Computer Dot Matrix Printer
Dot matrix printing
From Wikipedia, the free encyclopedia
Epson VP-500 Printer with its cover removed
Typical output from a dot matrix printer operating in
draft mode. This entire image represents an area of printer output approximately 4.5 cm × 1.5 cm (1.8 in × 0.59 in) in size
This is an example of a wide-carriage printer, designed for paper 14
inches wide, shown with legal paper loaded (8.5" x 14"). Wide carriage
printers were often used by businesses, to print accounting records on
11" x 14"
tractor-feed paper. They were also called
132-column printers, though this description was only true for a specific font size and type that was built into the printer's electronics
Dot matrix printing or
impact matrix printing is a type of
computer printing
which uses 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 the print mechanism on a
typewriter. However, 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.
Design
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 (pawls). Facing the ribbon and
the paper is a small guide plate pierced with holes to serve as guides
for the pins. This plate may be made of hard plastic or an artificial
jewel such as
sapphire or
ruby.
[1]
The portion of the printer containing the pins is called the print
head. When running the printer, it generally prints one line of text at a
time. There are two approaches to achieve this:
The common
serial dot matrix printers use a horizontally
moving print head. The print head can be thought of featuring a single
vertical column of seven or more pins approximately the height of a
character box. In reality, the pins are arranged in up to four
vertically or/and horizontally slightly displaced columns in order to
increase the dot density and print speed through interleaving without
causing the pins to jam. Thereby, up to 48 pins can be used to form the
characters of a line while the print head moves horizontally.
In a considerably different configuration, so called
line dot matrix printers
use a fixed print head almost as wide as the paper path utilizing a
horizontal line of thousands of pins for printing. Sometimes two
horizontally slightly displaced rows are used to improve the effective
dot density through interleaving. While still line-oriented, these
printers for the professional heavy-duty market effectively print a
whole line at once while the paper moves forward below the print head.
The printing speed of
serial dot matrix printers with moving heads varies from 50 to 550
cps. In contrast to this,
line matrix printers are capable of printing much more than 1000 cps, resulting in a throughput of up to 800 pages/hour.
Because the printing involves mechanical pressure, both of these types of printers can create
carbon copies and
carbonless copies.
These machines can be highly durable. When they do wear out, it is
generally due to ink invading the guide plate of the print head, causing
grit to adhere to it; this grit slowly causes the channels in the guide
plate to wear from circles into ovals or slots, providing less and less
accurate guidance to the printing wires. Eventually, even with
tungsten blocks and
titanium pawls, the printing becomes too unclear to read.
Although nearly all
inkjet,
thermal, and
laser printers
also print closely spaced dots rather than continuous lines or
characters, it is not customary to call them dot matrix printers.
Early history
Upper:
Inmac ink
ribbon
cartridge with black ink for Dot matrix printer. Lower: Inked and
folded, the ribbon is pulled into the cartridge by the roller mechanism
to the left
The
LA30 was a 30 character/second dot matrix printer introduced in 1970 by
Digital Equipment Corporation of
Maynard, Massachusetts. It printed 80 columns of uppercase-only 5x7
dot matrix characters across a unique-sized paper. The printhead was driven by a
stepper motor and the paper was advanced by a somewhat-unreliable and definitely noisy
solenoid
ratchet drive. The LA30 was available with both a parallel interface
and a serial interface; however, the serial LA30 required the use of
fill characters during the carriage-return operation.
The LA30 was followed in 1974 by the
LA36, which achieved far
greater commercial success, becoming for a time the standard dot matrix
computer terminal. The LA36 used the same print head as the LA30 but
could print on forms of any width up to 132 columns of mixed-case output
on standard
green bar fanfold paper. The carriage was moved by a much-more-capable
servo drive using a
DC electric motor and an optical encoder /
tachometer.
