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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
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.
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.
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.
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.
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.
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.
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.
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.
