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FREE online courses on the Basics of a Computer - HARDWARE AND SOFTWARE - Output

 

We have seen that there are several paths by which man can communicate with the machine. There are, also, several ways in which the machine can communicate with man.

 

Printed output

 

Line Printer

 

The most common method of obtaining output is via a device known as a line printer. Rows of character sets (fonts) are either wrapped around a drum or affixed to a chain. The drum or chain revolves across the path of a series of hammers each of which corresponds to a print position. As the character to be printed is selected, a magnetically controlled hammer presses it onto an ink ribbon and there onto paper, rather like a typewriter works.

 

A variety of stationery is used, with special designs to suit particular applications, such as pre-printed electricity bills and rate demands. Forms can also be multi-part (where copies are required for different department), with several sheets impregnated with carbon or separated by interleaved carbon paper. In general, though the paper used on drum and chain line printers is known as 'continuous stationery'. The paper can be plain or lined, is usually fan-folded and perforated for separation into convenient lengths (11 inch typically and about 15 inch wide) and comes folded in boxes, usually in ream multiples.

 

There are normally 132 or 136 print positions per line though some devices are able to print more. Character sets vary in content and size. The 64-set has more special characters than the 48-set and the 96 set prints in lower case letters as well as in upper case. It is even possible to print in Hebrew and in Japanese. The impact method of printing employed by line printers has limitations with respect to quality but it allows for very high speeds, ranging from 300 to 2500 lines per minute, and is associated particularly with large computer output requirements.

 

Serial Printer

 

Another category of printer is the serial printer which outputs one character at a time as opposed to one line at a time eg. A teletypewriter terminal. Some devices are print only machines with no keyboard for input purposes. A serial printing device can normally be operated using continuous stationery or separate sheets typically A4 size. A serial or character printer is much slower than a line printer. It is also much cheaper.

 

A special type is the daisywheel printer, so called because it uses a daisy-shaped disk made of metal or plastic which holds some 96 characters in its petals. Print heads are interchangeable, enabling the use of different character fonts. There are normally 132 or 136 print positions per line and typical printing speeds are 25 to 60 characters per second. Daisywheel printers are noted for their print quality and are often used with word processing systems and other applications where quality of print is desired.

 

The dot matrix printer is another common type of serial printer. The print head comprises a matrix of tiny needles, typically seven rows with nine needles in each, which hammers out characters in the form of patterns of tiny dots. The shape of each character, i.e. the dot pattern, is obtained from information held electronically in the printer. Matrix printers are faster than daisywheel printers in the range of 45 to 220 cps, but the quality of print inferior.

 

In addition there are devices which employ non-impact techniques. Thermal printers, for example, use heat to create characters in dot matrix form on special sensitized paper.

 

Serial printers in general are suitable for applications producing low volume output and are frequently used as output devices for small, special purpose computer systems and microcomputers.

 

Laser Printer

 

The most exciting new development is the laser printer. Using a combination of electronics, laser and copier technology, it is possible to design printers capable of converting computer information into print, page by page. Laser printers are very fast, produce very high quality print and can call upon a wide selection of character fonts. However there are few applications which can justify the present high cost of laser printing. Most laser printers operate at speeds between 30 and 250 pages per minute.

 

Graphical Output

 

Information can be output in graphical form using graph plotters. These are usually slow but the accuracy (up to within one thousandth of an inch) is more important than speed. Since there is a considerable mismatch between the speeds at which the CPU operates and the speed of the plotting device, output is often transferred to magnetic tape or paper tape first, and then plotted from the information on the tape. Computer systems dedicated to design work may send output direct to a plotter.

 

The most common type of plotter is flat bed device. As the name suggests, it plots on paper (or some other material) which rest on a flat bed. The pen moves in perpendicular direction across the bed. One directional movement is supplied by a gantry which straddles the bed and runs on rails at either side. The rails allow movement up and down the length of the bed. The other direction is supplied by a pen turret running to and for across the gantry itself. The turret may contain different colored pens (felt tip, ball point or ink) for multi-contain different colored plotting. The plot size is restricted by the area of the bed. Some may be as small as A2 size whilst some very large beds used in aircraft design, for such things as wing profiles, can be up to 20 ft by 50 ft. Plotters are used to trace out conventional graphs and to assist with design, e.g. in textiles. Some plotters can etch plastic or metal plates.

 

Visual Display Units - VDU

 

A visual display device uses a cathode ray tube (CRT) to display information. It looks like a television screen and is similar in other respects. VDUs are used particularly in situations where information is required quickly and where perhaps there is little advantage in having a permanent record of the information.

 

The VDU is really a type of terminal, with a keyboard for manual input of characters to the computer and with a screen for character display of the input or output. The screen displays information as it is keyed in enabling a visual check before the input is transferred to the computer. Information is displayed very much more quickly than by the convenient keyboard/printer terminal (teletypewriter) and it is almost silent in its operation. One disadvantage is that the device provides no hardcopy of the output, but it is normally possible to add a printing device which can be switched on  to provide a 'hard' copy of the display when it is needed. However, in the type of applications for which VDUs are particularly suited, where the display provides information on which action is taken immediately, there is often no requirement for printed output.