The paper was moved by a stepper motor. The LA36 was only available
with a serial interface but unlike the earlier LA30, no fill characters
were required. This was possible because, while the printer never
communicated at faster than 30 characters per second, the mechanism was
actually capable of printing at 60 characters per second. During the
carriage return period, characters were
buffered for subsequent printing at full speed during a
catch-up
period. The two-tone buzz produced by 60 character-per-second catch-up
printing followed by 30 character-per-second ordinary printing was a
distinctive feature of the LA36.
Digital then broadened the basic LA36 line onto a wide variety of dot matrix printers including:
- LA180: 180 c/s line printer
- LS120: 120 c/s terminal
- LA120: 180 c/s advanced terminal
- LA34: Cost-reduced terminal
- LA38: An LA34 with more features
- LA12: A portable terminal
In 1970,
Centronics (then of
Hudson,
New Hampshire)
introduced a dot matrix printer, the Centronics 101. The search for a
reliable printer mechanism led it to develop a relationship with
Brother Industries, Ltd of
Japan,
and the sale of Centronics-badged Brother printer mechanisms equipped
with a Centronics print head and Centronics electronics. Unlike Digital,
Centronics concentrated on the low-end
line printer marketplace with their distinctive units. In the process, they designed the
parallel electrical interface that was to become standard on most printers until it began to be replaced by the
Universal Serial Bus (
USB) in the late 1990s.
Printer head positioning
Mechanism with two wheels with rubber band and metal bar
The printer head is attached to a metal bar that ensures correct
alignment, but horizontal positioning is controlled by a rubber band
that attaches to
sprockets
on two wheels at each side which is then driven with an electric motor.
Actual position can be found out either by dead count using a
stepper motor,
rotary encoder
attached to one wheel or a transparent plastic band with markings that
is read by an optical sensor on the printer head (common on
inkjets).
Uses
Personal computers
An Epson MX-80, a classic model that remained in use for many years
In the 1970s and 1980s, dot matrix impact printers were generally
considered the best combination of expense and versatility, and until
the 1990s they were by far the most common form of printer used with
personal and
home computers.
The
Epson MX-80, introduced in 1979,
[2] was the groundbreaking model that sparked the initial popularity of impact printers in the personal computer market.
[citation needed]
The MX-80 combined affordability with good-quality text output (for its
time). Early impact printers (including the MX) were notoriously loud
during operation, a result of the hammer-like mechanism in the print
head. The MX-80 even inspired the name of a
noise rock band.
[3]
The MX-80's low dot density (60dpi horizontal, 72dpi vertical) produced
printouts of a distinctive "computerized" quality. When compared to the
crisp typewriter quality of a daisy-wheel printer, the dot-matrix
printer's legibility appeared especially bad. In office applications,
output quality was a serious issue, as the dot-matrix text's readability
would rapidly degrade with each
photocopy generation.
IBM sold the MX-80 as IBM 5125.
Initially, third-party software (such as the
Bradford
printer enhancement program) offered a quick fix to the quality issue.
The software utilized a variety of software techniques to increase print
quality; general strategies were doublestrike (print each line twice),
and double-density mode (slow the print head to allow denser and more
precise dot placement). Such add-on software was inconvenient to use,
because it required the user to remember to run the enhancement program
before
each printer session (to activate the enhancement mode). Furthermore,
not all enhancement software was compatible with all programs.
Early personal computer software focused on the processing of text,
but as graphics displays became ubiquitous throughout the personal
computer world, users wanted to print both text and images. Ironically,
whereas the daisy-wheel printer and pen-plotter struggled to reproduce
bitmap images, the first dot-matrix impact printers (including the
MX-80) lacked the ability to print graphics. Yet the dot-matrix print
head was well-suited to this task, and the capability, referred to as
"dot-addressable" quickly became a standard feature on all dot-matrix
printers intended for the personal and home computer markets. In 1981,
Epson offered a retrofit
EPROM
kit called GrafTrax to add the capability to many early MX series
printers. Banners and signs produced with software that used this
ability, such as
Broderbund's
Print Shop, became ubiquitous in offices and schools throughout the 1980s.
