A serial or character printer, whose name
resulted from the fact that they print one character at a time, was the display
mechanism first used with terminals. The first serial printers were ‘fully
formed’ impact printers in which different types of mechanisms, including a
daisy wheel, type-ball or rotating cylinder which formed characters from a
single piece of type, were used to strike a ribbon to produce a printed image.
The editing capability provided by the
terminal was minimal, typically permitting the operator to delete a previously
entered character or the current line, since the terminal transmitted and
received data on a line by line basis. A second type of impact printer which
grew in popularity during the 1970s and 1980s to where it has virtually
replaced fully formed printers is the dot matrix printer.
The dot matrix printer employs a matrix of
pins in its print head. The first dot matrix printers used a rectangular matrix
of dots, typically 7 dots high by 5 dots wide or 9 dots high by 7 dots wide.
The pins in the matrix are selectively ‘fired’ to form each character. Printing
of characters results from the movement of the print head containing a column
of 7 or 9 pins across the paper, with the printer selectively firing the pins
at 5 or 7 successive intervals to form each character.
Until the mid-1980s, the matrix of pins
used to form characters resulted in a considerable amount of white space
between dots. This space made the dot matrix pattern easily discernible to the
eye and limited the use of printed output produced by this type of printer to
draft correspondence.
By the mid 1980s, advances in print head
technology resulted in the inclusion of more pins on some print heads. The
additional pins were used to considerably reduce the space between pin impacts
in forming a character.
Other dot matrix developments included
two-pass printing in which the first pass of printing a line was followed by
the printer feeding the paper upward by a slight amount, perhaps 1/256th of an
inch, prior to the line being printed a second time.
One result of placing more pins on the
print head and using a two-pass printing technique was. a higher quality print.
Since this print resembled the letter quality print of a full impact printer,
it became known as near letter quality (NLQ).
The firing of additional pins to form a
better print image required additional time, resulting in NLQ printing being
slower than conventional dot matrix printing. Thus, most modern dot matrix
printers have two or more user selectable print modes – draft and NLQ – with
the draft print mode providing
a considerably faster print rate than the
NLQ print mode.
A second major category of printers employs
non-impact technology to form characters. Non-impact printers include thermal
matrix ink jet and laser devices. The thermal matrix printer forms characters
by applying a voltage to pins in a matrix, causing the pins to be heated. The
heated pins interact with heat-sensitive paper used in these printers,
resulting in the formation of characters.
The ink jet printer has a nozzle consisting
of a matrix of holes out of which ink is squirted to form characters. Thus,
both thermal matrix and ink jet printers are based upon dot matrix technology.
In comparison, the laser printer uses a rotating drum and a small amount of
current to generate a magnetic field, which results in toner from a cartridge
adhering to distinct locations on paper passing around the drum. Laser printers
have resolutions between 300 and 1200 dots per inch (dpi) and through software
can form characters in almost any shape.
The key limitation associated with the use
of printers for both input and output is the elementary editing capability
provided by this type of terminal device. Data entered from the keyboard are
either printed and transmitted as each character is pressed or stored in a
buffer area.
The buffer storage area contained in most
ASR and KSR terminals is only capable of holding one line of data or 72 to 80
characters depending upon the type of terminal. By using the backspace key to
eliminate a previously entered character, an operator can perform elementary
editing.
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