Peripheral Input and Output Devices

I/O Devices

1. Peripheral Input Devices

1. Keyboard

• The keyboard is a primary input device for any graphics system. It is used for entering text and numbers, i.e. on graphics data associated with pictures such as labels x‒y co‒ordinates etc.

• Keyboards are available in various sizes, shapes and styles. Fig. 4.9.1 shows standard keyboard. It consists of

■ Alphanumeric key

■ Function keys

■ Modifier keys

■ Cursor movement keys

■ Numeric keypad

• When we press a key on the keyboard, keyboard controller places a code corresponding to key pressed into a part of its memory, called keyboard buffer. This code is called scan code. The keyboard controller informs CPU of the computer about the key press with the help of interrupt signal. The CPU then reads the scan code from the keyboard buffer, as shown in Fig. 4.9.2.

 

2. Mouse

• A mouse is a palm‒sized box used to position the screen cursor. It consists of ball on the bottom connected to wheels or rollers to provide the amount and direction of movement. One, two or three buttons are usually included on the top of the mouse for signaling the execution of some operation. Now‒a‒days mouse consists of one more wheel on the top to scroll the screen pages.

 

3. Trackball

• A trackball works like an upside‒down mouse as a pointing device. Fig. 4.9.4 shows some trackball models. Two‒button and three‒button trackball models are available. Also right handed as well as left handed models are available.

• As shown in Fig. 4.9.4, trackball contains an exposed ball that can be rolled with an index finger or thumb to move the pointer around the screen. A trackball needs less space than a mouse as it is not necessary to move the whole device. The trackballs are mostly used on small work surfaces where sufficient space for mouse is not available. For example, trackballs on laptop computers are very popular.

 

4. Trackpads

• The trackpad is also called a 'touchpad'. It is a stationary pointing device. It uses a small touch‒sensitive surface. As you move a finger across this surface, the pointer moves on the computer screen. This surface is very small (1.5 or 2 inches square), so the required movement of the finger to move the pointer is very less. So the use of trackpad find less tiring to use as compare to mouse or trackball. Due to small size, they are useful in notebook computers. Some notebook computer models use built‒in trackpads. Fig. 4.9.5 shows an example of a built‒in trackpad on a desktop keyboard.

• In some cases, trackpads are separate from the desktop keyboards and are connected to the computer using a cord. There are two types of trackpads.

1. Buttons trackpads : They include two or three buttons that perform the same functions as mouse buttons.

2. Strike sensitive trackpads : They use pads which can be tapped with fingertip.

 

5. Touch Screens

• When you place a fingertip directly on the computer screen, touch‒screen accepts input. Generally, this method is used to make a choice from a menu. In most touch‒screen computers the sensors are used on the screen's surface to detect the touch of a finger. The touch‒screens need dirt‒free environment. The applications where intuitive interface is an important factor use touch‒screens. Some of them are listed below.

• Automated teller machines

• Public information kiosks

• Fast‒food restaurants

• Department stores

• Drugstores

• Supermarkets

• Lottery centers

• Touch input can be recorded using optical, electrical or acoustical methods.

• Optical touch screens use a line of infrared light emitting diodes along one vertical edge and along one horizontal edge of the screen. The opposite vertical and horizontal edges contain light detectors. These detectors are used to record which beams are interrupted when the panel is touched. The two crossing beams that are interrupted indicate the horizontal and vertical co‒ordinates of the screen position selected.

• In electrical touch screens, two transparent plates are used. These plates are separated by a small distance. One plate is coated with conducting material and other plate is coated with resistive material. When outer plate is touched, it is forced to contact with the inner plate. This contact creates a voltage drop across the resistive plate that is used to determine the co‒ordinate values of the selected screen position.

• An acoustic touch screens use high frequency sound waves in horizontal and vertical directions across a glass plate. Touching the screen causes partial reflection of each wave from finger to the emitter. The screen co‒ordinates of point of contact are then calculated by measuring time between the transmission of each wave and its reflection to the emitter.

 

6. Joystick

• A Joystick is a personal computer input device consisting of a handheld stick that pivots about one end and transmits its angle in two or three dimensions to a computer. It has a small, vertical lever called the stick mounted on the base and used to steer the screen cursor around.