Progressive hardware improvements to impact printers boosted the
carriage speed, added more (typeface) font options, increased the dot
density (from 60dpi up to 240dpi), and added pseudo-color printing.
Faster carriage speeds meant faster (and sometimes louder) printing.
Additional typefaces allowed the user to vary the text appearance of
printouts. Proportional-spaced fonts allowed the printer to imitate the
non-uniform character widths of a typesetter. Increased dot density
allowed for more detailed, darker printouts. The impact pins of the
printhead were constrained to a minimum size (for structural
durability), and dot densities above 100dpi merely caused adjacent dots
to overlap. While the pin diameter placed a lower limit on the smallest
reproducible graphic detail, manufacturers were able to use higher dot
density to great effect in improving text quality.
Several dot-matrix impact printers (such as the Epson FX series)
offered 'user-downloadable fonts'. This gave the user the flexibility to
print with different typefaces. PC software uploaded a user-defined
fontset into the printer's memory, replacing the built-in typeface with
the user's selection. Any subsequent text printout would use the
downloaded font, until the printer was powered off or soft-reset.
Several third-party programs were developed to allow easier management
of this capability. With a supported word-processor program (such as
WordPerfect 5.1), the user could embed up to 2
NLQ custom typefaces in addition to the printer's built-in (ROM) typefaces. (The later rise of
WYSIWYG software philosophy rendered downloaded fonts obsolete.)
Single-strike and
Multi-strike ribbons were an attempt
to address issues in the ribbon's ink quality. Standard printer ribbons
used the same principles as typewriter ribbons. The printer would be at
its darkest with a newly installed ribbon cartridge, but would gradually
grow fainter with each successive printout. The variation in darkness
over the ribbon cartridge's lifetime prompted the introduction of
alternative ribbon formulations.
Single-strike ribbons used a
carbon-like substance in typewriter ribbons transfer. As the ribbon was
only usable for a single loop (rated in terms of 'character count'), the
blackness was of consistent, outstanding darkness.
Multi-strike ribbons gave an increase in ribbon life, at the expense of quality.
The high quality of single-strike ribbons had two side effects:
- At least 50% and up to 99.9% of the given ribbon surface would be
wasted per character, since an entire fresh new region of ribbon was
needed to print even the smallest font shapes. Ribbon advance was fixed
to always span the largest character shape, so a row of periods would
consume as much fresh ribbon as a row of W's, with a large span of
unused carbon between each dot.
- Single-strike ribbons created a risk of espionage and loss of
privacy, because the used ribbon reel could be unwound to reveal
everything that had been printed. Secure disposal was required by
shredding, melting, or burning of used ribbon cartridges to prevent
recovery of information from garbage bins.
Pseudo-color
Several manufacturers implemented color dot-matrix impact printing
through a multi-color ribbon. Color was achieved through a multi-pass
composite printing process. During each pass, the print head struck a
different section of the ribbon (one primary color.) For a 4-color
ribbon, each printed line of output required a total of 4 passes. In
some color printers, such as the
Apple ImageWriter II,
the printer moved the ribbon relative to the fixed print head assembly.
In other models, the print head was tilted against a stationary ribbon.
Due to their poor color quality and increased operating expense,
color impact models never replaced their monochrome counterparts.
[citation needed]
As the color ribbon was used in the printer, the black ink section
would gradually contaminate the other 3 colors, changing the consistency
of printouts over the life of the ribbon. Hence, the color dot-matrix
was suitable for abstract illustrations and piecharts, but not for
photo-realistic
reproduction. Dot-matrix thermal-transfer printers offered more
consistent color quality, but consumed printer film, still more
expensive. Color printing in the home would only become ubiquitous much
later, with the ink-jet printer.The speed is usually 30-550 cps
Near Letter Quality (NLQ)
Text quality was a recurring issue with dot-matrix printers.
