BLOCK DIAGRAM OF COMPUTER SYSTEM

BLOCK DIAGRAM OF COMPUTER SYSTEM 

Unit-2

  INPUT - OUTPUT DEVICES

Input-output (I/O) devices are the primary means by which one can interact with computer. The computer devices are also called peripheral devices. Peripheral devices of computer can be divided into internal (installed inside the computer) & external (existing outside the computer but connected electronically to it). All the peripherals of the computer are connected to the ports. An input device converts incoming data & instructions into a pattern of electronic signals in binary code i.e. combination of 0s & 1s to a digital computer, whereas an output device reverses the processing, translating the digital signals into a form understandable to the user.

INPUT DEVICES

 The raw data & program are entered in the computer through input device. It reads the raw data prepared by the user & sends then into the computer as a series of electronic pulses. The devices which, read the data and program into the computer, are called input devices, i.e., data and programs are entered into the computer system for processing through input device. An input device converts input data into suitable form acceptable to a computer. So, it is a means of communication between user and the computer. Examples of input devices are keyboard, mouse, joysticks, optical character reader, light pen, touch panel.

KEYBOARD:

Christopher Latham Sholes (February 14, 1819 – February 17, 1890) was an American inventor who invented the QWERTY keyboard, and along with Frank Haven Hall, Samuel W. Soule, Carlos Glidden and John Pratt, has been contended as one of the inventors of the first typewriter in the United States.   Keyboard is the most common and very popular input device which helps to input data to the computer. The layout of the keyboard is like that of traditional typewriter, although there are some additional keys provided for performing additional functions. Keyboards are of two sizes 84 keys or 101/102 keys, but now keyboards with 104 keys or 108 keys are also available for Windows and Internet.

The keys on the keyboard are as follows –

Sn	Keys and Description
1	Typing Keys  These keys include the letter keys (A-Z) and digit keys (09) which generally give the same layout as that of typewriters.
2	Numeric Keypad It is used to enter the numeric data or cursor movement. Generally, it consists of a set of 17 keys that are laid out in the same configuration used by most adding machines and calculators.
3	Function Keys The twelve function keys are present on the keyboard which are arranged in a row at the top of the keyboard. Each function key has a unique meaning and is used for some specific purpose
4	Control keys These keys provide cursor and screen control. It includes four directional arrow keys. Control keys also include Home, End, Insert, Delete, Page Up, Page Down, Control(Ctrl), Alternate(Alt), Escape(Esc).
5	Special Purpose Keys Keyboard also contains some special purpose keys such as Enter, Shift, Caps Lock, Num Lock, Space bar, Tab, and Print Screen.

 How the Computer Accepts Input From the Keyboard?

=> A tiny computer chip, called the keyboard controller, notes that a key has been pressed. The Keyboard controller places a code into part of its memory, called the keyboard buffer, indicating which key was pressed. A buffer is a temporary storage area that holds data until it can be processed. This code is called the key’s scan code. The keyboard controller then signals the computer’s system software that something has happened at the keyboard. It does not specify what has occurred just that something has. The signal the keyboard sends to the computer is a special kind of message called an interrupt request. An interrupt is a signal; it notifies a program that an event has occurred. The keyboard controller sends an interrupt request to the system software when it receives a complete keystroke. For example, if you type the letter “r”, the controller immediately issues an interrupts request. If you hold down the shift key before typing the letter “R”, the controller waits until the whole key combination has been entered. When the system software receives an interrupt request, it evaluates the request to determine the appropriate response. When a key-press has occurred, the system reads the memory location in the keyboard buffer that contains the scan code of the key that was pressed. It then passes the key’s scan code to the CPU. The keyboard buffer can store many keystrokes at one time. This capability is necessary because some time elapsed between the pressings of a key and computer’s reading of that key from the keyboard buffer. With the keystrokes stored in the buffer the program can react to them when it is convenient.

