Output Devices

Output devices allow the computer system to provide the use with the results of processing the data. The particular application may require this to be in permanent form or the information may be temporarily displayed on a screen. In either case the characteristics of the output device used must match the requirements of the application. The following factors will need to be considered when selecting an output device.

• Whether or not permanent copy is required.
• Medium to be used.
• Initial cost of hardware.
• Running costs - consumables and maintenance.
• Speed at which output is produced.
• Quality or resolution of output.
• Working environment - size of and noise produced by the hardware.

VDU Displays

These are the most common output device. The display can be monochrome or colour and it is capable of presenting text and graphics. Running costs are low and output is silent.

The display is composed of a number of closely spaced dots. Dots that are more closely packed, being smaller, produce a higher quality display. Typically a VDU would have 1024 x 768 dots or pixels - this is the Super VGA standard.

A normal VDU has a picture created by firing an electron beam at a screen. When the beam hits a point on the screen a phosphorescent dot glows for a short time. The beam scans the whole screen so quickly that the eye does not notice the continual glowing and dimming of the dots. The beam is controlled by electromagnets. A fairly large space is needed behind the screen to provide a sufficiently long path to create and control the electron beam. This means that it is unlikely that this type of VDU will get much smaller. The power requirements of a VDU are also fairly large compared to those of the processor. This, together with the size of the display, makes them unsuitable for portable or battery operated systems.

A Fuller account of how these work is in the GCSE section

Liquid crystal displays are used for laptop computers. They have the advantages of low power consumption compared to cathode ray (electron beam) devices; they are lighter and have a flat profile. However they are more expensive to produce, particularly if a larger size screen is required and the display works more slowly than the traditional cathode ray screen

Some applications, such as CAD, require higher than normal resolution. In this situation a Graphics Workstation would be used.

LED's and Buzzers

Light emitting diodes are often used to display the status of a system or in control applications. One example is the use of LED's on a keyboard to indicate the status of the Number Lock and Caps lock keys. LED's require little power, do not produce light by heating and so do not get hot, are cheap to buy and have a long life.

Buzzers are useful to alert the user to some event - such as an error condition. They are used in POS terminals to indicate that a bar code has been correctly read.

Line Printers

Mainframe and mini computer systems often produce large amounts of output. An electricity board may, for example, need to print approaching two miles of bills each day. This requires something many times faster than the type of printer that would be attached to a PC.

Applications such as utility billing or payroll for a large firm use line printers. These are large, noisy and expensive. They can print from 600 to 1000 lines per minute but the quality if the output is poor and graphics are not possible. These large-scale printers are suitable for filling in account, invoice or payroll information on pre-printed stationery. They are only justified where large amounts of output are required. Line printers are impact printers - the characters are made by physical contact with ribbon and paper. This means that they can be used with multi-part stationery to produce second and third copies of the document. Multi-part stationery consists of several sheets. The second and other sheets are coated with a chemical that turns dark under pressure so that they take up an image of the characters formed on the top copy. One use of this would be to produce an invoice and delivery note in one print run, using two-part stationery.

Line printers use continuous stationery, which has sprocket holes in the side to allow it to be fed through the printer at high speed. The individual pages must be separated (burst) after printing.

Increasingly this type of output is being produced by large-scale laser printers. These are physically large printers which are capable of printing about 20 000 lines per minute. They are much quieter than a line printer but, because they are a non-impact printer, they cannot handle multi-part stationery. However a wider range of characters and graphics is available. Purchase and running costs are greater than for a line printer but the quality is greatly improved. Large-scale laser printers use continuous stationery.

Dot Matrix Printer

Characters are formed by pins, fired at a ribbon, producing dots on the paper. Using more and smaller pins produces higher resolution output. Two standards have emerged 9-pin and 24-pin. Dot matrix printers typically print up to 200 characters per second in draft mode. They usually have a slower higher resolution (NLQ - Near Letter Quality) mode. Graphics are possible as is colour with a special four colour ribbon.

