Editing Line printer (section)

====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>