Data Output Devices

Visual Display Unit

The most popular input/output device is the Visual Display Unit (VDU). It is also called the monitor. A Keyboard is used to input data and a Monitor is used to display the input data and to receive messages from the computer. A monitor has its own box which is separated from the main computer system and is connected to the computer by cable. In some systems, it is compact with the system unit. It can be color or monochrome.
Currently, there are two types of visual display units (VDU) available. These help in displaying what we have entered into the computer, as well as the output that is derived from the computer after a process.

• CRT – Cathode Ray Tube, which comes in monochrome, soft white, and color forms. These are normally used with Desktop computers.
• LCD – Liquid Crystal Display, also in soft white and color. These are sleek and mostly used in portable computers, viz. palm-tops, and laptops. On the same lines, there are LCD overhead projectors, which when interfaced with the computer the output could be projected on to a screen in a larger form for convenient viewing in meetings, demonstrations, and presentations.

Terminals

It is a very popular interactive input-output unit. It can be divided into two types: hard copy terminals and soft copy terminals. A hard copy terminal provides a printout on paper whereas soft copy terminals provide a visual copy on the monitor. A terminal when connected to a CPU sends instructions directly to the computer. Terminals are also classified as dumb terminals or intelligent terminals depending upon the work situation.

Printer

It is an important output device that can be used to get a printed copy of the processed text or result on paper. There are different types of printers that are designed for different types of applications. Depending on their speed and approach of printing, printers are classified as impact and non-impact printers. Impact printers use the familiar typewriter
approach of hammering a typeface against the paper and inked ribbon. Dot-matrix printers are of this type. Non-impact printers do not hit or impact a ribbon to print. They use electrostatic chemicals and ink-jet technologies. Laser printers and Ink-jet printers are of this type. This type of printers can produce colour printing and elaborate graphics. A computer printer is a computer peripheral device that produces a hard copy (permanent human-readable text and/or graphics, usually on paper) from data stored in a computer connected to it. The world’s first computer printer was a 19th-century mechanically driven apparatus invented by Charles Babbage for his Difference Engine.

• Non-impact printers: In these printers, the print head does not physically touch the paper; but makes impression on the paper by different techniques viz. heat, light and liquid. All these are versatile, high-speed letter-quality printers that work almost silently. Monochrome, colour, and photo printers – A monochrome printer can only produce an image consisting of one colour, usually black. A monochrome printer may also be able to produce gradations of the tone of that colour, such as a scale. A colour printer can produce images of multiple colours.
• Toner-based printers – Laser printers refer to the method used to adhere toner to the media. The advent of cost-effective, precision lasers has made them the dominant toner-based monochrome printer type for home and office applications. Another toner-based printer is the LED printer which uses an array of LEDs instead of a laser to cause toner adhesion.
• Inkjet printers – Inkjet printers spray very small, precise amounts of ink onto the media. For colour applications including photo printing, inkjet methods are dominant. Inkjet printers or bubble-jet printers are one of the most commonly used printers. They consist of nozzles that produce very small ink bubbles that turn into tiny droplets of ink. The dots formed are the size of tiny pixels. Ink-jet printers can print high-quality text and graphics. They are also almost silent in operation. They are cheaper than laser printers but are expensive to run as their cartridges need to be frequently replaced.
• Impact printers – These printers physically touch the paper and make the impression on it, hence these are called impact printers. These are very noisy printers. Impact printers rely on a forcible impact to transfer ink to the media, similar to typewriters, that are typically limited to reproducing text. A daisy wheel printer is a specific type of impact printer where the type is moulded around the edge of a wheel. A golf ball typewriter is similar to the daisy wheel type but has the characters distributed over the face of the globe shape.
• Dot-matrix printers – In the general sense many printers rely on a matrix of pixels, or dots, that together form the larger image. However, the term dot matrix printer is specifically used for impact printers that use a matrix of small pins to create precise dots. The advantage of dot-matrix over other impact printers is that they can produce graphical images in addition to text; however, the text is generally of poorer quality than impact printers that use letterforms. Dot-matrix printers can either be character-based or line-based (that is, a single horizontal
  series of pixels across the page), referring to the configuration of the print head. At one time Dot matrix printers were one of the more common types of printers used for general use – such as for home and small office use. Such printers would have either 9 or 24 pins on the print head. 24 pin print heads were able to print at a higher quality.
• Line printers – Line printers, as the name implies, print an entire line of text at a time. Two principle designs existed. In drum printers, a drum carries the entire character set of the printer repeated in each column that is to be printed. In chain printers (also known as train printers), the character set is arranged multiple times around a chain that travels horizontally past the print line. In either case, to print a line, precisely timed hammers to strike against the back of the paper at the exact moment that the correct character to be printed is passing in
  front of the paper. The paper presses forward against a ribbon which then presses against the character form and the impression of the character form is printed onto the paper. These printers were the fastest of all impact printers and were used for bulk printing in large computer centres. They were virtually never used with personal computers and have now been partly replaced by high-speed laser printers.
• Daisy wheel / Golf ball printer: A small Daisy-like structure/golf ball-like structure the fonts forming characters is embossed. The wheel/ball rotates at a speed and a hammer hits the wheel or ball, based on the instruction from the computer when a particular character is on the side of the paper, which in turn hits a ribbon which makes the character print on the paper. The print out is called a letter-quality printout, since complete characters are printed,
  unlike dots in a dot matrix printer. These cannot print graphs etc.