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Terminals
With all the forms of input
considered so far, the data is first prepared using a separate device. However,
it is possible to communicate with the computer directly by using a keyboard
machine. A teletypewriter terminal, sometimes called a teleprinter terminal or a
keyboard/printer terminal, usually combines a keyboard for manual input of
information with a printer for outputting a 'hard copy' (printer record) of the
input, system information and program results. The printing device outputs one
character at a time at rates between 20 and 50 characters per second on
continuous rolls of paper (typically 8 to 20 inch wide) or on fanfold paper,
according to the application.
Some terminals display information
on a screen as opposed to printing on paper. These are known as visual display
units. Some terminals also have a facility for punching and reading paper tape.
This enables information to be copied and retained in coded form so that it can
be used again later without the information having to be 're-typed'.
One of the features of a terminal
is that it can be situated some distance from the computer. It must, therefore,
include some means of transmitting information. Some terminals are designed only
to send information and some only to receive, but in general both functions are
carried out.
Terminals may be connected to a
computer in one of two ways. Some are connected locally, by direct cable line.
This is known as 'hard-wiring' and does not usually extend more than several
hundred feet from the computer itself. The second method is via a remote link,
either by telegraph or telephone line, or by microwaves. Whilst it is probable
that the 'remote' terminal would be some distance from the computer, it is
possible that it may be on the same site as the computer, even in the same room.
The terms 'local' and 'remote' refer to the way in which a terminal is linked to
the computer, with each terminal making use of the computer in turn. This is
known as time-sharing.
Terminals extended the use of the
computer to various places of work. They are widely used for such tasks as stock
control, entering others, updating accounts-of the computer to various places of
work. They are widely used for such tasks as stock control, entering orders,
updating accounts and seat reservations. The terminals found in sho9ps and
stores might combine several of these tasks while also acting as cash registers.
They can be sited at various points on a factory floor to record and receive
information on different stages of an industrial process. The development of
compact portable terminals as extended their usefulness still further.
Terminals now provide an extremely
important and effective communication link with the computer. Their versatility,
combined with the fact that they can be situated away from the computer, has led
to a rapid growth in their use, and for many applications they are not the
standard input device.
With the aid of micro technology
many modern terminals are provided with circuitry which enables them, without
being linked to a computer to perform some of the simple tasks normally carried
out by the computer, example the editing of data or text. Terminals which can
perform some computing are often referred to as intelligent terminals. The
degree of local intelligence given to terminals is growing rapidly. Refer figure
4.
Lines of bars can be arranged in a
code as illustrated in the following figure.
Bar codes are used particularly by
the retail trade for labeling goods and by supermarkets for labeling shelves and
in stock control. They are also used for numbering books in public libraries so
that when a book is borrowed or retuned it can be recorded using a computer.
Bar code reading is performed by a
scanner or light pen which is generally linked to a computer via a terminal
device. They scanner or light pen is stroked across the pattern of bars, a
sequence of bits is generated and information recorded.
Another important innovation is
the development of hand-held data entry terminals. A typical hand-held device
looks similar to a calculator and may be battery powered. Information is usually
entered via the keys. In addition a scanner or light pen may be incorporated in
the design to enable to capture of bar-coded data. The hand-held terminal may be
provided with up to 32 K or memory for temporary storage of information before
transmission to a computer. Typically the transmission is over ordinary
telephone lines. Hand-held device enables the collection of data at the place
where it is generated and avoids the delay and cost of data preparation at the
computer site. The hand-held terminal could be useful to a salesman in the field
for direct ordering or for such diverse applications as meter reading, road
traffic service, market research and control of medical supplies in hospitals.
A recent development is the
introduction of hand-print terminals, variously called data tablets or pads. The
device generates a representation of character direct to the computer as the
character is hand-printed on paper lying on top of the pad. Different types of
pad employ different sensing techniques and a special pen may or may not be
required. One method is based on the generation of pulses when to electrically
resistive layers beneath the surface of a pad are brought together by the
pressure of the pen as the characters are printed. A typical data pad
incorporates touch sensitive keyboard for entering non-written characters. It
may also be able to digitize, and hence input, drawings as well as characters.
Voice input system accept spoken
input. The wage form created by the spoken input is analyzed; patterns are
extracted and matched against pre-stored patterns to identify the input. Once
identified, the appropriate coding is generated, for handling within the
computer. Before a voice input system can be used it must first be provided with
a vocabulary. The words and phrases the system is to recognize are spoken with
the system operating in a so-called 'training mode'. In this mode the patterns
are created and stored for future matching. A system may be trained to recognize
both the voice of one or more operators and a given vocabulary for each
operator, in which case unwanted inputs or unauthorized speakers are rejected.
The voice linked to a system may
be a microphone, telephone or radio communication. Voice input systems are not
yet widely used but they are a reality. Vocabularies are comparatively small,
typically one hundred to three hundred words. In general as the size of a
vocabulary increases recognition reliability decreases. Possible applications or
situations in which an operator needs to be free to move about a work are or to
have his hands free or where an operator travels and relies on telephone contact
with his work base.