• Most joysticks are two‒dimensional, having two axes of movement (similar to a mouse), but three‒dimensional joysticks do exist. A joystick is generally configured so that moving the stick left or right signals movement along the X axis, and moving it forward (up) or back (down) signals movement along the Y axis. In joysticks that are configured for three‒dimensional movement, twisting the stick left (counter‒ clockwise) or right (clockwise) signals movement along the Z axis. These three axes XY and Z are, in relation to an aircraft, roll, pitch and yaw.

• An analog joystick is a joystick which has continuous states, i.e. returns an angle measure of the movement in any direction in the plane or the space (usually utilizing potentiometers) and a digital joystick gives only on/off signals for four different directions and mechanically possible combinations (such as up‒right, down‒left, and c). Digital joysticks were very common as game controllers for the video game consoles, arcade machines and home computers. Additional joysticks often have one or more fire buttons, used to trigger some kind of action. These are simple on/off switches.

• Most I/O interface cards for PCs have a joystick (game control) port. Modern joysticks mostly use a USB interface for connection to the PC.

• They are mostly popular in racing and flying games. They enable the user to 'drive' or 'fly' through a game. They direct to a character or a vehicle. A variant of joystick is the racing game controller. It includes an actual steering wheel and in some cases even foot pedals and gearshifts.

7. Light Pen

• A Light Pen is a pointing device shaped like a pen acts as computer input device. The tip of the light pen contains a light‒sensitive element which, when placed against the screen, detects the light from the screen enabling the computer to identify the location of the pen on the screen.

• Light pens have the advantage of 'drawing' directly onto the screen. It allows the user to point to displayed objects or draw on the screen, in a similar way to a touch screen but with greater positional accuracy. A light pen can work with any CRT‒based monitor, but not with LCD screens, projectors or other display devices.

• The light pen consists of photoelectric cell housed in a pen like case. It works by sensing the sudden small change in brightness of a point on the screen when the electron gun refreshes that spot. By noting exactly where the scanning has reached at that moment, the X,Y position of the pen can be resolved. This is usually achieved by the light pen causing an interrupt, at which point the scan position can be read from a special register, or computed from a counter or timer. The pen position is updated on every refresh of the screen.

• The response time of the pen is also important. For slow displays transistor type photo‒ cells such as photo‒diodes are used. These are small, inexpensive and suitable for hand held operation. The response time is about one microsecond. For the highspeed displays photo‒ multiplier tube is used. It is bulky and uses fibre‒optic cable. Now a days improved light pens are available. These consist of a matrix of fibre optic sensors.

 

8. Digitizers

• It is used for applications such as tracing. It consists of flat surface which can detect the position of a movable stylus.

 

9. Scanners

• The scanner is a device, which can be used to store drawing, graphs, photos or text available in printed form for computer processing. The scanners use the optical scanning mechanism to scan the information. The scanner records the gradation of gray scales or colour and stores them in the array. Finally, it stores the image information in a specific file format such as JPEG, GIF, TIFF, BMP and so on. Once the image is scanned, it can be processed or we can apply transformations to rotate, scale or crop the image using image processing software such as photo‒shop or photo‒paint. Scanners are available in variety of sizes and capabilities.

Working Principle of Scanners

• Fig. 4.9.10 shows the working of photo‒scanner. As shown in Fig. 4.9.10, the photograph is mounted on a rotating drum. A finely collimated light beam is directed at the photo and the amount of light reflected is measured by a photocell. As the drum rotates, the light source slowly moves from one end to the other, thus doing a raster scan of the entire photograph.

• For coloured photographs, multiple passes are made, using filters in the front of the photocell to separate out various colours.

• Other type of scanners are electro‒optical devices that use arrays of light sensitive Charge Coupled Devices (CCDs) to turn light reflected from, or transmitted through artwork, photographs, slides etc. into a usable digital file composed of pixel information.

Resolution and Colour Depth

• The optical resolution and colour depth are the two important specifications of the scanner. The photo‒scanners have resolution upto 2000 units per inch. Resolution of the CCD array is 200 to 1000 units per inch which is less than the photo‒scanners. The colour depth is expressed in bits. It specifies the number of colours scanner can capture.

