Computer:

Computer is an electronic machine that follows instructions, gets data from an input device, directs processing of the data and directs information to an output device.

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Computer System Units

Types of Computers
Personal computers
Minicomputers
Supercomputers
Servers

Personal computers

It can be defined as a small, relatively inexpensive computer designed for an individual user. Personal computers are desktop computers designed for an individual's use. They run programs designed to help individuals accomplish their work more productively.

Minicomputers

It is a midsize computer.Minicomputers are multi-user systems that can handle the computing needs of a smaller corporation or organization. Many people use them simultaneously by means of remote terminals or personal computers.

Supercomputers

Supercomputer is a broad term for one of the fastest computers currently available. Supercomputers are very expensive. Supercomputers are ultra fast computers designed to process hugh amounts of scientific data then display the underlying patterns that have been discovered.

Servers

Servers are not designed to be used directly. They make programs and data available for people hooked up to a computer network, a collection of computers connected together so that they can exchange data.

Input Devices

An input device lets you communicate with a computer. You can use input devices to enter information and issue commands. A keyboard, mouse, scanner, digital camera, touch pads and joystick are examples of input devices. 

Computer Input Devices:
Keyboard

• Used to type data into the computer
• This is most common input device today 
  
  Pointing Devices
  
• Common pointing devices
• Mouse - most common pointing device
• Track ball Joystick
  
  Scanner
  A scanner allows you to scan documents, pictures, or graphics and view them on the computer. You can also use software to edit the items you scan.
  
• Used to put printed pictures and text into a computer
• Converts an image into dots that the computer can understand
• To scan text, optical character recognition (OCR) software is needed
  
  Digital Camera
  
• Used to take electronic pictures of an object
• The pictures taken by a digital camera can be used directly by a computer
  
  Microphone
  
• Used to put sound into a computer
• Need sound recording software
  
  CD-ROM/DVD-ROM
  
• Can be used to put both sound and images into a computer
• Use a laser to read a Compact Disk (CD) or a DVD disk
  
  Video Capture Card
  
• Usually place inside the computer's case
• Need a video source, either a video camera or video recorder
  
  Out Devices
  output device displays information on a screen, creates printed copies or generates sound. A monitor, printer, and speakers are examples of output devices.
  
  Common Output Devices:
  
  Monitors
  
• Shows the processed information on a screen
      A monitor uses a Picture Tube like a television with the image displayed on the front of the tube, 
     which is called the screen.
      Displays are flat and use plasma, LCD, active-matrix, or some other technology.
• Monitors used to be called Cathode Ray Tubes (CRTs) because of the picture tube,
      which is a large vacuum tube.
• A monitor or display produce a soft copy. When the device is turned off the information goes away.
  
  Printers
  
• Printers produce a hard copy
• The information is printed on paper and can be used when the device is off. It is also called a printout.
  
  Dot-matrix printers
  
• Uses metal pins to strike an inked ribbon to make dots on a piece of paper.
• Can see the dots that make up the letters or images.
• Lowest print quality of all of the printers.
• Very low in cost per page to use.
• Rarely used today because of the poor print quality, but still used in business to print multi-part forms.
  
  Ink jet printers
  
• Use drops of magnetic ink to produce dots on a page to produce text or images.
• The print quality is almost the same as a laser printer's.
• Problems with the ink
      The ink is very expensive
      The ink is water soluble and will run if the paper gets wet.
• Very low in cost per page to use.
• Highest cost per page of all the printers.
• For producing color documents, it has the highest quality at a reasonable price.
  
  Laser printers
  
• How the laser printer produces an image
      A laser or LEDs make dots on a light sensitive drum
      Toner (very tiny particles of plastic) stick to the drum where the dots where made
      Paper is pressed against the drum and the toner is placed on the paper
      The paper is heated and the toner melts into the paper
• Produces the highest quality printout.
• For black and white printouts, very low cost per page.
• Printout is permanent.
• Color laser printers are still fairly expensive.
  
  Storage Devices
  
  Computer stores information
• The computer stores information as a string of zeros (0) and ones (1)
      The standard string length is eight 0's or 1's in a row
      This standard length is called a byte
      A byte equals one character 
           - A character is a letter, number, or symbol - it is about any thing that can be typed 
             on a keyboard
           - There are 256 standard characters used by almost all computers
  
• Information size measurements
  
  
• Kilobyte (KB)
      -One kilobyte equals about 1024 bytes
  
  
• Megabyte (MB)
      -One megabyte equals about 1000 KB
      -One megabyte equals about 1,000,000 bytes
  
  
• Gigabyte (GB)
     -One gigabyte equals about 1000 MB
     -One gigabyte equals about 1,000,000 KB
     -One gigabyte equals about 1,000,000,000 bytes
  
  Common Storage Devices
  
  Hard Drive
      The hard drive is the primary device that a computer uses to store information.
      The hard drive stores programs, data files, saves files, and organizes files.
      The hard drive is located inside the computer case.
      The hard drive, magnetically stores data on stacks of rotating disks called platters.
  
  Floppy Drive
     The floppy drive stores and retrieves information on a floppy disk.
  
  CD -ROM Drive
     CD-ROM is a device that reads information stored on a compact disc.
     CD-ROM stands for Compact Disc Read Only Memory.
     One CD is equal to the space in over 40 floppy disc.