 In many newer systems, the keyboard controller handles input from the computer’s mouse and stores setting for both the keyboard and the mouse.

 Fig: How Keyboard works? 

MOUSE 

 Mouse was invented by Douglas Engelbert of Standford Research Center in 1963 & pioneered by Xerox in the 1970's.

Mouse is the most popular pointing device. It is a very famous cursor-control device having a small palm size box with a round ball at its base, which senses the movement of the mouse and sends corresponding signals to the CPU when the mouse buttons are pressed. 

Generally, it has two buttons called the left and the right button and a wheel is present between the buttons. A mouse can be used to control the position of the cursor on the screen, but it cannot be used to enter text into the computer. 

There are basically three types of mice

Mechanical Mouse 

Mechanical mouse has a rubber or metal ball on its underside that can roll in all directions. Mechanical sensors within the mouse detect the direction of rolling ball & move on the screen accordingly. 

Optomechanical Mouse

The optical-mechanical or optomechanical mouse consists of a ball that rolls one of two wheels inside the mouse. Each wheel contains a circle of holes, allowing an LEDlight to shine through and be detected by a sensor. As the wheel spins these lights represent an X or Y axis for the mouse pointer on your screen. This mouse is more accurate than a mechanical mouse that uses only wheels and rollers, however, is not as good as an optical mouse.

Optical Mouse 

The optical mouse is used on a special pad having grids of altering light and dark lines. A LED on the bottom of the mouse directs a beam of light down onto the pad, from which it is reflected and sensed by the detectors on the bottom of the mouse. As the mouse is moved, the reflected light beam is broken each time a dark line is crossed. The number of pulses so generated, which is equal to the number of lines crossed, is used to report mouse movements to the computer. 

Mouse can be connected to PC in one of the three ways. 

1. Serial mouse connect directly to the serial port (9-pin). 

2. PS/2 mouse connect directly to the PS/2 port (6-pin). 

3. Cordless mouse does not connect physically but can be connected with infrared or radio waves. 

Using the mouse involves five techniques: 

1. Pointing: Move the mouse to move the on-screen pointer. 

2. Clicking: Press and release the left mouse button once. 

3. Double-clicking: Press and release the left mouse button twice. 

4. Dragging: Hold down the left mouse button as you move the pointer. 

5. Right-clicking: Press and release the right mouse button 

Trackball

A trackball is a pointing device that works like an upside-down mouse. A track ball is really a variation on the mouse. You rest your thumb on the exposed ball and your fingers on the buttons. The track ball is held stationary while the ball is manually rotated by hand in any direction. The track balls are intended for use where the desktop space is limited or not available. To move the pointer around the screen, you roll the ball with your thumb. The advantage over mice is that it is stationary so it does not require much space to use and it can be place on any type of the surface.

Joystick

A joystick is a device consisting of a hand held stick that pivots about one end and can be moved left or right, forward or backward. It lets the user move an object on the screen. A potentiometer senses the movements of the stick. A potentiometer is an instrument for measuring or comparing the movement of mouse from original position. As the stick is moved around, the movements are translated into binary instructions with the help of electrical contacts in its base. Children can play with a set in two crossed grooves & can be moved left or right, forward or backward. A joystick is generally used to control the velocity of the screen cursor movement rather than its absolute position.

Light Pen

Light pen is an input device that contains a photocell mounted in pen-shaped tube. When the pen is moved over the surface of the screen, it detects the light emitted from the screen and generates electric pulses. The pulses are transmitted to a processor that identifies the pixel (picture element) the light pen is pointing to. It enables a user to write words or draw pictures directly into the computer. It is useful for graphic work like CAD (Computer Aided Designing) package.

Because of the following drawbacks the light pens are not popular now-adays:

It obscures the screen image as it is pointing to the required spot.

The light pen cannot report the coordinates of a point that is completely black

It gives sometimes-false reading due to background lighting. Prolong use of it can cause arm fatigue.

