Display devices are peripheral equipment that contain a television like viewing screen. Most display devices fall into one of two categories: monitors and display terminals. A monitor is an output device that consists of only the viewing screen.
A display terminal is typically an input/output “communications workstation” that includes the screen (for output) and an attached keyboard (for input).
In practice, one commonly finds monitors plugged into and sitting on top of the system units of microcomputers. The keyboard is usually a separate input device that also connects to the system unit.
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Although the monitor is strictly an output device, the user can also see keyboard input on the display because the computer routes it to the monitor as output. Display terminals, on the other hand, are generally hooked up to either mainframes, minicomputers, or supercomputers in communications networks.
After all, communication is the primary function of these display devices. The keyboard unit is cabled directly to the display unit, which in turn is hooked up to the computer.
Display devices are handy when the user requires only small amounts of output and has to see what is being sent as input to the computer system.
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A student writing a program for a class, an airline clerk making inquiries to a flight information database, a stockbroker analyzing a security, and a bank teller checking the status of a customer account would each employ a display device.
However, the display is useful only up to a point. If, for example, the student writing the program wanted to take a copy of it home he or she would have to direct the output to a printer.
Many features differentiate the hundreds of display devices currently on the market. A discussion of some of the more noteworthy features follows
Text versus Graphics :
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Many of the display devices sold today are capable of providing both text and graphics output. Text output consists of only letters, digits, and special characters. Two examples are program listings and letters to friends Graphics output includes complex “picture images,” such as maps and drawings.
Resolution :
A key characteristic of any display device is resolution, or sharpness of the screen image. On many displays, images are formed by lighting up tiny dots on the screen. On such devices, resolution is measured by the number of these dots, or pixels (a contraction of the phrase picture elements).
The more pixels on the screen, the higher the resolution (i.e., the clearer the picture). A display resolution of, say, 640 by 480 means that the screen consists of 640 columns by 480 rows of dots- that is, 640 x 480 = 307,200 pixels.
Text characters are formed on the screen in a dot-matrix configuration. Generally, several specific matrix sizes- say, 5 by 7 (= 35 pixels) or 7 by 12 (= 84 pixels)- are available for users to display text.
For instance, a user may be able to display 25 rows of 65-character lines for one application and 50 rows of 132-character lines for another.
The more pixels used to form characters and the more pixels packed per square inch of screen, the higher the text resolution.