Types of Scanners

• According to construction the scanners also can be classified as :md

■ Flatbed scanners, also called desktop scanners, are the most versatile and commonly used scanners. In fact, this article focuses on the technology as it relates to flatbed scanners.

■ Sheet‒fed scanners are similar to flatbed scanners except the document is moved and the scan head is immobile. A sheet‒fed scanner looks a lot like a small portable printer.

■ Handheld scanners use the same basic technology as a flatbed scanner, but relay on the user to move them instead of a motorized belt. This type of scanner typically does not provide good image quality. However, it can be useful for capturing an image quickly.

■ Drum scanners are used by the publishing industry to capture incredibly detailed images. They use a technology called a Photo‒Multiplier Tube (PMT). In PMT, the document to be scanned is mounted on a glass cylinder. Located at the center of the cylinder is a sensor that splits light bounced from the document into three beams. Each beam is sent through a colour filter into a photo‒multiplier tube where the light is changed into an electrical signal.

 

2. Peripheral Output Devices

 

• The computer processes input data to produce useful information. This information can be displayed or viewed on a monitor, printed on a printer or listened through speakers or a headset, or it can be stored in the secondary memory device for further processing or future reference. These different ways of output can be broadly categorized as : Hard copy output and soft copy output.

1. Soft copy: The electric version of an output, which usually resides in computer memory and/or on disk, is known as soft copy. Soft copy output also includes audio and visual form of output generated using computer. The soft copy is not a permanent form of output, it can not be touched.

2. Hard copy: We obtain hard copy output for our image in several formats using printer or plotter. Therefore, printers and plotters are also called hard‒copy devices. The quality of pictures obtained from a hard‒copy device depends on dot size and the number of dots per inch, or lines per inch, that can be printed, i.e. it depends on the resolution of printer or plotter. Before going to see working principle of various plotters and printers we see the important characteristics of hardcopy devices.

• The hard copy output is permanent output and relatively stable form of output. This output is also highly portable.

• Based on the hard copy and soft copy outputs, the output devices are classified into : hard copy devices and soft copy output devices. The various types of output devices are as follows:

• Monitor: The computer sends processed data (i.e. output) to the monitor when the user needs to observe the output.

• Printer: When the user needs hard‒copy (i.e. paper‒copy) of an output, the computer sends output to the printer.

• Head‒phones or Speakers : When the user needs a sound‒ output, the computer sends output to the head‒phones or speakers.

1. Monitors

• Monitor is the most commonly used output device in personal computer systems. The text and graphics can be displayed on the monitor‒screen. The quality of image displayed on the monitor is determined by two hardware devices, the monitor itself and the video controller. We will discuss the details of these devices in further sub‒section.

Generally, the PCs use two types of monitors.

1. The monitor that looks and works in much the same say as a televison screen. It uses a vacuum tube, called a Cathode Ray Tube (CRT). Most desktop computers use this type of monitor.

2. Second type is flat‒panel display. Now days, this type is popular with desktop computers.

• The monitors can be categorized according to the colors which they display as, monochrome monitors, grayscale monitors and color monitors.

1. Monochrome monitors

• They display only one color such as green, amber or white against a contrasting back‒ground which is usually black. They are usually used when colors have no importance. For example, displaying only text.

2. Grayscale monitors

• These monitors display various shades of gray, i.e. from a very light gray to black against a white or off white background. They are especially used in handheld computers or low‒end portable systems so as to keep costs down.

3. Color monitors

• They can display different colors. The different types of color monitors display different number of colors ranging from 16 to 16 million colors. Also, they can be set to operate in monochrome or grayscale mode.

CRT Monitors

The CRT monitor uses a large vacuum tube, called Cathode Ray Tube (CRT). The working of this CRT is shown in Fig. 4.9.16.

• A CRT is an evacuated glass tube. An electron gun at the rear of the tube produces a beam of electrons passes through a magnetic coil called 'yoke' and is directed towards the front of the tube or screen. The inner side of the screen is coated with phosphor substance which gives off light when it is stroked by electrons. The screen's phosphor coating is arranged into a grid of dots. The smallest number of phosphor dots that the gun can focus on is called a 'pixel'. Each pixel has a unique address. The computer can locate the pixel and control its appearance with this address.