DIGITIZER

Digitizer is an input device which converts analog information into digital form. Digitizer can convert a signal from the television or camera into a series of numbers that could be stored in a computer. They can be used by the computer to create a picture of whatever the camera had been pointed at.

Digitizer is also known as Tablet or Graphics Tablet as it converts graphics and pictorial data into binary inputs. A graphic tablet as digitizer is used for fine works of drawing and image manipulation applications.

MICROPHONE

Microphone is an input device to input sound that is then stored in a digital form. The microphone is used for various applications such as adding sound to a multimedia presentation or for mixing music.

MAGNETIC INK CARD READER (MICR)

MICR input device is generally used in banks as there are large number of cheques to be processed every day. The bank's code number and cheque number are printed on the cheques with a special type of ink that contains particles of magnetic material that are machine readable.

This reading process is called Magnetic Ink Character Recognition (MICR).

The main advantages of MICR is that it is fast and less error prone.

OPTICAL CHARACTER RECOGNITION

Optical character recognition is a process of recognizing pre-printed characters & distinguishes one character from other provided characters to meet OCR standards. OCR can also read characters produced by typewriters. There are many OCR front standards such as OCR-A, OCR-B etc. It can be observed that OCR reader can read alphabets, numbers & special characters that are printed or typed on the paper. An optical reader uses photoelectric device to scan the input.

OCR is an input device, which is used to read an image, characters of different fonts printed on any paper. The printed characters are examined by passing them under a light and lens system. A light source converts the alphabets, number and special characters into electrical pulses, which are then sent to the computer for processing. If there is no dark spot it is represented by 0 and if there is a dark spot it is represented by 1.Such representation is called the bit map of the image. Nowadays, advanced OCR system can also read handwritten text.

BAR CODE READER

Bar Code Reader is a photoelectric scanner that read bar code, an identification code printed on product containers that gives its value (price) and other information (quality, manufacturer etc.).

Bar code is a printed series of parallel bars or lines of varying width that is used for entering data into a computer system. They are mostly used by supermarkets, bookshops etc.

The bars are typically black on a white background & their width & quantity vary according to applications. The bars are used to represent 0 & 1, sequence of which in turn can represent numbers from 0 to 9 & be processed by a digital computer. An optical scanner reads bar code information, which is apart of a computer system.

OPTICAL MARK READER (OMR)

OMR is a special type of optical scanner used to recognize the type of mark made by pen or pencil. It is used where one out of a few alternatives is to be selected and marked. It is specially used for checking the answer sheets of examinations having multiple choice questions.

DIGITAL CAMERA

Digital camera is the input device that input images into the computer. Digital camera uses digital photographic technique i.e. it outputs the images by means of digital phenomenon. Normally, film camera capture images on a specially coated films whereas the digital camera captures images electronically.

The digital camera implements the image sensor to convert the light directly into series of pixel values that make up the image to be taken. The more pixels your camera has, the more detail it can capture. Over the last few years, digital cameras have become more and more sophisticated enabling many advanced features including noise filtering, instant red-eye removal, highquality prints extracted from video, image and video stabilization, in-camera editing of pictures and wireless transmission of photos.

Scanner

Scanner is an input device that can read text or illustration printed on paper and translates the information into a form that the computer can use. The resulting image (text or illustration) can be stored in a file as Bitmap or JPEG (Joint Photographic Experts Group), displayed on a screen, and manipulated by programs.

A light source is moved across a printed page. The light bounces off the page and is passed through the lens and onto light-sensitive diodes, which convert the light to electricity. A circuit board converts the electricity to numbers and sends the information to the computer.

There are two types of scanners- Hand-held Scanner

Flatbed Scanner

Hand-held Scanner

A hand held scanner is around 13cm long and 15 cm wide with a handle to enable it to be held conveniently in hand. A set of light emitting diodes is enclosed in it. It is placed over the material to be scammed and slowly dragged from the top to the bottom.

