Editing Line printer (section)

===Timing-sensitive designs===
Several designs of printers have similar characteristics.

====Drum printer====
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[[Image:Drum-printer.jpg|right|thumb|Drum Printer]]
[[Image:TypefaceDrumPrinter1966.jpg|right|thumb|Typical print of a drum printer, showing the characteristic vertical misalignment of characters due to slight hammer timing errors (mainframe; about 1965)]]
[[Image:Drukarka wierszowa beben.jpg|right|thumb|Fragment of line printer drum<br>showing "'''%'''" characters.]]
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 can print. The wheels, joined to form a large drum (cylinder), spin at high speed. 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 has to be very fast. Typically, they are driven by [[voice coil]]s mounted on the moving part of the hammer.

Large mechanical and electric stresses occur when the line to be printed requires firing all of the hammers simultaneously. With simple type layouts, this happens when the line consists of a single character repeated in all columns, such as a line of dashes ("----...---") To avoid this problem, some printers use a staggered arrangement, with the characters in each column rotated around the drum by a different amount. Then simultaneous firing occurs only if the printed line matches the character layout on the drum, which should not happen in normal text.

Lower-cost printers do not use a hammer for each column. Instead, a hammer is provided for every other column, and the entire hammer bank is arranged to shift left and right, driven by an additional voice coil. For this style of printer, two complete revolutions of the character drum are required to print each line, with one revolution being used to print all the "odd" columns and another revolution being used to print all of the "even" columns. This requires only half (plus one) the number of hammers, magnets, and the associated channels of drive electronics.

At least one low-cost printer, made by [[Control Data Corporation|CDC]], achieves the same end by moving the paper laterally while keeping the hammer bank at rest.

[[Dataproducts]] was a typical vendor of drum printers, often selling similar models with both a full set of hammers (delivering, for example, 600 lines-per-minute of output) and a half set of hammers (delivering 300 LPM).<ref>{{cite web |title=Model 2230 Line Printer |type=Brochure |publisher=Dataproducts |url=http://www.bitsavers.trailing-edge.com/pdf/dataProducts/brochures/DataProductsLinePrinter2230.pdf}}</ref>

==== Printers with horizontally moving print elements ====

===== Chain printer =====

'''Chain Printers''' place the type on a horizontally-moving circular chain. As with the drum printer, as the correct character passes by each column, a hammer is fired from behind the paper. Compared to drum printers, chain printers have the advantage that the type chain can usually be changed by the operator. A further advantage is that vertical registration of characters in a line is much improved over drum printers, which need extremely precise hammer timing to achieve a reasonably straight line of print. By selecting chains that have a smaller character set (for example, just numbers and a few punctuation marks), the printer can print much faster than if the chain contains the entire upper- and lower-case alphabet, numbers, and all special symbols. This is because, with many more instances of the numbers appearing in the chain, the time spent waiting for the correct character to "pass by" is greatly reduced. Common letters and symbols appear more often on the chain, according to the [[frequency analysis]] of the likely input. IBM was probably the best-known chain printer manufacturer, and the [[IBM 1403]] is probably the most famous example of a chain printer.<!-- These also can print a complete line by using a chain with 2 moveable pulley different characters were embossed on the chain. -->

===== Train printer =====

'''Train printers''' place the type on a horizontally-moving circular train of print [[Slug (typesetting)|slugs]]. with multiple characters per slug, on a track, The technology is almost identical to print chains. 

===== Band printer =====
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[[Image:Printer band.jpg|right|thumb|Fragment of printer band, sitting on test printout for the characters (top) and hammer flight times (bottom)]]

'''Band printers''' are a variation of chain printers in which a thin steel band is used instead of a chain, with the characters embossed or etched onto the band. Again, a selection of different bands are generally available with a different mix of characters so a character set best matched to the characters commonly printed can be chosen. [[Dataproducts]] was a well known manufacturer of band printers, with their B300, B600, and B1000 range, the model number representing the lines per minute rate of the printer.<ref>{{cite web |title=Dataproducts (300/600 LPM) Line Printers Maintenance Guide |date=October 1984 |edition=2nd |id=741-0432 |publisher=Wang Labs |url=https://www.wang2200.org/docs/fiche/742-0432.MNL.2273LinePrinterMaintenance.85-07-19.pdf }}<br/>{{cite web |title=Dataproducts Band Printers |date=1997 |publisher=Dataproducts Corporation |url=http://www.dpc.com/market/band_dir.html |archive-url=https://web.archive.org/web/19970711163132/http://www.dpc.com/market/band_dir.html |url-status=dead |archive-date=1997-07-11 }}</ref> (The B300 is effectively a B600 with only half the number of hammers—one per two character positions. The hammer bank moves back and forth one character position, increasing the average number of band movements required for each line.)

===== Bar printer =====
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'''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]].

===== Common characteristics =====
In all four 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 resulted in printed lines that wandered vertically, albeit with characters correctly aligned horizontally in their columns. For train and bar printers, incorrect timing of the hammers resulted in characters shifting horizontally, printed closer or farther from the next character, or blurred on one side, albeit on vertically-level printed lines. The vertical misalignment of drum printers is more noticeable and annoying to human vision (see the sample pictured in this article).

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.