Near Letter Quality mode—informally specified as almost good enough to be used in a business letter
[4]—endowed
dot-matrix printers with a simulated typewriter-like quality. By using
multiple passes of the carriage, and higher dot density, the printer
could increase the effective resolution. For example, the Epson FX-86
could achieve a theoretical
addressable dot-grid of 240 by 216
dots/inch using a print head with a vertical dot density of only 72
dots/inch, by making multiple passes of the print head for each line.
For 240 by 144 dots/inch, the print head would make one pass, printing
240 by 72 dots/inch, then the printer would advance the paper by half of
the vertical dot pitch (1/144 inch), then the print head would make a
second pass. For 240 by 216 dots/inch, the print head would make three
passes with smaller paper movement (1/3 vertical dot pitch, or
1/216 inch) between the passes. To cut hardware costs, some
manufacturers merely used a
double strike (doubly printing each
line) to increase the printed text's boldness, resulting in bolder but
still jagged text. In all cases, NLQ mode incurred a severe speed
penalty. Not surprisingly, all printers retained one or more 'draft'
modes for high-speed printing.
NLQ became a standard feature on all dot-matrix printers. While NLQ
was well received in the IBM PC market, the Apple Macintosh market did
not use NLQ mode at all, as it did not rely on the printer's own fonts.
Mac word-processing applications used fonts stored in the computer. For
non-
PostScript
(raster) printers, the final raster image was produced by the computer
and sent to the printer, which meant dot-matrix printers on the Mac
platform exclusively used raster ("graphics") printing mode. For
near-letter-quality output, the Mac would simply double the resolution
used by the printer, to 144 dpi, and use a screen
font twice the point size desired. Since the Mac's screen resolution (72 dpi) was exactly half of the
ImageWriter's maximum, this worked perfectly, creating text at exactly the desired size.
Due to the extremely precise alignment required for dot alignment
between NLQ passes, typically the paper needed to be held somewhat taut
in the tractor feed sprockets, and the continuous paper stack must
perfectly aligned behind or below the printer. Loosely held paper or
skewed supply paper could cause misalignments between passes, rendering
the NLQ text illegible.
24-pin printers
By the mid-1980s, manufacturers had increased the pincount of the
impact printhead from 7, 8, 9 or 12 pins to 18, 24, 27 or 48, with 24
pins being most common. The increased pin-count permitted superior
print-quality which was necessary for success in Asian markets to print
legible
CJK characters.
[5] In the
PC market, nearly all 9-pin printers printed at a defacto-standard vertical pitch of 9/72 inch (per printhead pass, i.e. 8
lpi).
Epson's 24-pin LQ-series rose to become the new de facto standard, at
24/180 inch (per pass - 7.5 lpi). Not only could a 24-pin printer lay
down a denser dot-pattern in a single-pass, it could simultaneously
cover a larger area.
Compared to the older 9-pin models, a new 24-pin impact printer not
only produced better-looking NLQ text, it printed the page more quickly
(largely due to the 24-pin's ability to print NLQ with a single pass).
24-pin printers repeated this feat in bitmap graphics mode, producing
higher-quality graphics in reduced time. While the text-quality of a
24-pin was still visibly inferior to a true
letter-quality printer—the daisy wheel or laser-printer, the typical 24-pin impact printer outpaced most daisy-wheel models.
As manufacturing costs declined, 24-pin printers gradually replaced
9-pin printers. 24-pin printers reached a dot-density of 360x360 dpi, a
marketing figure aimed at potential buyers of competing ink-jet and
laser-printers. 24-pin NLQ fonts generally used a dot-density of
360x180, the highest allowable with single-pass printing. Multipass NLQ
was abandoned, as most manufacturers felt the marginal quality
improvement did not justify the tradeoff in speed. Most 24-pin printers
offered 2 or more NLQ typefaces, but the rise of WYSIWYG software and
GUI environments such as
Microsoft Windows ended the usefulness of NLQ.