Flatbed Scanner

A flatbed scanner consists of a box with a glass plate on top and cover, which covers the glass plate. The document to be scanned is placed above the glass plate. The light beam is situated below the glass plate and is moved from left to right horizontally.

Output Devices

CPU processes the information that is entered through the input devices and produces the meaningful information. These information can appear in variety of form – as a binary number, as picture, as text, as sound or as printed pages. An output device is any device that is capable of representing that information from the computer. Output device includes monitors, speaker, printers, plotters, etc.

Classification of Outputs – based on type of output produced

Softcopy Output

Softcopy output refers to the output displayed on the screen. The output on the screen is lost when computer is turn off. The most common output device is monitor. Sound produced by voice output device (speaker) is also softcopy output.

Hardcopy Output

Hardcopy output refers to recording letters, graphics or pictures on a permanent medium such as paper. Such output can be read immediately or stored and read latter. The most commonly used hardcopy output devices are printer and plotters.

Monitors

Monitor, also called display screen, is a most commonly used output device that produces soft copy output. Monitor is also known as Visual Display Unit (VDU).

Classification of Monitors – based on colors

Monochrome Monitor

Monochrome Monitor can display only two colors. One is foreground and another is for background. The color can be black and white, black and green, amber (brown yellow color) and black.

Gray-scale Monitor

A Gray-Scale monitor is special type of monochrome monitors capable of displaying different shade of Gray.

Color Monitor

Color monitor can display anywhere from 16 colors to 16 million colors. Colors monitors are sometimes called RGB monitors because they accept three signals – red, green, and blue.

Classification of monitors – based on technology

CRT (Cathode Ray Tube) Monitors

Flat Panel Display Monitors

CRT Monitors

A monitor consists of the display device- the picture tube along with the related circuitry that converts the signal send by the computer in a form that can be used by the display. The picture tube, called cathode ray tube or CRT, is a core of monitor.

It has many other elements such as a shadow mask and a phosphors coating that enables it to display picture along with the display electrons. The simplest way of explain the functioning of the monitor screen is by what we call CRT electron’s guns. The electrons are filtered through a shadow mask and hit the phosphors- coated screen to produce the image. This process is monitored and controlled by various display electronics.

The most important component in the component in the monitor’s electronic is the video amplifier increase the strength of single volt signals that are received from your PC, to the thousands of voltage that are required to drive the electron beam.

The Cathode- Ray Tube is a major component of any monitor. It essentially is a glass tube partially evacuated and filled with inert gas at a very low pressure. The cathode (a negatively charged electrode) beams a ray of electrons towards a positively charged the anode. At the end of their flight, the electron crash into a coating made from phosphor compounds that convert the kinetic energy of the electron into visible light- glowing to produce the picture. Color CRTs use thousands of triangle painted across the inner surface of the tube. Every triad consists of three dots of the primary color red, green and blue (RGB) arrayed next to each other. One triad of the dots makes up a ‘picture cell’ called a pixel.

To generate beam that light up the phosphor on the screen, a CRT uses one or more electron guns. An electron gun is an electron emitter (a cathode) that draw the electron into sharp and high speed beams. These are three separate electron beam, one each for red, blue and green colors. These beam have to go through a shadow mask, which is a layer between the phosphors triad and electron guns. The shadow mask cause the three beams to land on the phosphors in a distinct triangle so that each beam illuminates only the right – color dot. Each triad has a corresponding hole in a shadow mask, which prevent the beam from illuminating the wrong color phosphors.

To move the beam across the breadth of the tube face, a group of powerful electromagnets are arranged around the tube, forming a yoke. They bend the electron beam in the course of its flight. Monochrome CRTs have a single electron gun that continuously sweeps across the screen. Most color tube has three guns arranged in a triangle. They emit their electrons continuously and the three resulting beam are steered by the yoke.

