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									                                       Output Devices
          An Output device is any machine capable of representing information from a
computer. Output devices can be classified into hard copy devices (printers, plotters etc.) and
soft copy devices (visual display unit, Liquid Crystal Displays etc.).
Visual Display Unit(VDU)
The terminals that use video screens to displayinformation are commonly called visual
display units(VDU). VDUs are of two types: Cathode Ray Tube(CRT) and lat Panel Display.
(i) Cathode Ray Tube
     Two types of CRTs are available-monochrome and colour. The monochrome monitors
     display two colours (black and white). Colour monitors or RGB monitors use three
     diferent colours Red, Green and Blue to display from 16 to over 1 million dierent colours.
     Quality of the monitor is judged in terms of resolution which is a measure of the number
     of picture elements(Pixels). The higher the number of pixels, the clearer and sharper is the
     image.
          An RGB monitor consists of a vacuum tube with three electron guns(red, green and
     blue) at one end and the phosphor coated screen at the other end. When the three electron
     guns fire electrons at the screen, they excite the phosphors and make them glow. They
     glow in red, green or blue depending on the colour of the beam that excites them. The
     display memory is read constantly and the screen image created is refreshed at the rate of
     50 or 60 times each second.
          The disadvantage of CRT monitors is that they are heavy nf bulky. They consume
     more power and emits heat.
(ii) Flat Panel Display
     Flat panel displays are suitable for portable computers. They are thinner and lighter in
     weight. They consume less power and does not emit heat. Some of the flat panel displays
     are Liquid Crystal displays and Plasma panels.
     (a) Liquid Crystal Display(LCD)
          Liquid Crystal Displays consists of a thin layer o liquid crystal between electrically
     conducting plates. A liquid crystal is a liquid that exhibits crystalline properties. By
     applying proper electrical signals across the plates, various segments of the liquid crystal
     can be activated, causing the segments to either transmit or block the light. An image is
     produced by passing light through selected segments of the liquid crystal and then
     reflecting it back from the mirror to the viewer.
     (b) Plasma Panels
          Plasma panels consists of two glass plates separated by a thin gap filled with a gas
     such as neon. Each plate has several parallel electrodes running across it. The electrodes
     at the two plates are placed at right angles to each other. A voltage pulse applied between
     two electrodes, one on each plate, causes the small segment of the gas at the intersection
     of the two electrodes to glow.

   Printers
   Printers are used to produce hard copy of output. Printers are broadly classified into
   impact printers and non-impact printers.
   Impact Printers
   An Impact printer makes contact with the paper. It forms the print image on the paper by
   pressing an inked ribbon against paper with a hammer like mechanism. The impact
   printers can be classified into Line printers and Character printers.
   (1) Line Printers
       Line printers print a compete line at a time. They print 150 to 2500 lines per minute.
       The different types of line printers are drum printers and chain printers.
             (a) Drum Printer
                 It uses a rapidly rotating drum which contains a complete raised character set.
                 A set of print hammers, one for each character in a line, is mounted in front of
                 the drum. The printer buffer gets a line to be printed from memory. A
                 complete line is printed within one revolution of the drum. These printers have
                 a fixed font.
             (b) Chain Printer
                 A chain printer has a steel belt on which the character sets are embossed. For a
                 printer with 132 characters per line, 132 hammers will be positioned to strike
                 the carbon ribbon. The belt is rotated at high speed. When the desired
            
								
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