DOT MATRIX PRINTER

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 adot matrix, and thus, varied fonts and arbitrary graphics can be produced.

    Design

  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 rubyThe 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 printersare capable of printing much more than 1000 cps, resulting in a throughput of up to 800pages/hour.

Because the printing involves mechanical pressure, both of these types of printers can create carbon copies and carbonless copies.

  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.

Although nearly all inkjet, thermal, and laser printers also print closely spaced dots rather than continuous lines or characters, it is not customary to call them dot matrix printers.

   Early history

  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 theparallel 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).