Flat Panel Monitors

There are several types of flat-panel of monitors, but the most common is the liquid crystal display monitor. The LCD monitors creates images with a special kind of liquid crystal that is normally transparent but become opaque when charged with the electricity. One disadvantage of LCD monitors is that, unlike phosphors, the liquid crystal does not emit light, so there is not enough contrast between image and background to make them clear under all conditions. The problem is solved by backlighting the screen. Although this makes the screen easier to read, it requires additional power.

Another disadvantage of LCD monitors is their limited viewing angle – i.e. the angle from which the display images can be viewed clearly. LCD monitors use transistor for pixel but not the electron beam. The electricity coming from transistor determines the color displayed by a pixel. Depending upon the arrangement of transistors, flat-panel screens are divided into Passive Matrix LCD & Active Matrix LCD

Passive Matrix LCD is less expensive than Active matrix monitors are. One Disadvantage is that, Passive matrix have narrow viewing area as compared to that the Active matrix. In a passive-matrix display, a transistor controls a whole row or column of pixels.

Active Matrix LCD technology assigns a transistor to each pixel, and each pixel is turned on and off individually. These enhancements allow the pixel to be refreshed much more rapidly. In an active matrix display, each pixel is controlled by its own-transistor. Active matrix screens are much brighter & sharper than passive matrix screens.

Gas-Plasma Display

It is like a neon bulb, in which the display uses a gas that glows & emits light in the presence of an electric current. At present, ELD & gas-plasma technologies are more expensive & they are not used as often as LCD technology.

Characteristics of Monitors

•  Size

•  Resolution

•  Refresh Rate

•  Dot Pitch Convergences

Size

The most important aspect of monitors is its screen size. A typical size of small VGA monitors is 14 inches. Just as the television set, monitors screen size are measured by the length of diagonal of their picture tubes

Refresh Rate

Display monitors must refresh many times per second. The refresh rate determines how many times per second the screen is to be refreshed. The refresh rate for a monitor is measured in Hertz (HZ) and is called vertical frequency or vertical refresh rate. The faster the refresh rate, the less the monitors flickers.

Convergence

Convergence refers to how sharply an individual color pixel appears. If the dots are badly miss converged, the pixel will appear blurrily.

Resolution

The resolution of monitors indicates how densely the pixels are packed. The clarity or sharpness of a display screen is called its resolution. Pixel is short for “picture element”. The resolution of a computer monitors is classified by the number of pixel on the screen, expressed as matrix. For example, a resolution of 640 X 480 means that there are 640 pixels horizontally across the screen and 480 pixels vertically down the screen. The number of bit is used to represent single pixel that determine how many color or shades of grey can be displayed. For example, an 8-bit color monitors use 8 bit for each pixel, making it possible to display 2 to the 8_th power (i.e. 256) different colors of grey.

Dot Pitch

Dot pitch is the distance between the two phosphors dots that makes up the single pixel. Each RGB triad contains three phosphors dot - red, green and blue. If these dots are not close enough together, the image on the screen will not be crisp. Smaller dot pitch means clearer and sharper pictures. In general dot pitch should not be greater than 0.28mm.

Printers

Printer is a hard o/p device that prints text or other information on paper and in many cases transparencies and other media. There are mainly two types of printers which fall into following categories Impact Printer & Non -impact Printer.

When evaluating printers, following criteria must be considered-

Image Quality

Image quality, also known as print resolution, is usually measured in dots per inch (dpi). The more dots per inch a printer can produce, the higher its image quality. Most common print resolution is 300 or 600 dpi.

Initial Cost

Consumer printers cost $250 or less, but professional printers can cost thousands of dollars.

Speed

Printer speed is measured in the number of pages per minute (ppm) the device can print. Print speed goes up, so does cost.

Cost of Operation

The cost of ink or toner and maintenance varies with the types of printer. Many different types of printer paper are available, too, and the choice can affect the cost of operation.

