Output Devices


Monitor is an output device that resembles the television screen and uses a Cathode Ray Tube (CRT) to display information. The monitor is associated with a keyboard for manual input of characters and displays the information as it is keyed in. It also displays the program or application output. Like the television, monitors are also available in different sizes.

Liquid Crystal Display (LCD)

LCD was introduced in the 1970s and is now applied to display terminals also. Its advantages like low energy consumption, smaller and lighter have paved its way for usage in portable computers (laptops). Liquid Display Unit / Liquid Crystal Display (LCD) – we usually find this installed in laptops and handheld computers. The technology used is different from that of the CRT. LCD uses tiny capsules filled with liquid crystals. When the molecules of the liquid are subjected to an electric field, the liquid crystals align and light reflects off them.

Without the field, their alignment reverts to its original, nonreflecting arrangement, so the elements appear dark. Combinations of LCD’s are arranged to form patterns of reflected light that spell out numbers, symbols or letters.

TFT (Thin Film Transistor) is a variant of LCD which uses thin film transistor technology to improve image quality.



 Impact printers leave an image on the paper by physically striking an inked ribbon against the surface of the paper

– Used for multipart forms

– Relatively slow and noisy


Type Mode of Printing Speed


Dot – Matrix printer Prints the character in dotted pattern through printer ribbon using either 24 pin or 9 pin


200/300 to 700


Ink Jet printer Work by spraying ionized ink Slow, 90 CPS
Laser printer Also called page printer. Uses laser beam to produce an image. 6 to 12 PPM


Line printer Prints lines at a time instead of single characters. 300 to 600 LPM
Plotter Produces drawings or graphs through

pens which are filled with different



( CPS: Characters Per Second; PPM: Pages Per Minutes; LPM : Lines Per Minute)

Impact Printers

An impact printer is a type of printer that operates by striking a metal or plastic head against an ink ribbon. The ink ribbon is pressed against the paper, marking the page with the appropriate character, dot, line, or symbol. Common examples of impact printers include dot matrix, daisy-wheel printers, and ball printers.

Dot matrix printers work by striking a grid of pins against a ribbon. Different characters are printed by using different pin combinations. Daisy-wheel printers use a circular wheel with “petals” that each have a different character or symbol on the end. In order to print each character, the wheel spins to the appropriate petal and a hammer strikes the petal against the ribbon and the page. Similarly, ball printers use a spherical ball with raised characters on the outside. The ball spins to each character before printing it on the page.


Non-Impact Printers

Most printers are now non-impact printers. These printers, such as laser and inkjet printers are much quicker than impact printers and can print more detailed images.


  Dot-matrix printers

DotMatrix Printer

– More flexibility than daisy-wheel printers

– Use an array of pins known as printwires to strike an inked printer ribbon and produce images on paper.

– The case that holds the print wires is called the printhead

– Use either 9-pin(draft quality) or 24-pin (letter or near-letter quality)

Dot-matrix printers are two important characteristics :

Speed : Given in characters per second (cps), the speed can vary from about 50 to over 500 cps. Most dot-matrix printers offer differen speeds depending on the quality of print desired.

Print Quality : Determined by the number of pins (the mechanisms that print the dots), it can vary from 9 to 24. The best dot-matrix printers (24 pins) can produce near letter-quality type, although you can still see a difference if you look closely.


Advantages :

 can print on multi-part stationery or make carbon copies.

Impact printers have one of the lowest printing costs per page.

They are able to use continuous paper rather than requiring individual sheets.

The ink ribbon also does not easily dry out.

Disadvantages :

 Impact printers are usually noisy.

They can only print low resolution graphics, with limited colour performance, limited quality and comparatively low speed.

They are prone to bent pins (and therefore a destroyed printhead) caused by printing a character half-on and half-off the label.


