The line printer is a form of high speed impact printer in which one line of type is printed at a time. They are mostly associated with the early days of computing, but the technology is still in use. 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 is 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.
Often the character sequences are staggered around the drum, shifting with each column. This obviates the situation whereby all of the hammers fire simultaneously when printing a line that consists of the same character in all columns, such as a complete line of dashes ("----").
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 (train) printer
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. Common letters and symbols would appear more often on the chain, according to the frequency analysis of the likely input. It was also possible to play primitive tunes on these printers by timing the nonsense of the printout to the sequence on the chain, a rather primitive piano. IBM was probably the best-known chain printer manufacturer and the IBM 1403 is probably the most famous example of a chain printer.
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