Line printer

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The line printer is a form of high speed impact printer in which one line of type is printed at a time. Print speeds of 600 to 1200 lines-per-minute (approximately 10 to 20 pages per minute) were common.

Four principal designs existed:

• Drum printers
• Chain (train) printers
• Bar printers
• Comb printers

In a typical drum printer design, a fixed font character set is engraved onto the periphery of a number of print wheels, the number matching the number of columns (letters in a line) the printer could print. The wheels, joined to form a large drum (cylinder), spin at high speed and paper and an inked ribbon are stepped (moved) past the print position. As the desired character for each column passes the print position, a hammer strikes the paper from the rear and presses the paper against the ribbon and the drum, causing the desired character to be recorded on the continuous paper. Because the drum carrying the letterforms (characters) remains in constant motion, the strike-and-retreat action of the hammers had to be very fast. Typically, they were driven by voice coils mounted on the moving part of the hammer.

The character sequences are staggered around the drum, shifting with each column. This avoided the need to fire all of the hammers simultaneously when, for example, a complete line of dashes ("----") needed to be printed.

Lower-cost printers did not use a hammer for each column. Instead, a hammer was provided for every other column and the entire hammer bank was arranged to shift left and right, driven by another voice coil. For this style of printer, two complete revolutions of the character drum were required with one revolution being used to print all the "odd" columns and another revolution being used to print all of the "even" columns. But in this way, only half the number of hammers, magnets, and the associated channels of drive electronics were required.

Dataproducts was a typical vendor of drum printers, often selling similar models with both a full set of hammers (and delivering, for example 600 lines-per-minute of output) and a half set of hammers (delivering 300 LPM).

Chain printers (also known as train printers) placed the type on moving bars (a horizontally-moving chain). As with the drum printer, as the correct character passed by each column, a hammer was fired from behind the paper. Compared to drum printers, chain printers had the advantage that the type chain could usually be changed by the operator. By selecting chains that had a smaller character set (for example, just numbers and a few punctuation marks), the printer could print much faster than if the chain contained the entire upper- and lower-case alphabet, numbers, and all special symbols. This was because, with many more instances of the numbers appearing in the chain, the time spent waiting for the correct character to "pass by" was greatly reduced. IBM was probably the best-known chain printer manufacturer and the IBM 1403 is probably the most famous example of a chain printer.

Bar printers were similar to chain printers but were slower and less expensive. Rather than a chain moving continuously in one direction, the characters were on fingers mounted on a bar that moved left-to-right and then right-to-left in front of the paper. An example was the IBM 1443.

In all three designs, timing of the hammers (the so called "flight time") was critical and was adjustable as part of the servicing of the printer. For drum printers, incorrect timing of the hammer caused the print line to wander up-and-down but the characters were always correctly aligned in their columns. For train and bar printers, incorrect timing of the hammers caused the characters to wander left-and-right of their correct position, but the print line was always level.

Most drum, chain, and bar printers were capable of printing up to 132 columns, but a few designs could only print 80 columns and some other designs as many as 160 columns.

Comb printers represent the fourth major design. These printers were a hybrid of dot matrix printing and line printing. In these printers, a comb of hammers printed a portion of a row of pixels at one time (for example, every eighth pixel). By shifting the comb back and forth slightly, the entire pixel row could be printed (continuing the example, in just eight cycles). The paper then advanced and the next pixel row was printed. Because far less motion was involved than in a conventional dot matrix printer, these printers were very fast compared to dot matrix printers and were competitive in speed with formed-character line printers while also being able to print dot-matrix graphics as well as variable-sized characters.

Printronix was a well-known vendor of comb printers.

Because all of these printing methods produced a lot of noise, lineprinters of all designs were always enclosed in sound-absorbing cases of varying sophistication.

[edit] Paper (forms) handling

All line printers used paper provided as boxes of continuous fan-fold forms rather than cut-sheets. The paper was usually perforated to tear into cut sheets if desired, and was commonly printed with alternating white and light-green areas, allowing the reader to easily follow a line of text across the page. This was the iconic "green bar" form that dominated the early computer age. Pre-printed forms were also commonly used (for printing cheques, invoices, etc.). A common task for the system operator was to change from one paper form to another as one print job completed and another was to begin. Some lineprinters had covers that opened automatically when the printer required attention.

These continuous forms were advanced through the printer by means of tractors (sprockets or sprocket belts). Depending on the sophistication of the printer, there might simply be two tractors at the top of the printer (pulling the paper) or tractors at the top and bottom (thereby maintaining paper tension within the printer). The horizontal position of the tractors was usually adjustable to accommodate different forms. High-speed servomechanisms usually drove the tractors, allowing very rapid positioning of the paper, both for advancing line-by-line and slewing to the top of the next form. The fastest line printers also used "stackers" to re-fold and stack the fan-fold forms as they emerged from the printer.

The high-speed motion of the paper often developed large electrostatic charges. Line printers frequently used a variety of discharge brushes and active (corona discharge-based) static eliminators to discharge these accumulated charges.

Many printers supported ASA carriage control characters which provided a limited degree of control over the paper, by specifying how far to advance the paper between printed lines. Various means of providing vertical tabulation were provided, ranging from punched paper tape to fully electronic (software-controlable) tab simulation.

[edit] Current applications

This technology is still in use in a number of applications. It is usually both faster and less expensive (in total ownership) than laser printers. In printing box labels, medium volume accounting and other large business applications, line printers remain in use. Multi-part paper forms (carbon copies) are sometimes useful when exact copies are needed for legal accountability or other reasons. Because of the limited character set engraved on the wheels and the fixed spacing of type, this technology was never useful for material of high readability such as books or newspapers.

Laser printers became popular when word processing replaced typewriters. In high volume printing, continuous form laser printers have become popular. These no longer had fixed columns or monospaced type and offered a range of fonts as well as graphics. The technology operates in a way similar to single sheet laser printing.

The names of the lp and lpr commands in Unix were derived from the term "line printer". Many other systems also called the lineprinting devices "LP", "LPT", or some similar variant. These references served to distinguish formatted final output from normal interactive output from the system, which in many cases was also printed on paper (as by a teletype) but not by a line printer.