Printers
are the most commonly used output devices for producing hard copy output.
The
various types of printers in used today are
§ Dot-Matrix Printers
§ Inkjet Printers
§ Drum Printers
§ Laser Printers
Dot-Matrix
Printers
A
dot matrix printer or impact matrix printer is a type of computer printer with
a print head that runs back and forth, or in an up and down motion, on the page
and prints by impact, striking an ink-soaked cloth ribbon against the paper,
much like a typewriter. Unlike a typewriter or daisy wheel printer, letters are
drawn out of a dot matrix, and thus, varied fonts and arbitrary graphics can be
produced. Because the printing involves mechanical pressure, these printers can
create carbon copies and carbonless copies.
Each
dot is produced by a tiny metal rod, also called a "wire" or
"pin", which is driven forward by the power of a tiny electromagnet
or solenoid, either directly or through small levers. Facing the ribbon and the
paper is a small guide plate (often made of an artificial jewel such as
sapphire or ruby) pierced with holes to serve as guides for the pins. The
moving portion of the printer is called the print head, and when running the
printer as a generic text device generally prints one line of text at a time.
Most dot matrix printers have a single vertical line of dot-making equipment on
their print heads; others have a few interleaved rows in order to improve dot
density.
Inkjet
Printers
Inkjet
printers form characters and images by spraying small drops of ink on to the
paper. They are the most common type of computer printer for the general user
due to their low cost, high quality of output, capability of printing in
different colors, and ease of use.
If
you ever look at a piece of paper that has come out of an inkjet printer, you
know that:
§ The dots are extremely small, so small that they are tinier
than the diameter of a human hair (70 microns).
§ The dots are positioned very precisely, with resolutions of
up to 1440x720 dots per inch.
§ The dots can have different colors combined together to
create photo-quality images.
Lasre
Printers
A
type of printer that utilizes a laser beam to produce an image on a drum. The
light of the laser alters the electrical charge on the drum wherever it hits.
The drum is then rolled through a reservoir of toner, which is picked up by the
charged portions of the drum. Finally, the toner is transferred to the paper
through a combination of heat and pressure. This is also the way copy machines
work.
Because
an entire page is transmitted to a drum before the toner is applied, laser
printers are sometimes called page printers. There are two other types of page
printers that fall under the category of laser printers even though they do not
use lasers at all. One uses an array of LEDs to expose the drum, and the other
uses LCDs. Once the drum is charged, however, they both operate like a real
laser printer.
One
of the chief characteristics of laser printers is their resolution -- how many
dots per inch (dpi) they lay down. The available resolutions range from 300 dpi
at the low end to 1,200 dpi at the high end. By comparison, offset printing
usually prints at 1,200 or 2,400 dpi. Some laser printers achieve higher
resolutions with special techniques known generally as resolution enhancement.
Laser
printers produce very high-quality print and are capable of printing an almost
unlimited variety of fonts. Most laser printers come with a basic set of fonts,
called internal or resident fonts, but you can add additional fonts in one of
two ways:
Font
Cartridges : Laser printers have slots in which you can insert font cartridges,
ROM boards on which fonts have been recorded. The advantage of font cartridges
is that they use none of the printer's memory.
Soft
Fonts : All laser printers come with a certain amount of RAM memory, and you
can usually increase the amount of memory by adding memory boards in the
printer's expansion slots. You can then copy fonts from a disk to the printer's
RAM. This is called downloading fonts. A font that has been downloaded is often
referred to as a soft font, to distinguish it from the hard fonts available on
font cartridges. The more RAM a printer has, the more fonts that can be
downloaded at one time.
Laser
printers are controlled through page description languages (PDLs). There are two
de facto standards for PDLs:
PCL
: Hewlett-Packard (HP) was one of the pioneers of laser printers and has
developed a Printer Control Language (PCL) to control output. There are several
versions of PCL, so a printer may be compatible with one but not another. In
addition, many printers that claim compatibility cannot accept HP font
cartridges.
PostScript
: This is the de facto standard for Apple Macintosh printers and for all
desktop publishing systems.
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