Impact Printer

In impact printer, the hammer of the printer strikes against an ink ribbon to make marks on the paper. This includes the working of an impact printer is exactly similar to a typewriter. Examples are

1. Character Printers

a.  Dot-Matrix printers

b.  Daisy-Wheel printers

2 Line Printers

a. Drum printers

Character Printers

A character printer produces character at a time as a printed output. Depending upon the type of print head, the character printers can be broadly classified as: Daisy Wheel Printer and Dot matrix Printer.

Daisy Wheel Printer

The letter printer or continuous character printer is daisy wheel printer. It is a very high quality but equally slow printer. The daisy wheel is a disc made of plastic or metal on which characters stand out along the outer edge. While printing, the wheel is moved across the paper to the printing position & rotated until the required character on the top is found. The speed of a daisy wheel printer is 90 cps (character per second). It is only for text printing.

Dot Matrix Printers

This type of printer creates character from a set of dots. The print head consists of vertical array of pins. While printing, the head moves across the paper to the printing position & selected pins are activated to strike against an inked ribbon to form a pattern of dots on the paper. To print the full character, the print head moves a predetermined number of positions to complete the matrix. The matrix contains 7 rows & 5 columns. The numbers of pins in the print head are either 9 or 24, the bottom 2 pins being used to form a lower case letters. It is a very versatile device because it is capable of printing in various languages with fronts of different types & sizes. It can also give graphic output. The major reason for its popularity is that dot matrix printers are cheaper than other printers. The speed of a typical dot matrix printer has 200-500 cps.

Advantages

1.  They are generally cheaper as they use simple technology.

2.  It only requires paper & the carbon ribbon.

3.  It does not require air condition environment.

Disadvantages

1.  They are noisy.

2.  The quality of print is poor.

3.  Not good for continuous printing

Fig: How a Dot Matrix Printer creates an image.

Line Printers

Line printer is a high-speed printer capable of printing an entire line at a time. The one of the  line printer is  Drum Printer.

Drum Printers

Drum printer consists of drum having letters marked on them, & rotates at uniform speed. The paper passes between the drum & the hammers strike the character image through a carbon, when the required characters pass the print location. It is capable of printing at a very high-speed ranging 125 to 3000 lines per minute & is very expensive.

Non-impact Printer

In Non-Impact printer, the ink is sprayed to the paper so that the letters or objects get printed. This includes Ink-Jet printer, Laser printer and Thermal printer.

Ink-Jet Printer

An ink-jet printer produces a character by shooting small droplets of ink onto the paper. One simple technique is the thermal heating process. In this method, ink is heated from one end to more than 300₀c & the vaporized ink forms a bubble. When the bubble expands, a drop of ink is forced out. The print quality of an ink-jet is generally 300 dpi to 600 dpi. An ink-jet printer can also generate colored graphics images.

Fig: How an Ink-jet printer creates an image.

Advantages

•  High quality print.

•  Price is lower than Laser printer

•  Color ink-jet printers provide an inexpensive way to print full color documents.

•  Performance: 2-4 page per minutes.

Disadvantages

•  Expensive than dot–matrix printer.

•  It needs special type of ink.

•  Ink clogging (blocking) in the printer head is the main problem with an ink-jet printer.

Thermal Printer

As the name suggests this printer uses heat to make a mark on the heat sensitive paper. Similar to the dot-matrix printer, the print head containing needles is pressed against the paper. By applying heat to the selected pins, the paper changes color to for a pattern of dots. The printer is comparatively quicker than the dot matrix printers. The disadvantage is that a special heat sensitive paper is required.

Laser Printer

LASER – Light Amplification Stimulated Emission and Radiation

Laser printer uses a laser beam to produce an image on a drum, coated with photosensitive material. With this, certain parts of the drum surface get electrically charged and special ink is sprinkled on the drum, which sticks electro statically to the charged area of the drum, forming the image. As the drum rotates, the image is transferred onto the paper through a combination of heat and pressure. Laser printer is reliable printers with high quality output. The resolution of laser printer ranges from 300 dpi to 1600 dpi. The standard speed of laser printer is 4 to 8 pages per minute.

