Dot matrix printing
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Dot matrix printing or impact matrix printing is a type of computer printing which uses a print head that moves 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 1
- Early history 2
- Printer head positioning 3
- Personal computers 4.1
- Pseudo-color 4.2
- Near Letter Quality (NLQ) 4.3
- 24-pin printers 4.4
- Contemporary use 4.5
- Advantages and disadvantages 5
- See also 6
- References 7
- External links 8
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.
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.
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.
A variation on the dot matrix printer was the cross hammer dot printer, patented by Seikosha in 1982. The smooth cylindrical roller of a conventional printer was replaced by a spinning, fluted cylinder. The print head was a simple hammer, with a vertical projecting edge, operated by an electromagnet. Where the vertical edge of the hammer intersected the horizontal flute of the cylinder, compressing the paper and ribbon between them, a single dot was marked on the paper. Characters were built up of multiple dots.
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 5×7 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
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
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).
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, 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. The MX-80's low dot density (60 dpi horizontal, 72 dpi 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 60 dpi up to 240 dpi), 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 100 dpi 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.
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. 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—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 be perfectly aligned behind or below the printer. Loosely held paper or skewed supply paper could cause misalignments between passes, rendering the NLQ text illegible.
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. In the PC market, nearly all 9-pin printers printed at a de facto-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 printed more quickly than most daisy-wheel models.
As manufacturing costs declined, 24-pin printers gradually replaced 9-pin printers. Twenty-four pin printers reached a dot-density of 360×360 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.
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, 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.
One often overlooked application for dot-matrix printers is in the field of IT security. Various system and server activity logs are typically stored on the local filesystem, where a remote attacker - having achieved their primary goals - can then alter or delete the contents of the logs, in an attempt to "cover their tracks" or otherwise thwart the efforts of system administrators and security experts. However, if the log entries are simultaneously output to a printer, line-by-line, a local hard-copy record of system activity is created - and this cannot be remotely altered or otherwise manipulated. Dot-matrix printers are ideal for this task, as they can sequentially print each log entry, one entry at a time, as they are added to the log.
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 low printing cost 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. 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 lower risk of damaging the paper.
The advantages are: low purchase cost, can handle multipart forms, cheap to operate, only needs fresh ribbons, rugged, low repair cost and the ability to print on continuous paper. This makes it possible 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, slowness and more prone to jamming - with jams that are more difficult to clear. This is because paper is fed in using two sprockets engaging with holes in the paper. A small tear on the side of a sheet can cause a jam, with paper debris that is tedious to remove.
- Character matrix printer
- Daisy wheel printing
- Dye-sublimation printer
- Typeball printer
- Line printer
- Printer (computing)
- Thermal printer
- IBM Proprinter
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- High speed, near letter quality dot matrix printers Popular Science Dec 1983.
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- Flatbed Dot Matrix Printers