Output Devices

To get processed information out of a computer, you need

an output device. ‘Output’ means to produce the results

of processed data

The most common types of output are:

• Soft copy: this is not permanent copy

• It includes output from a computer monitor, audio (sound) from speakers, electrical signals from modems and output from one computer to another.

The most common types of output are:

• Hard copy: this is also called permanent output since it is printed for you to review away from the computer. Examples are printed reports and pictures.

Display devices

• Computers display output on a screen or monitor.

• Different names have been used for this device, including VDT (video display terminal), CRT (cathode ray tube),

Display devices• VDU (visual display

unit), LCD (liquid crystal display) flat screens on laptops and even LED (light-emitting diode) screens on palmtop computers.

How do they work?

• A monitor contains a matrix or array of luminescent dots of red, green and blue (known as RGB).

• These can be blended to display millions of colours. Mapping the location and colour information of each bit of data creates a computer image.

• This is known as a bitmap (bmp).The bitmapped image seen on a monitor is made up of thousands of pixels. Pixel stands for picture (pix) element.

Features of a computer screen

• Size• This is the dimension of the screen which shows

the output. Common desktop screens are from 14 inches to 19 inches measured diagonally. Larger sizes are now available.

• Resolution• This determines how clear and detailed the

output on the screen can be. Pictures on a screen are made up of tiny dots (one dot = 1 pixel). The more pixels per inch, the clearer and more detailed the graphic.

• Colour• The number of colours displayed can vary from

16 to 256 to 64 thousand to 16.7 million. The more colours, the smoother the graphics appear, especially photos.

• Cursor/pointer• The cursor is a symbol that shows where you

are working on the screen. It may appear as ~ for text and k for the mouse pointer location.

• Scrolling• This allows the text or graphic to be moved or up

or down or brought into view on the screen.

Printing devices

• No matter how simple or complex the data-processing operation is, the final results must be made available in a user-friendly form,

• usually in the form of a permanent record. (hard copy)

Printers

• Printers are devices which output a hard copy of your work.

• The choice of printer depends on the speed, quality and quantity you want, and the cost.

• Two kinds of printer are impact and non-impact.

Impact Printers

• Impact printers strike through a carbon or inked ribbon, like a typewriter.

• They are noisy and do not usually print acceptable graphics.

• However, they are useful for printing multiple copies using carbon paper.

Train printer• the characters are located on a

continuous belt.

• The characters move horizontally across the page and as the required character moves into position, it is hammered onto the paper through a carbon ribbon.

• If the hammer hits a little early or late, the character will appear slightly to the right or left of its proper position.

Daisy wheel printer• The daisy wheel printer uses a wheel with

up to 100 characters.• Each character is on an individual arm,

and the wheel rotates to bring the required character into position.

• The hammer hits a single letter, guaranteeing accurate positioning and uniform striking.

• This allows fast, quiet and reliable operation with excellent quality

Dot-matrix printer• A dot—matrix printer uses pins to print a

pattern of dots on paper. The main advantages of using matrix printers is the relatively low cost. The main disadvantage is the

• relatively poor quality of printing, which can be read but cannot be used for reproduction or for business letters.

Line printers

• The characters in line printers are on a cylinder. Each segment of the cylinder has the full set of characters in raised form around its edge.

• One complete line of text is printed during one revolution of the cylinder.

• All the ‘A’s found in the text on that particular line are printed first, then all the ‘B’s, and so on until finally all the letters that make up the words have been printed and the line is complete.

• Whereas 200 to 600 lines per minute maximum are produced by the other printers, some line printers will operate at up to 3000 lines per minute.

Non-impact printers• These printers do not involve actually

striking the paper.• Instead, ink spray or toner powder is

used. • The characters are then fixed onto the

paper by heating, for example.• Because the printing element is simple

and has no moving parts, these printers are inexpensive to manufacture and print quickly and silently

Ink-jet printer

• An ink—jet printer operates by projecting small ink droplets and deflecting them electrostatically. Speeds of 4 to 6 pages per minute may be achieved.

• It is now possible to buy a low-priced, good quality black-and-white or colour ink-jet printer. The quality of printout can be nearly as good as that of a laser printer,

• Ink-jet printers are slower, their ink filled print heads have to be cleaned and their cartridges need to be replaced more frequently than the toner cartridges of laser printers.

• Colour ink jet printers are ideal for use at home, where small-quantity output is required for greeting cards, photographs and school work.

• Ordinary paper can be used but for the best results, particularly in colour, special glossy paper must be used. This can be quite costly

Laser printer• Laser printers produce attractive

documents at a high resolution and are much faster than ink jets. They are used in many workplaces because they are quiet, print quickly,

• can be stocked with a large number of sheets of paper, and produce very high-quality documents.

• The printout from most laser printers tends to be 300 to 1200 dots printed per inch (dpi).

• In the laser printer, the paper is charged electrostatically and attracts dry ink powder. The pattern is then baked on the paper. Many lines are printed simultaneously, and speeds of 8 to 12 pages per minute can be attained.

• Colour laser printers are also available. They are far more costly to buy than a colour ink-jet printer,

• but produce higher quality images and are cheaper to run. Some laser printers also perform the functions of a scanner, a photocopier and fax machine.

Thermal printers• Thermal printers use heat on chemically

treated paper to form characters.

• Fax machines and supermarket check out registers that use rolls of paper are also of this type.

• Although these printers are quiet, they are relatively slow, expensive since they require special paper, cannot use multiple-copy paper, and the print eventually fades.

Plotters• Car designers, architects and engineers who

wish to print accurate charts, diagrams and 3D drawings, output not to a laser printer but to a plotter.

• A plotter uses coloured pens or toner to draw an image on paper (Fig 1.10).The paper is handled in different ways depending on the type of plotter. Flatbed plotters hold the paper

• still while the pens move. Drum plotters roll the paper over a cylinder, while pinch—roller plotters are a mixture of the two. The advantage of a drum plotter is that it can produce very large drawings.