Dot matrix printer
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A dot matrix printer is an impact printer that prints using a fixed number of pins or wires. In contrast, inkjet and laser printers technically exhibit dot matrix printing, but they are not considered "dot matrix printers".
The first impact dot matrix printer was the Centronics 101. Introduced in 1970, it led to the design of the parallel electrical interface that was to become standard on most printers until it was displaced well over a decade later by the Universal Serial Bus (USB).
Digital Equipment Corporation (DEC) was another major vendor, albeit with a focus on use with their PDP minicomputer line. Their LA30 30 character/second (CPS) dot matrix printer, the first of many, was introduced in 1970.
DEC's dot matrix printersEdit
Unlike the LA30's 80-column, uppercase-only 5x7 dot matrix, DEC's product line grew. New models included:
- LA36 (1974): supported upper and lower case, with up to 132 columns of text (also 30 CPS)
- LA34: a lower-cost alternative to the LA36
- LA38: an LA34 with more features
- LA180: 180 CPS
- LS120: 120 CPS
- LA120: 180 CPS (and some advanced features)
- LA12: a portable terminal
It printed 80 columns of uppercase-only 7×5 dot matrix characters across a unique-sized paper. The printhead was driven by a stepper motor and the paper was advanced by a noisy solenoid ratchet drive. The LA30 was available with both a parallel interface (LA30-P) and a serial interface (LA30-S); however, the serial LA30 required the use of fill characters during the carriage-return. In 1972, a receive-only variation named LA30A became available.
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, quickly copied by many other manufacturers well into the 1990s. Most efficient dot matrix printers used this buffering technique.
Digital technology later broadened the basic LA36 line into a wide variety of dot matrix printers.
The DEC LA50 was designed to be a "compact, dot matrix" printer. When in graphic mode (as opposed to text mode), the printhead can generate graphic images. When in (bitmap) graphics mode, the LA50 can receive and print Sixel graphics format.
- Print speed: 165 characters per second
- Weight: 155 pounds (70.3 kg)
- Size: 27 ½" W x 11 ¼" H x 19 ¼ D (approx. 70 cm x 29 cm x 49 cm)
- Shipping: 200 pounds (approx. 91 kg), wooden crate, unpacked by removal of 36 screws
- Characters: 62, 10 numeric, 26 upper case and 26 special characters (no lower case)
- Character size: 10 characters per inch
- Line spacing: 6 lines per inch
- Vertical control: punched tape reader for top of form and vertical tab
- Forms thickness: original plus four copies
- Interfaces: Centronics parallel, optional RS-232 serial
Impact vs. non-impact is one way in which printers are divided into two classes. Dot-matrix impact printers, "the most common type still sold as of October of 2012," use "a vertical column of pins which fire" (and thus force contact between the paper and "an ink-soaked ribbon").
Low-cost dot-matrix printersEdit
In the mid-1980s, dot-matrix printers were dropping in price, and, being "faster and more versatile than daisywheel printers" (including becoming even more flexible in what they can do because of 24-pin print heads, versus the earlier 9-pin models) they've continued to sell.
There are techniques for obtaining what some call "Near Letter Quality" (NLQ) output from a dot matrix printer
- use more pins (typically 24, sometimes 18, compared to 9)
- print and then slightly shift & overprint. This takes more time.
To obtain the maximum output speed, albeit at a lower quality, each character and line is only printed once. This is called "draft mode".
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 and home computers.
Also known as "serial dot matrix printers", the 1985 statement "for the average personal computer user dot matrix remains the most workable choice" was still quite valid over a quarter of a century later. At the time, IBM sold Epson's MX-80 as their IBM 5152.
Another technology, Inkjet printing, which uses the Razor and blades model (give away the razor handle, make money on the razor blade) has reduced the value of the low cost for the printer: "a price per milliliter on par with liquid gold" for the ink/toner.
In June 1978, the Epson TX-80/TP-80, an 8-pin dot-matrix printer mainly used for the Commodore PET computer, was released. This and its successor, the 9-pin MX-80/MP-80 (introduced in 1979/1980), were the models that sparked the initial popularity of impact printers in the personal computer market. The MX-80 combined affordability with good-quality 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. The MX-80 even inspired the name of a noise rock band. The MX-80's low dot density (60 dpi horizontal, 72 dpi 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. IBM sold the MX-80 as IBM 5152.