• Raster scan is the most common method of displaying images on the CRT screen. In this method, the horizontal and vertical deflection signals are generated to move the beam all over the screen as shown in Fig. 4.9.17.

• Here, the beam is swept back and forth from the left to the right across the screen. When the beam is moved from the left to the right, it is ON. The beam is OFF, when it is moved from the right to the left as shown by dotted lines in Fig. 4.9.17.

• When the beam reaches the bottom of the screen, it is made OFF and rapidly retraced back to the top left to start again. This scanning process is similar to reading different lines on the page of a book. During scanning, the monitor‒driving circuitry adjusts the intensity of each beam. In a monochrome monitor, this intensity determines whether a pixel is ON (white) or OFF (black). In a gray scale monitor, this intensity determines the brightness of each pixel.

Color CRT Monitors

• A CRT monitor displays color pictures by using a combination of phosphors that emit different‒coloured light. It generates a range of colors by combining the emitted light from the different phosphors. 'Shadow‒mask' is the commonly used technique in raster‒scan displays including color TV. In a shadow mask technique, CRT has three phosphor color dots at each pixel position. One phosphor dot emits a red light, another emits a green light and the third emits a blue light. Fig. 4.9.18 shows the shadow mask CRT.

• The shadow mask CRT has three electron guns, one for each color dot and a shadow mask grid just behind the phosphor coated screen. The shadow mask grid consists of series of holes aligned with the phosphor dot pattern. Three electron beams are reflected and focused as a group onto the shadow mask and when they pass through a hole in the shadow mask, they excite a dot triangle. A dot triangle consists of three small phosphor dots of red, green and blue color. These phosphor dots are arranged so that each electron beam can activate only its corresponding color dot when it passes through the shadow mask. A dot triangle when activated appears as a small dot on the screen which has color of combination of three small dots in the dot triangle. By varying the intensity of the three electron beams we can obtain different colors in the shadow mask CRT.

Size : A standard CRT monitor may be more than 16 inches deep and weigh about 30 pounds. Now a days significantly thinner and lighter CRT monitors compared to old‒ fashioned CRT monitors are available.

Power : CRT monitors require a lot of power to run.

Flat‒Panel Monitors

• Flat‒panel monitors are very popular among users of desktop systems. The viewable area of both, CRT monitors and flat‒panel monitors is same, but the CRT monitors take more space than the flat‒panel monitors. Fig. 4.9.19 shows this difference.

• Liquid Crystal Display (LCD) is the common type of flat‒panel monitor. It creates images with a special kind of liquid crystal, which is normally transparent but becomes opaque when charged with electricity.

Disadvantages of LCD monitors

• It is difficult to see images on it in bright light.

• The viewing angle of LCD monitor is limited.

With the improvements in the technology, the viewing angle of flat‒panel monitors is extended.

There are two main types of liquid crystal displays, passive matrix LCD and active matrix LCD.

1. Passive Matrix LCD

They use transistors for each row and each column of pixels. This creates a grid to locate each pixel. The electricity coming from the transistors at the end of the row and the top of the column determines the color displayed by a pixel. These monitors are inexpensive to manufacture.

Disadvantages

• They have a narrow viewing angle.

• Refreshing rate of pixels is less.

• The quick movement of the pointer disappears it. This effect is called 'submarining'.

• Animated graphics can appear blurry on these monitors.

Advancement

Now most passive matrix LCDs use dual‒scan LCD technology. This technology scans the pixels twice as often. It also reduces the submarining and blurry effects.

2. Active Matrix LCD

This technology assigns a transistor to each pixel. Each pixel is turned ON and OFF individually. Using this technology, refreshing rate of pixels is improved. The submarining effect is not present. The viewing angle is wider in active matrix screens than passive matrix screens. These displays use Thin‒Film Transistor (TFT) technology, which uses as many as four transistors per pixel. Now a days most notebook computers use TFT displays.

