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Revision as of 17:00, 17 July 2005

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 uses the power of a tiny electromagnet or solenoid to drive it forward, 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 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. After about a million characters, even with tungsten blocks and titanium pawls, the printing becomes too unclear to read.

Nearly all inkjet, thermal, and laser printers use a dot matrix to describe each character or graphic. 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 produced 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 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 -- 120 c/s advanced terminal
• LA34 -- Cost-reduced terminal
• LA38 -- An LA34 with more features
• LA12 -- A portable terminal

Meanwhile, Centronics (then of Hudson, New Hampshire) was reselling a printer mechanism produced by Brother Industries, Ltd. of Japan. 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

Certain models produce double-wide or bold characters by printing each vertical slice of a character twice. Some can produce higher resolutions by moving the print head more slowly while keeping the same "dot rate". They can also produce graphics by printing stripes of dots, each the same height as its standard character matrix. (Because of the particular six-bit dot-pattern encoding used by Digital, Digital referred to these dot columns as "sixels".) Though most dot matrix printers print in black and white, a few produce colour by making extra passes, shifting a multi-color striped ink ribbon between passes.

Dot matrix usage

In the 1970s and 1980s, dot matrix 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 groundbreaking model that drove their initial popularity in the consumer market was the Epson MX-80. However, they were notorious in homes and offices for their loud buzzing sound when printing (finally softened in some later models). They were also known for unattractive, spotty printouts which were difficult to photocopy clearly (especially when the inked ribbon was running dry). Printing enhancement programs such as Bradford were sometimes used, which worked by slowing the print head allowing more precise dot placement, and overprinting to remove some of the gaps between dots.

Later in the 1980s, models using more pins in the printhead (18 or 24 instead of the original 9) were released by a number of manufacturers, billed as near letter quality (NLQ). These printers produced better quality printouts (though clearly inferior to letter-quality printers—daisy wheel or laser printing—and still shy of inkjet printing quality) and were usually much faster in high-quality mode. But Hewlett-Packard's patents expired on steam-propelled photolithographically-produced ink-jet heads, and by the middle of the 1990s, falling laser printer and inkjet prices had almost completely wiped the dot matrix printer from the mainstream market.

Dot matrix impact printers remain in common use in devices such as cash registers, ATM printouts, and in industries where a carbon copy is required (for printing on to multi-part stationery), although thermal printing has largely supplanted them even in these applications. However, some companies still produce serial and line printers, such as TallyGenicom ; this particular corporation eventually bought Digital's line of printers and video terminals. Impact printers are used because of their low cost per page.

• Computer printers
• Impact printers
• DEC hardware
• Human-computer interaction