser beam to detect

direction of rolling.

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Mechanical mouse shown with

the top cover removed

A standard wireless mouse with

connector

Computer mouse built between 1986 to 2007

Scanner

A scanner is a device that optically scans images,

printed text, handwriting, or an object, and

converts it to a digital image.

Flat-bed Scanner

Hand-held Scanner

Drum Scanner

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Optical Mark Reader (OMR): An OMR is a special

scanning device that can read carefully placed pencil

marks on specially designed documents. OMR is

frequently used in forms, questionnaires and answer-

sheets.

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Magnetic Ink Character Recognition (MICR): MICR is an

input device widely used by banks to process large volume

of checks and deposit forms. It consists of special ink called

magnetic ink, used to write characters on the check and

deposit forms to be processed by MICR. It is capable of

recognizing characters written on paper by magnetic ink. It

interprets the characters and sends the corresponding data

directly to the computer for processing.

Joystick

A joystick is also a pointingdevice which is providedwith a spherical ball onwhich a stick is mounted andmoves in a socket. The usermove the spherical ball withthe help of the stick withhis/her hand. The stick can bemoved forward or backward,left or right to positioned thecursor at the desired location.It is provided with a buttonon the top to select theoption.

Output Devices

An output device is a peripheral device

used to communicate the results of

data processing carried out by a

computer which converts the

electronically generated information

into human-readable form.

Monitors

A monitor is an electronic visual display for computers. It

is used to display the video and graphics information

generated by the computer through a video card. Images

are represented on monitors by individual dots called

pixels. A pixel is the smallest unit on the screen that can

be turned on and off or made different shades. The density

of the dots determines the clarity of the images, the

resolution. Monitors are very similar to televisions but

usually display information at a much higher resolution.

Monitors come in two major types:CRT (Cathode ray tube)TFT-LCD (Thin film transistor – Liquid crystal display)

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CRT (Cathode ray tube): CRT is a vacuum tube

containing one or more electron guns and a fluorescent

screen used to view images. A CRT works by moving an

electron beam back and forth across the back of the

screen. Each time the beam makes a pass across the

screen, it lights up phosphor dots on the inside of the

glass tube, thereby illuminating the active portions of

the screen. By drawing many such lines from the top to

the bottom of the screen, it creates an entire screenful of

images. CRTs are relatively heavy, bulky and take up lot

of space in comparison to other display technologies.

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TFT-LCD (Thin-film transistor – Liquid crystal display): A TFT-

LCD is a flat-panel display, that has replaced the old CRT. It uses

thin-film transistor technology in which a separate transistor

works for each pixel along with the liquid crystal material. The

liquid crystal material exhibits properties of both a liquid,

because of its ability to change quickly, and a crystal, because of

its ability to remain in an arranged position. The transistor applies

the voltage to the pixel, determining its colour and intensity. TFT

monitors consumes less power, deliver sharp images and looks

more elegant than CRT monitors. However, they tend to offer

relatively poor viewing angles. It means they look best when

viewed head-on, and if viewed from the side, it can be difficult to

see.

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Printers

A printer is a peripheral device that produces a

text or graphics of documents stored in

electronic form, usually on paper.

Two broad categories are:

Impact Printers

Non-impact Printers

Impact Printers

Impact printers are the oldest printing technology

that work by banging a head against an ink

ribbon to produce a mark on the paper. The three

most common forms of impact printers are:

Dot-matrix printers

Daisy-wheel printers

Line printers

Dot-Matrix Printers

Dot-matrix printers, are so named because printer characters are in

the form of group of dots. Each dot is produced by moving a metal

rod, called wire, that strikes a ribbon against the paper.

Its features are:

Low purchase cost.

Lower quality with low resolution.

Generate carbon copies.

Good, reliable and ideal for situations where printed contents is

more important than quality.

Noisy

Slow speed.

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Daisy-Wheel Printers

Daisy-wheel printers, consist of a circular wheel about 3inches in diameter

with arms or spokes. The character shape are embossed at the outer ends of the

spokes. The shape of the printer wheel resembles the petals of a daisy flower

and hence, it is named daisy wheel printer.

To print a character, the daisy wheel rotates so that the required character is

positioned just in front of the printer ribbon. The spoke containing the

character is hit by hammer which strikes the ribbon leaving an impression on

the paper.

Its features are:

Better printing quality than dot-matrix printers.

Font types can be changed by changing the wheel.

Low speed & noisy.

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Line Printers

Line-printers are somewhat similar to the daisy-wheel printer. Instead of a

print wheel, line printers have a mechanism that allows multiple characters to

be simultaneously printed on the same line.

The mechanism uses a large spinning print drum. As the drum is rotated over

the paper’s surface, electromechanical hammer behind the paper pushes the

paper along with a ribbon onto the surface of the drum marking the paper with

the shape of the character on the drum.

Its features are:

Faster than dot-matrix and daisy-wheel printers.

Noisy.

Limited multi-font capability.

Lower print quality

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Non-impact Printers

Non-impact printers does not operate by striking

a head against a ribbon, as there is no direct

contact between the printhead and the paper. The

three most common forms of non-impact printers

are:

Ink-jet printer

Thermal printer

Laser printer

Ink-jet Printers

Ink-jet printers, creates a digital image by spraying droplets of ink onto the paper. The

characters are produced as a result of electrically charged or heated ink being sprayed

from fine nozzles onto the paper. Individual nozzles produces high resolution (upto 400

dpi) dot-matrix characters.

Ink-jet printers can use coloured cartridges to create coloured images. The printer can

generate upto 200 characters per second, and allows for good quality and cheap

printing.

Its features are:

Less noisy

Lower cost per page

Expensive

With time, the ink-jet prints fades and colour balance may get changed.

Fine ink-jet nozzles are prone to blocking which requires periodic cleaning.

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Thermal Printers

Thermal printers, uses heat that adheres a wax-based ink onto paper. It has a

thermal printhead that melts wax-based ink from the ribbon onto the paper.

When cool, the wax ink becomes permanent. These printers are popular for

printing bar codes, price tags etc.

Its features are:

Quiet with good printing speed.

Good quality printouts.

Weather resistant as it can last for longer time even with extreme

temperatures.

Light weight and easy to carry.

It requires a specialized paper, which are more expensive than normal bond

paper.

Unable to print coloured images.

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Laser Printers

Laser printers, rapidly produces high quality text or graphics on plain paper. When a

document is sent to the printer, a laser beam draws the document on a selenium-coated drum

using electrical charges. After the drum is charged, it is rolled in tonner (a dry powder type

ink), which is picked up by the charged portions of the drum. The tonner is then transferred

onto the piece of paper through the combination of heat and pressure. After the document is

printed, the electric charge is removed from the drum and the excess toner is collected.

Its features are:

Very good printing quality.

High printing speed.

Noiseless.

Light weight and compact.

Colour printing is possible.

Cartridges are expensive.

Carbon copies cannot be generated simultaneously.

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Melting toner onto paper

using heat and pressure