Inkjet printers

InkJet Printer

Inkjet printers are simple devices that consist of the printhead mechanism, support electronics, a transfer mechanism, and a paper feed component

– Work by ejecting ink through tiny tubes

Ink is heated by tiny resistors or electroconductive plates

at the end of each tube

The resistors or plates boil the ink which creates a tiny air

bubble that ejects a droplet of ink on the paper

Ink inside the jets tend to dry out when not used

Most color printers are ink-jet andproduce a high-quality image


Advantages of inkjet printers:

1) Low cost

2) High quality of output, capable of printing fine and smooth details

3) Capable of printing in vivid color, good for printing pictures

4) Easy to use

5) Reasonably fast

6) Quieter than dot matrix printer

7) No warm up time


Disadvantages of inkjet printers:

1) Print head is less durable, prone to clogging and damage

2) Expensive replacement ink cartridges

3) Not good for high volume printing

4) Printing speed is not as fast as laser printers

5) Ink bleeding, ink carried sideways causing blurred effects on some papers

6) Aqueous ink is sensitive to water, even a small drop of water can cause blurring

7) Cannot use highlighter marker on inkjet printouts


Laser Printer

A laser printer is a printer that uses a focused beam or light to transfer text and images onto paper. Though contrary to popular belief, the laser does not actually burn the images onto the paper. Instead, as paper passes through the printer, the laser beam fires at the surface of a cylindrical drum called a photoreceptor. This drum has an electrical charge (typically positive), that is reversed in areas where the laser beam hits it. By reversing the charge in certain areas of the drum, the laser beam can print patterns (such as text and pictures) onto the photoreceptor.

Once the pattern has been created on the drum, it is coated with toner from a toner cartridge. The toner is black in most cartridges, but may be cyan, magenta, and yellow in color laser printers. The positively charged toner clings to areas of the drum that have been negatively charged by the laser. When the paper passes through the printer, the drum is given a strong negative charge, which allows the toner to transfer and stick to the paper. The result is a clean copy of the image written on the paper.


Advantages of laser printers:

1) High resolution

2) High print speed

3) No smearing

4) Low cost per page (compared to inkjet printers)

5) Printout is not sensitive to water

6) Good for high volume printing


  Disadvantages of laser printers:

1) More expensive than inkjet printers

2) Except for high end machines, laser printers are less capable of printing vivid colors and high quality images such as photos.

3) The cost of toner replacement and drum replacement is high

4) Bulkier than inkjet printers

5) Warm up time needed


Tharmal Printer

thermal printer (or direct thermal printer) produces a printed image by selectively heating coated thermochromic paper, or thermal paper as it is commonly known, when the paper passes over the thermal print head. The coating turns black in the areas where it is heated, producing an image. Two-color direct thermal printers can print both black and an additional color (often red) by applying heat at two different temperatures.

Thermal transfer printing is a related method that uses a heat-sensitive ribbon instead of heat-sensitive paper.

A thermal printer comprises these key components:

Thermal head — generates heat; prints on paper

Platen — a rubber roller that feeds paper

Spring — applies pressure to the thermal head, causing it to contact the thermo-sensitive paper

Controller boards — for controlling the mechanism

In order to print, thermo-sensitive paper is inserted between the thermal head and the platen. The printer sends an electrical current to the heating elements of the thermal head, which generate heat. The heat activates the thermo-sensitive coloring layer of the thermo-sensitive paper, which changes color where heated. Such a printing mechanism is known as a thermal system or direct system. The heating elements are usually arranged as a matrix of small closely-spaced dots—thermal printers are actually dot-matrix printers, though they are not so called.



A graphics printer that draws images with ink pens. It actually draws point-to-point lines directly from vector graphics files. The plotter was the first computer output device that could print graphics as well as accommodate full-size engineering and architectural drawings. Using different colored pens, it was also able to print in color long before inkjet printers became an alternative.

Pen plotters are still the most affordable printing device for CAD use and offer resolution unlike any other printer. The lines are not made up of dots. They are actually drawn, providing infinite resolution.