Contemporary use
The desktop impact printer was gradually replaced by the
inkjet printer. When
Hewlett-Packard's patents expired on steam-propelled photolithographically produced ink-jet heads,
[when?]
the inkjet mechanism became available to the printer industry. For
applications that did not require impact (e.g., carbon-copy printing),
the inkjet was superior in nearly all respects: comparatively quiet
operation, faster print speed, and output quality almost as good as a
laser printer. By the mid-1990s, inkjet technology had surpassed
dot-matrix in the mainstream market.
As of 2005, dot matrix impact technology remains in use in devices such as
cash registers,
ATMs,
fire alarm systems, and many other point-of-sales terminals.
Thermal printing is gradually supplanting them in these applications. Full-size dot-matrix impact printers are still used to print
multi-part stationery, for example at bank tellers and auto repair shops, and other applications where use of
tractor feed paper is desirable such as
data logging and
aviation.
Some are even fitted with USB interfaces as standard to aid connection
to modern computers without legacy ports. Dot matrix printers are also
more tolerant of the hot and dirty operating conditions found in many
industrial settings. The simplicity and durability of the design, as
well as its similarity to older
typewriter technology, allows users who are not "
computer literate" to easily perform routine tasks such as changing ribbons and correcting paper jams.
Some companies, such as Printek, DASCOM, WeP Peripherals, Epson,
Okidata, Olivetti, Lexmark, and TallyGenicom still produce serial and
line printers. Today, a new dot matrix printer actually costs more than
most inkjet printers and some entry level laser printers. However, not
much should be read into this price difference as the printing costs for
inkjet and laser printers are a great deal higher than for dot matrix
printers, and the inkjet/laser printer manufacturers effectively use
their
monopoly
over arbitrarily priced printer cartridges to subsidize the initial
cost of the printer itself. Dot matrix ribbons are a commodity and are
not monopolized by the printer manufacturers themselves.
Advantages and disadvantages
Dot matrix printers, like any impact printer, can print on multi-part
stationery
or make carbon-copies. Impact printers have one of the lowest printing
costs per page. As the ink is running out, the printout gradually fades
rather than suddenly stopping partway through a job. They are able to
use continuous paper rather than requiring individual sheets, making
them useful for data logging. They are good, reliable workhorses ideal
for use in situations where printed content is more important than
quality. The ink ribbon also does not easily dry out, including both the
ribbon stored in the casing as well as the portion that is stretched in
front of the print head; this unique property allows the dot-matrix
printer to be used in environments where printer duty can be rare, for
instance, as with a Fire Alarm Control Panel's output.
Impact printers create noise when the pins or typeface strike the ribbon to the paper.
[6]
Sound-damping enclosures may have to be used in quiet environments.
They can only print lower-resolution graphics, with limited color
performance, limited quality, and lower speeds compared to non-impact
printers. While they support fanfold paper with tractor holes well,
single-sheet paper may have to be wound in and aligned by hand, which is
relatively time-consuming, or a sheet feeder may be utilized which can
have a lower paper feed reliability. When printing labels on
release paper,
they are prone to paper jams when a print wire snags the leading edge
of the label while printing at its very edge. For text-only labels
(e.g., mailing labels), a
daisy wheel printer or
band printer
may offer better print quality and a lesser chance of damaging the
paper. The advantages are: low purchase cost, can handle multipart
forms, cheap to operate, just new ribbons, rugged and low repair cost
and the paper is completely connected. This allows to print long banners
that span across several sheets of paper.
The disadvantages are: noisy, low resolution. You can see the dots
making up each character, not all can do color, color looks faded and
streaky, slow and dot matrix printers are more prone to jamming, and
their jams are more difficult to clear. This is because paper is fed in
using two wheels and holes set into the paper. A small tear on the side
of a sheet can create a jam and the bits of paper between the wheels
make it tedious to fix.
Dot
Matrix Printer Manufacturers include Epson, Okidata
and Brother
Laser
Printer Manufacturers include HP, Lexmark, Xerox
Centronics 
USB_B
