Dot matrix printer


Dot matrix printer
Epson VP-500 Printer (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 × 1.5cm (1.75 × 0.6 inches) 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.

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 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. 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 (pawls). Facing the ribbon and the paper is a small guide plate (often made of an artificial jewel such as sapphire or ruby[1]) 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.

These machines can be highly durable.[2] 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 print closely spaced dots rather than continuous lines or characters, it is not customary to call them dot matrix printers.

Contents

Early Dot Matrix Printers

Upper image: Inmac ink ribbon cartridge with black ink for Dot matrix printer
Lower image: Inked and folded lengthy ribbon squeeze in the cartridge, zoom in of inside part, pull ribbon in mechanism and ribbon.

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 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.

Dot matrix usage

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 was the groundbreaking model that sparked the initial popularity of impact printers in the personal computer market. 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.

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. 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.

24-pin printers

By the mid 1980s, manufacturers had increased the pincount of the impact printhead from 9 pins to 18, or 24. (At 27 pins, the Apple ImageWriter LQ held the record for consumer market). 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.

Use of dot matrix printers today

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, 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, ATM, 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 legacy-free computers. 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 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 subsidise the initial cost of the printer itself. Dot matrix ribbons are a commodity and are not monopolised by the printer manufacturers themselves.

Advantages and disadvantages

Advantages

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.

Disadvantages

Impact printers create noise when the pins or typeface strike the ribbon to the paper.[6] Sound dampening 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.

Future of dot-matrix printers

The main use of dot-matrix printers is in areas of intensive transaction-processing systems that churn out quite a lot of printing. Many companies who might have started off with dot-matrix printers are not easily convinced to go for paperless options or printers based on other technologies because of the speed advantage of dot-matrix printers, as well as simply having many years of experience with dot matrix and being reluctant to change to a new, unproven method.

See also

Media related to Dot matrix printers at Wikimedia Commons

References

  1. ^ "United States Patent 4194846". 1980-03-25. http://www.freepatentsonline.com/4194846.html. Retrieved 2009-07-16. 
  2. ^ "HP Computer Museum". http://www.hpmuseum.net/display_item.php?hw=315. "This highly-reliable dot-matrix printer..." 
  3. ^ "MX-80 SOUND". http://www.trouserpress.com/entry.php?a=mx-80_sound. 
  4. ^ Dot Matrix, InfoWorld Jul 28, 1986. http://books.google.com/books?id=Vy8EAAAAMBAJ&lpg=PA30&dq=dot%20matrix%20letter%20quality&pg=PA40#v=onepage&q&f=false. 
  5. ^ High speed, near letter quality dot matrix printers Popular Science Dec 1983. http://books.google.com/books?id=kawCnk4051wC&lpg=PA139&pg=PA139#v=onepage&f=false. 
  6. ^ "Panasonic KX-P2123. (dot-matrix printer) (Hardware Review) (Evaluation)". http://www.atarimagazines.com/compute/issue144/G10_Panasonic_KXP2123.php. 

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