Fig: How a Laser printer creates an image.

Difference Between Impact printer and Non-Impact Printer

Impact printer / Dot-matrix Printer	Non impact Printer / Laser Printer
1. It prints characters or images by striking print hammer or wheel against an inked ribbon.	1. It prints characters and images without striking the papers.
2. Its speed is slower.	2. Its speed is faster.
3. Its printing quality is lower.	3. Its printing quality is higher.
4. It normally uses continuous paper sheet.	4. Its normally uses individual paper sheet.
5. It generates noise during printing.	5. It does not generate noise during printing.
6. It uses inked ribbon for printing.	6. It uses toner or cartridge for painting.
7. It is less expensive.	7. It is more expensive.
8. Dot matrix is an impact printer.	8. Laser printer is a nonimpact printer.

Speech Output Unit

A speech output unit is one, which reads stings of characters stored in a computer’s memory and converts them into spoken sentences. Electronic chip have been designed which accepts ASCII characters of a sentence, combine sequence of words into sound, amplifies them and output them through a speaker.

Sound Cards

Sound card is a circuit board that converts sound from analog to digital form, and viceversa, for recording or playback. It actually has both input and output functions. If you want to use your computer’s microphone to record your voice, for instance, you connect the microphone to the sound card’s input jack.

Headphones and Headsets

Headphones include a pair of speakers, which are attached to an adjustable strap that can be custom fitted to the wearer head. A headset includes one or two speakers and a microphone, all mounted to an adjustable head strap.

How a computer uses a speaker to produce a sound?

 

Plotters

A plotter is a special kind of output device that looks like a printer that is used for drawing pictures on paper based on commands from a computer. It is typically used to print largeformat images, such as construction or engineering drawing created in a CAD where precision is mandatory.

Plotter differs from printer in that they draw lines using a pen as a result they can produce continuous lines whereas printers can only simulate lines by printing closely spaced series of dots. Multicolor plotter use different colored pens to draw different colors. Plotter is considerably expensive than printer.

Plotters differ from printers in that they draw lines using a pen. As a result, they can produce continuous lines, whereas printers can only simulate lines by printing a closely spaced series of dots. Multicolor plotters use different colored pens to draw different colors.

Plotters are more expensive than printers. They are used to print large-format images, such as construction or engineering drawing.

There are two types of plotters:

•  Drum Plotters

•  Flat-Bed Plotters

Drum Plotters

In a drum plotter, the paper on which the graph is to be drawn is mounted on a rotating drum. A pen, which can move linearly, i.e. perpendicular to the direction of drum rotation, is mounted on a carriage. The drum can rotate in either clockwise or anti-clockwise direction under the control of the plotting instructions sent by the computer. The pen can move left to right or right to left. The pen can also move up or down. The graph-plotting program controls the movement of the pen and drum. The program can thus draw various graphs an also annotate them by using the pen to draw characters.

Flat-Bed Potters

A flat bed plotter has a stationary horizontal plotting surface on which paper is fixed. The pen is mounted on a carriage which can move in either X or Y direction. The pen can also be moved up or down. A graph plotting computer program is used to move the pen to trace the desired graph.

Advantages of plotters

•       Plotters can work on very large sheets of paper while maintaining high resolution.

•       They can print on a wide variety of flat materials including plywood, aluminum, sheet steel, cardboard, and plastic.

•       Plotters allow the same pattern to be drawn thousands of times without any image degradation.

Disadvantages of plotters

•       Plotters are quite large when compared to a traditional printer.

•       Plotters are also much more expensive than a traditional printer.

PC Projectors

A PC projector connects to a PC and is used to project images on a large screen. Many PC projectors provide the same resolutions and color levels as high-quality monitors. Digital light processing (DLP) projectors use a microchip containing tiny mirrors to produce very sharp, bright images.

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