Working of output devices.

Q2. What is the role of an output unit? Discuss various types of output devices along with their working mechanism.

Ans. Once data has been input into a computer and processed, it is of little use unless it can be retrieved quickly and easily from the system. To allow this the computer must be connected to an output device. The output devices produce output of the machine in human understandably form.

Types of output devices along with their working mechanism are:

  1. Monitors: Monitors or “screen” is the most common form of output from a computer. The picture is made up of thousands of tiny colored dots called pixels. The quality and detail of the picture on a monitor depends on the number of pixels that it can display. The denser are the pixels, the greater the clarity of the screen image. The two types of monitors are a Liquid Crystal Display(LCD) and a Cathode-ray Tube (CRT) monitor.

  • Cathode-ray Tube (CRT): The CRT works in the same way as a television- it contains an electron gun at the back of the glass tube, This fires electrons at groups of phosphor dots, which coat the inside of the screen. When the electrons strike the phosphor dots they glow to give the colors.
  • Liquid Crystal Display (LCD) (or “TFT” Display): Liquid Crystal is the material used to create each pixel on the screen. The material has a special property- it can ‘polarize’ light depending on the electrical charge across it. This feature allows the pixels to be created. Each tiny cell of liquid crystal is a pixel.


TFT Display (Thin Film Transistor) is the device within each pixel that sets the charge. TFT displays refer to the tiny transistors that make them work.


  1. Printers: Printer is an output device used for creating paper copies of output from the computer. Printers can produce text and images on paper, plastic or even textiles such as T-shirts. Printers can be divided into two distinct categories:
  • Impact printer: In these printers, there is mechanical contact between the print head and paper.
  • Non-impact printers: In these printers, there is no mechanical contact between the print head and paper.


  • Impact printer: The impact printers come in lot of varieties such as line printers, drum printers, daisy wheel printers and dot-matrix printer.


  1. Dot-matrix printer: A dot matrix printer (DMP) is the most popular serial printer i.e., it prints one character at a time. In DMPs, the printing head contains a vertical array of pins. As the head moves across the paper, selected pins fire against an inked ribbon the form a pattern of dots on the paper. In the 9-pin DMPs, the capital letters are formed by suing 5 x 7 matrix of dots i.e., 5-dot rows and 7-dot columns. Though there are 9 pins in the print head. The bottom 2 pins are used to form the descended of small letters link f,g,j,p,q,y etc.


  • Non-impact printer: There are two non-impact printers- Inkjet and laser printers.
  1. Inkjet and bubble jet printers: An inkjet printer is any printer that fires extremely small droplets of ink onto paper to create impression of text or image. The inkjet printers direct a high-velocity stream of ink toward the paper. This stream is deflected, generally by passing it through an electrostatic field such as that used to deflect beams in oscilloscopes.


In a bubble jet printer, tiny resistors create heat, and this heat vaporizes ink to create a bubble. The expansion that creates the bubble causes a droplet to form and eject form the print head.

  1. Laser Printer: These printers make use of office copier technologies. The desired output image is written on a copier drum with the help of a light beam controlled by a computer. With this certain parts of the drum surface get electrically charged, and then this drum surface is exposed to the laser beam. These laser exposed areas attract a toner that forms the image by attaching itself (toner ink) to the laser generated charges on the drum. The toner is then permanently fused on paper with hot or pressure.

The laser printers are quiet and are capable of producing very high point quality. The speed of laser printer can be up to 10-20 pages per minute (ppm).


  1. Plotters: Plotters are the output devices that produce good quality drawing and graphs. There are two types of plotters: Drum plotters and flat bed plotter.


In a drum plotter, the output paper is mounted on a rotating drum. A pen capable for linear motion is mounted on a carriage. The drum can rotate in either clockwise or anti-clockwise direction under the control of plotting instructions sent by the computer. The pen can either move in either direction- left to right or upwards or downwards. The movement of pen and drum is controlled by the graph plotting program. The program can thus draw various graph and also annotate them using the pen to draw characters.


A flat-bed plotter consists of stationary horizontal plotting surface on which paper is fixed. The pen is mounted on a carriage which can move horizontally or vertically or leftwards or rightwards. A graph plotting program is used to move the pen to trace the desired graph.

  1. Speakers: A computer system having sound capabilities cannot work without a sound card. Speakers receive the sound in the form of electric current from the sound card and then convert it to sound format.

Speakers receive constantly changing electric current from the sound card. This current is transferred to a magnet which pushes the speaker’s core back and forth. This way pressure vibrates are generated that create sound.


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