Drum and Flatbed Plotters

Both types of plotters actually “draw” the images. The drum plotter (left) wraps the paper around a drum with pin feeds. It moves the paper back and forth for one direction of the plot. The pens move across the paper, creating the other axis. The bed of the flatbed unit (right) determines the maximum size of the total drawing.



Monitor or Visual Display Unit (VDU)

The monitor is the most common type of output device and is also called Visual Display Unit (VDU).

1. Monochrome – This type of display uses only one type of colour or a greyscale

2. Colour – This type of display uses more than one type of colour and is generally called RGB monitors. The RGB stand for Red, Green and Blue and a combination of these colours are used to display the image on the monitor. Colour monitors can display range from 16 to 16.7 million colours.


This term refers to the clarity or sharpness of a printout or display screen. The more pixels there are per square inch, the better and greater the resolution.

Visual Display Units

One can find different standards for monitors. They support different colour depths (number of colours it supports). The most common standards for monitors are:

1. VGA (Video Graphics Array) mode appeared in 1987. It offered a resolution of 720×400 in text mode and a resolution of 640 by 480 (640×480) in 16-colour graphics mode. It also offered a resolution of 1024 x 768.

The VGA quickly became the baseline display mode for PCs.

2. SVGA (Super Video Graphics Array) is a graphics mode which can display 256 colours at resolutions of 640×200, 640×350 and 640×480. SVGA can also display higher definitions such as 800×600 or 1024×768 by using fewer colors.

3. XGA (eXtended Graphics Array). is an IBM display standard introduced in 1990. Today, it is the most common appellation of the 1024 × 768 pixels display resolution, but the official definition is broader than that. It was not a new and improved replacement for Super VGA, but rather became one particular subset of the broad range of capabilities covered under the “Super VGA” umbrella.

The initial version of XGA expanded upon IBM’s VGA, adding support for two resolutions:

• 800 × 600 pixels with high color (16 bits per pixel, i.e. 65,536 colors).

• 1024 × 768 pixels with a palette of 256 colors (8 bits per pixel)

The Different Types of Monitors

The different types of monitors are:

Cathode Ray Tube (CRT) – uses the same technology as used in television sets. The CRT uses a vacuum tube in which an electron gun is installed. When electrons fired by the electron gun touch the phosphor layer situated at the internal side of the screen, this will glow. This dot of light is called a Pixel.

Liquid Display Unit / Liquid Crystal Display (LCD) – we usually find this installed in laptops and handheld computers. The technology used is different from that of the CRT. LCD uses tiny capsules filled with liquid crystals. When the molecules of the liquid are subjected to an electric field, the liquid crystals align and light reflects off them. Without the field, their alignment reverts to its original, nonreflecting arrangement, so the elements appear dark. Combinations of LCD’s are arranged to form patterns of reflected light that spell out numbers, symbols or letters.

TFT (Thin Film Transistor) is a variant of LCD which uses thin film transistor technology to improve image quality.

Summary of Video Glossary

CRT Cathode ray tube (monitor)

VDU Video display unit (monitor)

LCD Liquid crystal display – as available on hand held games and potables Pixel The smallest area on a screen, seen closely as a tiny dot 14” 15” 17” 21” This is the length measured in inches (1 inch = 2.54cm) of the screen measured diagonally, that across two corners. VGA Video graphics array, this provided,

1. Introduced analogue monitors to the computer system; that is colour was no longer restricted to fixed colours but allowed different hues and colour blending.

2. Graphics resolution of 1024 x 768 pixels

3. Allows 256 colours SVGA Super Video Graphics Array  As above but allows 16.7 million colour hues Bit map Computer memory that represents a video image

Side NOTE: A terminal is an integration of a VDU with a keyboard. There are two types of terminals, namely INTELLIGENT TERMINLAS (having a CPU) AND DUMB TERMINALS (no CPU)

- Intelligent terminals can do some local processing of data and then communicate with a main computer just to relay the results

- In a dumb terminal, all processing must be done through the main computing system


















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