Comparison of Standard CRT Monitors with Flat‒Panel Monitors

CRT monitors

 1. These are bigger than flat‒panel monitors, so they take more desk‒top space.

2. A standard CRT monitor may be more than 16 inches deep and weigh about 30 pounds.

3. These are difficult to move.

4. These monitors require a lot of power to run.

5. Because of the above reason, they are not practical for use with notebook computers.

6. The viewing angle is greater. You can see the image clearly even when standing at an angle to the screen.

7. They use CRT.

 Flat‒panel monitors

1. These are smaller than CRT monitors, so they take less desk‒top space.

2. A standard flat‒panel monitor is only a few inches deep and usually weigh less than 10 pounds.

3. These are easy to move.

4. These can run on battery power that is built into the computer.

5. Note‒book computers use flat‒panel monitors.

6. The viewing angle is limited. As you increase your angle to the screen, the image becomes fuzzy quickly.

7. They use special kind of liquid crystal.

Other Types of Monitors

• In addition to CRT and flat‒panel monitors, other types of displays are also used in PC systems. Their operation is based on specialized technology and they are used in specific applications. Some of them are discussed below.

• Paper‒white displays

These displays can create high‒quality printed documents. Usually the display on monitor is such that a black text or graphics is created on a paper‒white background. An LCD version of paper‒white display is called a 'page‒white display'. It is based on a special technology, called supertwist to create higher contrasts. The document designers such as desktop publishing specialists, newspaper or magazine compositors and the persons who create high quality printed documents use the paper‒white displays.

• Electro‒Luminescent Displays (ELDs)

These displays use a phosphorescent film. This film is held between two sheets of glass. An image is created by sending current through this film using a grid of wires.

• Plasma displays

A special gas, such as neon or xenon is sandwiched between two sheets of glass in plasma displays. This gas glows when it is electrified via a grid of small electrodes. The voltage applied at various points on the grid can be controlled. Each point acts as a pixel to display an image.

Comparing Monitors

• A number of different types of monitors are available in the market. To make a correct choice, following points should be considered

■ Graphics display on a monitor : It should be nice and easy on your eyes so that you can work longer and more comfortably. A poor monitor can reduce the productivity. Also it causes the strain on eyes.

■ Crispness of the letters : Observe a screen full of text and especially, examine the crispness of the letters at the corners of the screen. The low‒cost and low‒quality monitors cause some distortion around the edges and especially in the corners.

■ Colors : Observe the familiar picture on the monitor and check for the accuracy of colors.

The following most important specifications should be checked while buying a monitor

• Size

• Resolution

• Refresh rate

• Dot pitch

size

• The size of the monitor is measured diagonally, across the front of the screen, in inches. For example, 18 inch monitor means 18 inches from the lower left corner to the upper right corner. In case of CRT monitor, the viewing area is less than the size of the monitor. The viewing area of flat‒panel display is larger than that of same‒sized CRT monitor. As the size of the monitor increases, the objects on the screen appear bigger, more of them can be viewed simultaneously and you can see them well.

Resolution

• The sharpness or clarity of an image is called 'resolution'. In case of monitor, the number of pixels on the screen determine the resolution of it. It is expressed as a matrix. As the number of pixels increase, the resolution increases and the image becomes cleaner and smaller. However, the eyestrain and squinting problems increase with higher resolution. For example, 640 × 480 resolution means, there are 640 pixels horizontally across the screen and 480 pixels vertically down the screen. The actual resolution is determined by the video controller and not by the monitor itself. Most monitors can be operated at several different resolutions. The five commonly used resolution settings are,

640 × 480, 800 × 600, 1024 × 768, 1152 × 864 and 1280 × 1024.

The different standards for monitor resolution are available.

• Video Graphics Array (VGA)

This standard is 640 × 480 pixels.

• Super VGA

This standard is 800 × 600 and 1024 × 768.

Refresh Rate

• The electron gun scans every pixel on the screen at the rate called 'refresh rate'. It is given by the number of times per second. The phosphor dots fade quickly after the electron gun charges them with electrons. Refreshing is essential and refresh rate is important. If the screen is not refreshed often enough, the flicker occurs and it causes an eyestrain. Refresh rate is measured in Hertz (Hz), i.e. cycles per second. For example, refresh rate is 100 Hz means it refreshes its pixels 100 times every second. The refresh rate should be 72 Hz or higher. It should be adequate for the resolution you will be using. Some monitors have different refresh rates for different resolutions.