The main advantages of dot matrix printers are that they are cheap to buy and run. They are an impact printer so they can be used with multi-part stationery. One application of dot matrix printers is in POS terminals where they are used to print the itemised receipt for a customer. Multi-part stationery is used and the second copy is retained at the till to provide an audit trail.

Ink-Jet Printer

Ink Jet printers work by firing small bubbles of ink from a series of jets. The dots formed when the ink hits the paper build up the image. Ink Jet printers are faster than a dot matrix printer, and the quality of the image formed is much better although it can depend on the paper used. If the paper is not absorbent then the ink will smear. Ink jet printers have a resolution of 300 dots per inch although in practice if the ink smears, the final result is not this good. The inks are water based and will run if the paper gets wet.

Good quality colour is available with many monochrome printers having a colour upgrade option. Colour printing is more economical when there is a separate black cartridge, otherwise black has to be produced by mixing all three colours. Since colour cartridges are more expensive than black this adds to the cost.

A monochrome ink-jet printer does not cost much more than a dot matrix to buy, but the running costs are much greater. The ink cartridges are expensive and it may be necessary to buy special high quality absorbent paper to obtain the best results.

Ink jet printers are non-impact and are therefore quiet in operation. They are an option when high quality output is required, but the total amount of output does not justify the cost of a laser printer. Most ink jet printers use cut sheet stationery some can be adapted to take continuous stationery. Like the dot matrix printer, they produce text one character at a time.

Laser Printers

Laser printers set up the image of the complete page before printing. This means that there is an initial delay while the page is processed but once printing starts the page is completed within a second or so. For this reason we call laser printers page printers.

The image is formed by small particles of toner that are distributed electrostatically on the paper. The laser is used to build up a pattern of charge on a roller that then transfers the toner to the paper. The toner is then heated so that it melts onto the paper forming a permanent image.

The resolution of a typical laser printer is 600 dots per inch. This refers to the number of dots per inch used when the laser beam marks out the pattern of electric charge that is used to distribute the toner. If the toner particle size is too large then the effective resolution is reduced but a laser printer still produces the highest quality output of all the printers described. Special fine tone toner is used to obtain the best results in laser printers.

The initial cost of buying a laser printer is two to three times that of an ink jet. The running costs are however much less. In addition laser printers can print overhead transparencies and they have a higher print speed compared to an ink jet.

Laser printers are non-impact and all of them use cut sheet paper. Because of the way that they produce the image it would be very expensive to produce a laser printer handling large size paper. Most will take A4 paper and it is possible to buy an A3 laser printer. Colour is also an expensive option.

Laser printers are used when high quality output is required and there is sufficient output to justify the initial outlay or where speed of printing is important. The quality is sufficiently good for CAD applications but these often require output to sheets larger than the A4 that the normal laser printer can handle.

Graph Plotters

Graph plotters use pens to draw on the paper. Two types are available:

Flatbed plotters have an arm that moves the pen over the flat sheet of paper in both the x and y directions to form the image.

Drum Plotters have the paper draped over a drum. The drum rotates to provide y-direction movement while the pen is moved back and forth in the x-direction. This arrangement takes up less floor space than the flat bed plotter. The plotter can automatically select pens from a rack allowing a variety of colours and nib sizes to be used.

The quality of output is very high. Initial purchase price is high and running costs are moderate. The plotter produces output very slowly however. Its main purpose is as an output device in CAD applications, producing architectural and engineering drawings. These are commonly larger than the A4 sheet size that a normal laser printer can accommodate.

Computer Output on Microform (COM)

Photographic film is used as the output medium in this method of output. The output device exposes the image onto the film which is then developed to provide a permanent record. Two forms of film are use. Microfilm which looks like 35 mm camera film and Microfiche which is about 6 inches by 4 inches. A single microfiche sheet will contain the equivalent of 200 A4 pages.

The images are too small to be read unaided so a special reading device is used to magnify the image and project it onto a screen.

The principal advantage of COM is that it compresses a large amount of data into a small space. Catalogues and reference material are often output to microfilm or microfiche although increasingly this type of information is output to CD-ROM.

© LEV