 

The most common display method is to generate characters from a 'dot matrix'. A selected pattern of dots is illuminated to form a character. Screens vary in size (12 in. and 15 in. are common) and in the number of characters which they can display. A maximum display is typically 24 lines of 80 characters, with sometimes an optional display of 132 characters per line.

 

Initially used in situations where information is required quickly, for example in airline seat reservations where speed is the essence in handling customer enquirer, VDUs  are now widely used for general data entry and retrieval of stored information. The VDU is fast becoming as common a piece of office equipment as the typewriter. Many of today's models are intelligent terminals, incorporating microprocessors, and able to carry out some computing functions.

 

There are not some VDUs equipped with touch sensitive screens which allow data to be input by touching the screen with the fingertip. The screen surface consists of a number of 'touch points' as defined by the program in use. When touched the terminal sends the co-ordinates of the point to the computer.

 

Graphics VDU

 

This type of VDU is able to display graphics and diagrams as well as alphanumeric characters. It is a specialized piece of equipment normally very much more expensive than the conventional VDU, and is used particularly as an aid to design. Via suitable software it can enable a design to be viewed from different angles and can display intricate detail at varying levels of magnification. The design can then be modified as necessary. Designs may be enhanced by different shades of grey and some device display in color. Graphics devices are used as an aid in car and boat design, in constructional and civil engineering applications and by architects and interior designers. Graphics often make considerable programming demands on the system and these sophisticated types of display are usually linked to large, powerful computer systems, or to smaller machines which are used solely for this one purpose.

 

A copying device can be used in conjunction with a graphics VDU to provide hard copy of any display. A link up with a Computer Output Microfilm Device is particularly significant. It enables the recording of a whole series of graphs or designs which can then be viewed later at leisure to assist in the selection of an optimum design.

 

Computer Output Microfilm - COM

 

A computer output microfilm device translates information normally held on magnetic tape into miniature images on microfilm. The device displays the information as characters on a CRT screen and then using photographic methods records the display onto film, usually 16 or 35 mm. Drawings or pictures can usually be displayed as well as narrative text. A full display (perhaps equivalent to a page of line printer output) is recorded as a single frame.

 

A special reader or reader/printer can be used subsequently to view  the processed film. The reader operates on a 'back projection' principle displaying a frame at a time on a translucent screen, typically about A4 size. The printer can then produce a hard copy of what is presented on the screen, probably using an electrostatic method.

 

Microfilm, in roll form or microfiche, is small and easily stored and the speed of recording is some 25 - 30 times faster than the average line printer. The equivalent of thousands of pages of computer output can be stored in a small drawer and the cost of the microfilm for a page of output is less than a sheet of line printer paper. Once the film has been processed it can easily be duplicated and full size hard copy prints made quickly and inexpensively.

 

A COM system is ideal for use in applications where there is a large amount of information to be retained which is required only for manuals, industrial catalogues and archives. Companies may need to retain records of such things as bills and invoices for a number of years before destroying them. COM provides an easy way of retaining the information, of retrieving it in a matter of seconds using a compact desk top viewer and is ideal when multiple copies of reports or information are required. The information may be distributed in microfiche form.

 

Complete COM systems are relatively expensive to install and are associated more with big computer users. Small and medium-sized computer users who need microfilm are more likely to take their files for conversation into microfilm to a bureau offering a COM processing service. Desk top microfiche readers, ideal for use on workshop benches as well as in offices, are comparatively inexpensive. There are also portable readers small enough to fit into briefcases which run off mains or batteries.

 

Attention has now turned to providing a mechanism to input directly from microfilm. Some KIM (Computer Input Microfilm) equipment is already on the market but it is not yet widely used.

 

Audio Response Unit

 

The computer can be used effectively to trigger verbal communication via an audio response unit and this may be an appropriate method to use if standard replies to requests for information are all that are required.

 

Messages are composed and transmitted in coded form, may be over telephone lines, using perhaps a keyboard for input or even a voice-input system. The unit analysis the input assembles the response from pre-recorded words and phrases and delivers the verbal reply. For the sake of clarity the response is delivered more slowly than words are normally spoken. The digitized format in which the spoken word is retained by the system makes heavy demands on storage and typical systems can only store around 400 spoken words or phrases. However, within the context of a single application, the limitation of a small vocabulary may not be unduly restrictive.

 

Speech synthesis is getting cheaper and voice output is starting to make sense in a variety of applications. Transient information, that is information which only needs to be conveyed once, may be a good candidate for voice output. A good test of whether information is transient is whether it can usefully be conveyed over the telephone. Potential application areas include remote enquiry of stock quotations and product availability, and instructional sequences for various procedures. An audio response unit, sometimes called a voice output terminal may be attached to a large computer system or be supported by a minicomputer dedicated to the one purpose. Very small devices in the form of micro chips or boards can be added to some micros systems to provide limited voice output, for example with spelling instruction and learning games.

 

 

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