Dot Pitch

• The distance between the similar colored phosphor dots of adjacent pixels, is called 'dot pitch'. It is the specification of a color monitor. It is measured as a fraction of a millimeter (mm). It ranges from 0.15 mm which is very fine to 0.40 mm or higher which is coarse. As the dot pitch decreases, the image appears finer and more detailed on monitor.

• The dot pitch should not be greater than 0.28 mm for a 15 inch monitor. For a larger monitor, it should be 0.22 mm or less.

2. Printers

• Printers can be classified according to their printing methodology: Impact printers and Non‒impact printers. Impact printers press formed character faces against an inked ribbon onto the paper. A line printer and dot matrix printer are the examples of an impact printers. Non impact printers and plotters use laser techniques, ink‒jet sprays, xerographic processes, electrostatic methods, and electro‒thermal methods to get images onto the paper. A ink‒jet printer and laser printer are the examples of non‒impact printers.

Line Printers

• A line printer prints a complete line at a time. The printing speed of line printer vary from 150 lines to 2500 lines per minute with 96 to 100 characters on one line. The line printers are divided into two categories: Drum printers and chain printer.

Drum Printers

• A drum printers consists of a cylindrical drum. One complete set of characters is embossed on all the print positions on a line, as shown in Fig. 4.9.20. The character to be printed is adjusted by rotating drum.

• The codes of all characters to be printed on line are transmitted from the memory of the computer to a printer memory, commonly known as printer buffer. This printer buffer can store 132 characters. A print drum is rotated with high speed and when printer buffer information matches with the drum character, character is printed by striking the hammer. Thus to print one line drum has to rotate one full rotation. A carbon ribbon and paper are in between the hammer and the drum therefore when hammer strikes the paper an impression is made on the backside of the paper by the ribbon mounted behind the paper. In drum printers to get good impression of the line on paper it is necessary to synchronize the movements of drum and the hammer.

Chain Printers

• In these printers chain with embossed character set is used, instead of drum. Here, the character to be printed is adjusted by rotating chain. To print line, computer loads the code of all characters to be printed on line into print buffer. The chain rotated and when character specified in the print buffer appears in front of hammer, hammer strikes the carbon ribbon. A carbon ribbon is placed between the chain, paper and hammer. In this printer to get good printing quality the movement of hammer and chain must be synchronized.

Dot Matrix Printers

• Dot matrix printers are also called serial printers as they print one character at a time, with printing head moving across a line. In dot matrix printer the print head consists of a 9 x 7 array of pins. As per the character definition pin are moved forward to form a character and they hit the carbon ribbon in front of the paper thereby printing that character, as shown in Fig. 4.9.21.

• In these printers character definition can be changed to get different font as shown in Fig. 4.9.22.

• An other advantage of dot matrix printers is that they can print alphabets other than English, such as Devangari, Tamil etc.

Comparison between line printer and dot matrix printer

Line printer

1. Prints one line at a time.

2. Characters are embossed on the drum or chain.

3. Characters cannot be printed with different fonts.

4. Better printing quality.

5. Better printing speed.

6. Heavy duty printers.

Dot matrix printer

1. Prints a character at a time.

2. Characters are formed by combination of dots.

3. Characters can be printed with various fonts.

4. Poor printing quality as characters are formed by combination of dots.

5. Poor printing speed.

6. Light duty printers.

Ink‒Jet Printer

An ink‒jet printer places extremely small droplets of ink onto paper to create an image. If we ever look at a piece of paper that has come out of an ink‒jet printer, we know that: the dots are extremely small (usually between 50 and 60 microns in diameter), so small that they are thinner than the diameter of a human hair (70 microns). The dots are positioned very precisely, with resolutions of upto 1440 × 720 dots per inch (dpi). The dots can have different colours combined together to create photo‒quality images.

Ink jet printers print directly on paper by spraying ink through tiny nozzles as shown in Fig. 4.9.23.

• As shown in Fig. 4.9.23, the ink is deflected by an electric field with the help of horizontal and vertical charged plates to produce dot matrix

Features of ink‒jet printer

1. They can print from two to four pages per minute.

2. Resolution is about 360 dots per inch, therefore better printing quality is achieved.

3. The operating cost is quite low, the only part that needs replacement is the ink cartridge.

4. Colour ink‒jet printers have four ink nozzles with colours cyan, magenta, yellow and black, because it is possible to combine these colours to create any colour in the visible spectrum.

