Dot matrix printing

Dot Matrix is also the name of a character in the TV series ReBoot, and of a character in the Mel Brooks film Spaceballs.

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 garnet) 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, but eventually wear out. Ink invades 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.

Strictly speaking, "dot matrix" in this context is a misnomer, as nearly all inkjet, thermal, and laser printers produce dot matrices. However, in common parlance these are seldom called "dot matrix" printers, to avoid confusion with dot matrix impact printers.

Early dot matrix printers

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 printhead 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

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 Printronix.

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-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 printjobs. First offered on the 9-pin Epson FX-80. Later high-end 24-pin printers supported 2 or more simultaneous user-fonts, allowing a printjob 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.

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, readibility. Generally slower to print, especially on 9-pin printers. (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.

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 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-image 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

Personal Computers

A Tandy 1000 HX with a Tandy DMP-133 dot-matrix printer.

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 printhead. 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 printhead to allow denser and more precise dot placement.) Such add-on software was inconvenient to use, because they 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 printhead 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.

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 printhead 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 printhead assembly. In other models, the printhead 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 pie-charts, but not for photo-realistic reproduction. Dot-matrix thermal-transfer printers offered more consistent color quality, but consumed even-more-expensive printer film. Color printing in the home would only become ubiquitous much later, with the ink-jet printer.

NLQ

Text quality was a recurring issue with dot-matrix printers. Near letter quality (NLQ) 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 addressible 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 curiously did not use NLQ mode at all, as it did not rely on the printer's own fonts. For this quality of output, a Mac would simply set the printer to the same resolution as the printer would have used for NLQ. 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 Macintosh dot-matrix printers exclusively used raster ("graphics") printing mode. The Mac's WYSIWYG philosophy foreshadowed the direction the PC market would later follow.

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 kanji 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 (tills), 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 found in printing of carbon copy multi-part stationery, and other applications where use of tractor feed paper is desirable such as computer programming and data logging. Dot matrix printers are also more tolerant of hot, 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 [1], Epson [2], Okidata [3], and TallyGenicom [4], 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.

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 cost per page. As the ink is running out, the printout gradually fades rather than suddenly stopping part way 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.

Disadvantages

Impact printers are usually noisy, to the extent that sound dampening enclosures are available for use in quiet environments. They can only print text and graphics, with limited color performance, limited quality and comparatively low speed.