A dot matrix printer
or impact matrix printer
refers to a type of computer printer
with a print head that runs back and forth 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 (pawls). 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.
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 produce dot matrices, in common parlance these are seldom called "dot matrix" printers, to avoid confusion with dot matrix impact printers.
Early Dot Matrix Printers
was a 30 character/second dot matrix printer introduced in 1970 by Digital Equipment Corporation
. 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 dot matrix printers (indeed, most printers in general) until it started to be replaced by the Universal Serial Bus (USB) in the late 1990s.
Dot matrix features
As with any technology product, feature-sets for dot-matrix impact printers vary by price, intended market, and year of introduction.
Industrial-market printers are designed for high-volume printing and offer construction, feed paths, and carriage configurations suited for this task. The carriage assembly typically houses multiple printheads, permitting rapid printing of the entire paper-width with only a partial carriage displacement. Industrial printers are often cabinet-sized, with their own housing for blank paper, the printer, and printed output. Suppliers of industrial impact printers include Mannesmann-Tally
Personal computer market
The 1980s saw a wide variety of printers from many different manufacturers. Nearly all consumer printers are desktop sized. Common features included:
- alphanumeric mode (text) - ASCII/ANSI character printing mode, encoded as 1 byte per printed character. Early IBM PC printers had limited typeface definitions, later printers had fully ANSI-compliant typeface definitions. Standard on all dot-matrix printers. (Note: 'Windows-only' printers no longer support text mode printing from MS-DOS.)
- bitmap mode (graphics) - freeform bitmap printing. Controller transcribed any host-provided bitmap sequence. Standard on all but the earliest dot-matrix models.
- boldface (text) - Usually implemented by printing selected text segment in printer's double-density mode. (For NLQ fonts which already used double-density mode, the printer controller digitally 'widened' the typeface bitmap.)
- color printing (text+graphics)- multi-color output, generally achieved with multiple printhead passes. Required a color ribbon to be installed.
- condensed cpi (text) - characters per inch, standard-sized text was 10cpi (or 80 columns for letter-width paper.) Many printers offered condensed text modes of 12cpi, 15cpi, 17cpi, and 20cpi. If supported in NLQ mode, the condensed typefaces generally used extra storage (ROM) in the printer controller to hold different versions of the typeface. (Early printers limited NLQ mode to 10cpi or 12cpi.)
- doublestrike (text) - double printing (two-pass) of a selected text segment. Sometimes used to simulate boldface or cheap, poor-quality NLQ.
- downloadable font (text) - ability to accept and store a user-defined typeface. The user-downloaded typeface was downloaded into printer's onboard RAM, where it becomes available to subsequent (alphanumeric) text print jobs. First offered on the 9-pin Epson FX-80. Later high-end 24-pin printers supported 2 or more simultaneous user fonts, allowing a print job to use any combination of user or built-in typefaces.
- draft (text) - high-speed print mode, characters were formed with openly spaced dots. Some models had multiple draft modes with differing speed and dot density. Many printers could print draft mode text bidirectionally - every other line was printed backwards, so that no time was lost for moving back the print head to the start of the next line.
- italics (text) - built-in italics capability. Printer controller created the effect through digital processing of the typeface.
- NLQ (text) - Near Letter Quality, ASCII typeface with improved darkness and readability. Generally slower to print, especially on 9-pin printers, which used two passes of the print head and usually turned off bidirectional printing to achieve this quality. (Available on 24-pin printers and later model 9-pin printers.)
- outline (text) - printed selected text segment in a hollowed outline of the typeface. Printer controller created the effect through digital processing of the typeface. Found on a handful of late-model 24-pin printers (such as Epson LQ-570/870/1070.)
- proportional-space font (text) - non-uniform (horizontal) spacing between characters. Some models only allowed PS in conjunction with specific typefaces. Word processing programs needed to know the width of every character used by the printer in this mode to determine where to break lines; thus software support was often limited.
- quiet-mode (text+graphics) - reduced the printhead's acoustic noise during printing. Generally reduced the speed of printing.
- scalable font (text) - allowed user control of a font's printed-size (continuously variable point-size.) Unlike the traditional bitmap representation of typeface data, scalable typefaces used a vector-based definition. Found on a handful of late-model 24-pin printers (such as Epson LQ-570/870.)
- shadow (text) - added a shadow to a selected text segment. Printer controller created the effect through digital processing of the typeface. Found on a handful of late-model 24-pin printers (such as Epson LQ-570/870/1070.)
- subscript/superscript (text) - built-in typeface for simulating raised and lowered letters. (Usually implemented as a separate typeface.)
- typeface (text) - the typeface is the bitmap (image) definition of a font. Common printer typefaces were Courier, Roman, Serif, and Sans Serif. Early and low-end printers offered a single typeface. High-end and later models offered 2 or more distinct typefaces, as well as user-downloading of custom typefaces.
- wide-carriage (carriage size) - could print to wider (132 column) paper. Standard-carriage printers printed on letter-width (8.5") paper.
- dot-density (printhead and controller) - "dot matrix resolution" for 9-pin printers: Vertical is 72dpi, horizontal (dpi):60, 66, 72, 80, 96, 120, 132, 144, 160, 180, 240. For 24-pin printers (in native 24-pin mode), Vertical is 180dpi, horizontal (dpi): 60, 90, 120, 180, 240, 360. Many models could achieve higher vertical-density through fractional linefeeds (1/144" or 1/216" for 9-pin, 1/360" for 24-pin), for a maximum-rating of 240x216, or 360x360.
- 9-pin (printhead configuration) - the standard printhead config until the introduction of 24-pin printers. Later models offered dot density up to 240dpi (horizontal.)
- 18-pin, 27-pin (printhead configuration) - uncommon in consumer market.
- 24-pin (printhead configuration) - square pixel for high bitmap resolution (180x180dpi standard), faster and higher-quality NLQ mode. Later models offered dot density up to 360dpi (horizontal.)
Dot matrix usage
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 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 solid 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. Furthermore, 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 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, they were not 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 computer-generated images. Yet the dot-matrix print head was well-suited to this task, and the capability quickly became a standard feature on all PC-oriented dot-matrix printers.
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 downloaded 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.
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 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. The Mac's WYSIWYG philosophy foreshadowed the direction the PC market would later follow.
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
. In the PC market, nearly all 9-pin printers printed at a defacto-standard vertical pitch of 9/72 inch (per printhead pass.) Epson's 24-pin LQ-series rose to become the new de-facto standard, at 24/180 inch (per pass.) 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 quicker (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 misleading 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. 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 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 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 (see Inkjet printer -> Underlying business model). Dot matrix ribbons are a commodity and are not monopolised by the printer manufacturers themselves.
Dot matrix printers are the dominant type of printer in small firms and offices in some parts of India.
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 are usually noisy, to the extent that sound dampening enclosures are available for use in quiet environments. They can only print low resolution graphics, with limited color performance, limited quality and comparatively low speed. While they support fanfold paper with tractor holes, single-sheet paper usually has to be wound in and aligned by hand, which is relatively inconvenient and time-consuming. While far better suited to printing on labels than a laser printer or an inkjet printer, they are prone to bent pins (and therefore a destroyed printhead) caused by printing a character half-on and half-off the label; for text-only labels (ie. mailing labels), a daisy wheel printer
offers most of the advantages of a dot matrix, with better print quality and a lesser chance of being damaged.
Future of Dot-Matrix Printers
The main use of Dot-Matrix Printers are 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 so easily convinced to go for printers based on other technologies because of the speed advantage that they have with dot-matrix printers.