• An ink‒jet printer places extremely small droplets of ink onto paper to create an image. If we ever look at a piece of paper that has come out of an ink‒jet printer, we know that: the dots are extremely small (usually between 50 and 60 microns in diameter), so small that they are thinner 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 colours combined together to create photo‒quality images. Ink‒jet printers print directly on paper by spraying ink through tiny nozzles.

• An ink‒jet printer has a print cartridge with a series of tiny electrically heated chambers. These cartridges are attached to print heads with a series of small nozzles that spray ink onto the surface of the paper. As print head moves back and forth across the page, the software tells the position where the dots of ink should be 'sprayed'. It also gives instructions regarding the type and the quantity of colours. There are two main ways to drop the ink droplets, namely, the bubble‒jet and piezo‒electric technology. Bubble‒jet printers use heat to fire ink onto the paper. There are three main stages with this method.

• Initially the bubble is created by heating the ink.

• The pressure force is applied to burst and then hit the bubble on the paper.

• The bubble then collapses as the element cools and the resulted vacuum in the nozzle draws ink from the reservoir to replace the ink that was ejected.

• In the piezo‒electric method, piezo crystal is used and it is located at the back of ink reservoir. When electric current flows through the piezo crystal, it contracts and thus forces a drop of ink out of the nozzle.

Laser Printers

• The line, dot matrix and ink‒jet printers need a head movement on a ribbon to print characters. This mechanical movement is relatively slow due to the high inertia of mechanical elements. In laser printers these mechanical movements are avoided. In these printers, an electronically controlled laser beam traces out the desired character to be printed on a photoconductive drum. The exposed areas of the drum gets charged, which attracts an oppositely charged ink from the ink toner on to the exposed areas. This image is by then transferred to the paper when it comes in contact with the drum with pressure applied by the pressure roller. The charge on the drum decides the darkness of the print. When the charge is more, more ink is attracted and we get a dark print.

• This pressure roller transfers the black toner onto the paper. Since the paper is moving at the same speed as the drum, the paper picks up the image pattern precisely. Finally, the printer passes the paper through a pair of heated rollers called fuser. As the paper passes through these rollers. The loose toner power gets melted and fuses with the fibres in the paper. The paper is then brought out of the printer.

• A colour laser printer works like a single colour laser printer, except that the process is repeated four times with four different ink colours : Cyan, magenta, yellow and black. Laser printers have high resolution from 600 dots per inch upto 1200 dots per inch. These printers print 4 to 16 page of text per minute. The high quality and speed of laser printers make them ideal for office environment.

Advantages of Laser printer

• The main advantages of laser printers are speed, precision and economy. A laser can move very quickly, so it can "write" with much greater speed than an ink‒jet. Because the laser beam has an unvarying diameter, it can draw more precisely, without spilling any excess ink. Laser printers tend to be more expensive than ink‒jet printers, but it doesn't cost as much to keep them running. Its toner power is cheap and lasts for longer time.

Thermal Transfer Printer

• In thermal transfer printer, wax paper and plain paper are drawn together over the strip of heating nibs. The heating nibs are selectively heated to cause the pigment transfer.

• In case of colour thermal transfer printers, the wax paper is placed on a roll of alternating, cyan, magenta, yellow and black strips, each of a length equal to the paper size. It is possible to create one colour hardcopy with less than 1 minute. This is possible because the material used to manufacture nib heats and cools very rapidly. Modern thermal transfer printers accept a video signal and digital bitmap input, making them convenient for creating hardcopy of video images.

Review Questions

1. Write a short note on

a) Keyboard

b) Mouse

c) Scanner

2. Write a short note on

a) Cathode‒Ray Tubes

b) Vector scan display

c) Raster scan display

d) Beam penetration technique

e) Shadow mask technique

3. Explain the working of direct‒view storage tubes.

4. List the advantages and disadvantages of DVST.

5. Write a short note on

a) Flat panel display b) Plasma panel display

6. List the important characteristics of video display devices.

7. Write a short note on dot matrix printer.

8. Write a short note on desk jet printer.

9. Write a short note on laser printer.

10. Write short notes on I/O devices.