26-06-2013, 12:32 PM
Dot matrix printing
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INTRODUCTION
This is an example of a wide-carriage printer, designed for paper 14 inches wide, shown with legal paper loaded (8.5" x 14"). Wide carriage printers were often used by businesses, to print accounting records on 11" x 14" tractor-feed paper. They were also called 132-column printers, though this description was only true for a specific font size and type that was built into the printer's electronics
Dot matrix printing or impact matrix printing is a type of computer printing which uses a print head that runs 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
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.[1] The moving portion of the printer is called the print head, and when running the printer 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.
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 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.[citation needed] 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[4]—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.