Initially, third-party software (such as the 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, not all enhancement software was 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 graphics. Yet the dot-matrix print head was well-suited to this task, and the capability, referred to as "dot-addressable" quickly became a standard feature on all dot-matrix printers intended for the personal and home computer markets. In 1981, Epson offered a retrofit EPROM kit called Graftrax to add the capability to many early MX series printers. Banners and signs produced with software that used this ability, such as Broderbund's Print Shop, became ubiquitous in offices and schools throughout the 1980s.
Progressive hardware improvements to impact printers boosted the carriage speed, added more (typeface) font options, increased the dot density (from 60 dpi up to 240 dpi), and added 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 100 dpi 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 uploaded 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 nylon ribbons gave an increase in ribbon life, at the expense of quality.
The high quality of single-strike ribbons had two side effects:
- At least 50% and up to 99.9% of the given ribbon surface would be wasted per character, since an entire fresh new region of ribbon was needed to print even the smallest font shapes. Ribbon advance was fixed to always span the largest character shape, so a row of periods would consume as much fresh ribbon as a row of W's, with a large span of unused carbon between each dot.
- Single-strike ribbons created a risk of espionage and loss of privacy, because the used ribbon reel could be unwound to reveal everything that had been printed. Secure disposal was required by shredding, melting, or burning of used ribbon cartridges to prevent recovery of information from garbage bins.
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 up to 3 passes depending on color. In some color printers, such as the Apple ImageWriter II and Star Micronics NX-1000C "Rainbow" or the optional color kits for the NEC Pinwriter series, the printer moved the ribbon vertically 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 became as popular as 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 printer film, which was more expensive. It would take much longer for color printing in the home to become ubiquitous, with the ink jet printer.
Near Letter Quality (NLQ)Edit
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—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.
In 1985, The New York Times described the use of "near letter-quality, or N.L.Q." and "near letter quality" as "just a neat little bit of hype" but acknowledged that they "really show their stuff in the area of fonts, print enhancements and graphics."
By the mid-1980s, manufacturers had increased the pincount of the impact printhead from 7, 8, 9 or 12 pins to 18, 24, 27, 36 or 48, with 24 pins being most common. The increased pin-count permitted superior print-quality which was necessary for success in Asian markets to print legible CJKV characters. In the PC market, nearly all 9-pin printers printed at a de facto-standard vertical pitch of 9/72 inch (per printhead pass, i.e. 8 lpi). Epson's 24-pin LQ-series rose to become the new de facto standard, at 24/180 inch (per pass - 7.5 lpi). 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 more quickly (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 printed more quickly than most daisy-wheel models.
Market acceptance of 24-pinEdit
As manufacturing costs declined, 24-pin printers gradually replaced 9-pin printers. Twenty-four pin printers reached a dot-density of 360×360 dpi, a marketing figure aimed at potential buyers of competing ink-jet and laser-printers. 24-pin NLQ fonts generally used a dot-density of 360×180, 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 two or more NLQ typefaces, but the rise of WYSIWYG software and GUI environments such as Microsoft Windows ended the usefulness of NLQ.
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,[when?] the inkjet mechanism became available to the printer industry. For applications that did not require impact (e.g. carbon-copy printing), 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 2016[update], dot matrix impact technology remains in use in devices and applications such as:
- cash registers,
- fire alarm systems,
- many point-of-sale terminals
- Dot matrix impact printers such as the Epson tm-u220b (which don't need thermal paper) are more tolerant of the hot and dirty operating conditions found in many industrial settings, allowing them to be used even in settings such as restaurant or cafeteria kitchens.
- Dot matrix printers, when used with continuous forms to produce hard copy, can be used for a computer room console log's output, producing a "permanent" record that cannot be remotely altered or otherwise manipulated.
Thermal printing is gradually supplanting them in some of these applications, but full-size dot-matrix impact printers are still used to print multi-part stationery. For example, dot matrix impact printers are still used at bank tellers and auto repair shops, and other applications where use of tractor feed paper is desirable such as data logging and aviation. Some of these printers are even fitted with USB interfaces as standard to aid connection to modern computers without legacy ports.
Some companies, such as Printek, DASCOM, WeP Peripherals, Epson, Okidata, Olivetti, Compuprint, Lexmark, and TallyGenicom still produce serial printers. Printronix is now the only manufacturer of line printers. Today, a new dot matrix printer actually costs more than most inkjet printers and some entry level laser printers. Despite this initial price difference, 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 subsidize the initial cost of the printer itself. Dot matrix ribbons are a commodity and are not monopolized by the printer manufacturers themselves.
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