Output Devices
Output Devices: Information for the User
Output devices provide the results after processing, in a suitable form. In many cases this will be in the form of a hard copy (printout), or on screen (soft copy). With the widespread use of electronic mail (e-mail), output could be in the form of an electronic message to another computer.
Display Screens (Monitors)
Today the computer monitor is the most common form of output, which is also referred to as the VDU or Visual Display Unit that is the most prolific form of output.
Pixel is the name given to a picture element and refers to the smallest area of the screen. The entire screen is made of a lot of pixels. The clarity or clearness of a computer screen is measured in terms of resolution. Screens are normally classed as low resolution, medium resolution or high resolution. High-resolution monitors use more pixels than low resolution.
The quality of a computer monitor is based on the following properties:
• The resolution
• The number of possible colors
• The size (normally the monitors come in sizes of 15" and 17")
• Energy consumption and radiation
Different types of display screens:
Standard television set - An ordinary home television can be used for computer output.
Standard computer monitors - These have better resolution than TV monitors. Larger monitors with high resolution are used for specialized applications such as Desktop Publishing and CAD.
Liquid Crystal Displays - These are screens made up of two glass plates with liquid in between.
Printers
There are many types of different printers used for computer output. We will consider 3 types in this section.
• Dot-matrix printers
• Ink-jet printers
• Laser printers
Dot-Matrix Printers
Dot-matrix printers are impact printers, which can transfer print through layers of paper. This means that they are able to print multi-part stationery.
Example:
If you want to print a multi-part sheet where, the white top sheet goes to the customer, the yellow sheet goes to accounts and the blue sheet to the store sheet, then you will need to use a dot-matrix printer. (similar to the blue, yellow and green layers of bills found in shops when they write the bill).
A dot-matrix printer is very cheap and has the lowest running costs of any printer. It has a matrix of small pins in its print head. As the head moves across the paper the correct pins are fired out to hit an inked ribbon and the shape of the letter required is formed. The greater the number of pins, the higher the quality of the print.
Ink-Jet Printers
An ink-jet printer is a non-impact character printer. They are silent in operation, have good quality printing and have become a very popular printer for PCs. The print head of an ink-jet printer consists of nozzles (usually 64). The ink flows through the appropriate nozzle, where it is heated and a bubble forms. This expands and breaks, releasing a very small ink droplet. These dots are much smaller, and there are more of them, than in a dot-matrix printer. These printers produce printouts that are almost comparable to that produced by laser printers and therefore can print high quality text and graphics.
Laser Printers
These are non-impact page printers. A laser beam is used to form an image on a rotating charged metal drum. Laser printers have toner cartridges, which contain a fine powder called toner. The charged image then picks up the toner particles, which are transferred to the paper, which is also charged. Once the image has been transferred, heat and pressure are used to stick it to the paper permanently.
Since they are page printers they are very fast. The speed of a laser printer is typically about 8 ppm (pages per minute). Color laser printers are now available. Laser printers offer both high speed and excellent print quality of text and graphics. Although they are expensive, they have become widespread in many offices where quality printouts are needed because of the many advantages it has.
Graph Plotters
A plotter is a device commonly used in producing printing plans, maps, line diagrams and three-dimensional drawings, which are particularly line diagrams or graphical output on paper.
Graph plotters use pens to produce images and different pens containing different colored inks may be used. Plotters are generally classified as pen plotters or as pen less plotters. Pen less plotters use various kinds of different technologies. At the moment high quality work for publication is done on electrostatic plotters.
Voice Synthesis
Voice synthesis is used when you get the output as sound. This method is usually very useful for handicapped people.
Example:
Visually handicapped people, find the spoken word from a computer invaluable. For instance, each letter as it is typed can be heard on headphones when special software with a word processor is used.
Projector
These are special devices, which can be connected to the computer and used as a substitute to the monitor. These are used when the output has to be shown to a large audience by projecting the computer output on a white screen or wall.
Input Devices
Input Devices: The user connection
Most of the data comes in hand written forms that cannot be directly input into the computer. These documents need special data preparation devices that translate the source document into a medium that the computer can read, such as a magnetic disk. Input devices are used to input or capture data into a computer system. The ideal input device would be able to get data into a system as accurately as possible in the least amount of time.
There are wide ranges of input devices used today:
Keyboard
The keyboard of all input devices is the oldest and most familiar. Keyboards are devices that contain their own chips. Basically each key in the keyboard acts as a switch, which is switched on when the key is pressed.
The microprocessor scans the keyboard hundreds of times a second to see if a key has been pressed; if it has, a code that denotes which key has been pressed is sent to the processing unit. The CPU translates this code into an ASCII code (the code that computers use to represent characters on the computer keyboard), which is then used by a computer program.
Mouse
A mouse is an input device that usually has one, two or three buttons, which are used to make selections on the screen. It is a pointing device. A mouse translates its movements on the desktop into digital information; this is fed to the computer, which in turn causes the cursor to move on the screen. The cursor is the blinking line on the screen, which indicates that the computer is ready to type. Underneath the mouse is a rotating ball and as the mouse is moved on the desk, and sensors pick up this movement to move the pointer on the screen. (The pointer helps the user point and click on an icon or menu on the screen).
Trackball
What do you get when you turn a computer mouse upside down? A trackball! The ‘mouse' stays still while the user rotates the ball. Trackballs are often seen on laptop computers because it has the advantage of taking up much less space compared with a conventional mouse. It is a pointing device.
Joystick
A joystick is similar to a trackball. Whenever the stick is moved, the pointer moves in a similar direction on the screen. Joysticks are commonly used for games, but you can also see them being used for scanning purposes in hospitals.
Scanner
Scanners are used to scan text or pictures into a computer's memory. Then the scanned image is manipulated in some way before being printed. There are cheap hand held scanners and or flatbed A4 sized ones. Both black and white and color scanners are available.
Touch Screen
A touch screen is a special kind of screen. This is sensitive to touch. A selection is made from usually a menu present on the screen, which is activated by touching part of it. These screens are ideal for use in banks, restaurants and bars, where customers who are not use to keyboards can obtain information about the services offered.
Light Pen
A light pen is a pen-shaped device held in the hand. The presence or absence of light is detected by it. The pen is used to select a particular point on the screen. The screen is refreshed about every 1/50th of a second by a point of light travelling rapidly across it. The pen detects this point of light and the computer can work out by precise timing where the pen is. Light pens need special software to make them work and are mainly used for design work using CAD packages.
Digital Camera
Digital cameras allow the user to transfer the photographs taken, directly into the computer. Previously, if a photograph had to be input into the computer, it had to first scan using a computer scanner and then viewed on the screen. Digital cameras eliminate the requirement for a scanner. No film or photographic paper is needed. The quality of photographs obtained even from low cost digital cameras is quite good. Digital cameras are very efficient to use especially if you want to take photos and edit them later on the computer.
Sensors
For industrial computer applications, such as process control, the computer is required to interact directly with the plant. Since the computer can only communicate with the outside world by using electrical signals, any devices connected to it must be made compatible using analog to digital converters (ADC) and digital to analog converters (DAC).
Sensors are designed to pick up analogue values of the physical variables and feed them into the computer.
Example:
• temperature
• pressure
• intensity of light
• electrical voltages
• electrical currents
• switch positions
Depending on the values received by the computer, it activates other output devices like motors, valves etc.
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