Printer (computing)
Line printers

Line printers print an entire line of text at a time. Four principal designs exist.

In each case, to print a line, precisely timed hammers strike against the back of the paper at the exact moment that the correct character to be printed is passing in front of the paper. The paper presses forward against a ribbon which then presses against the character form and the impression of the character form is printed onto the paper. Each system could have slight timing issues, which could cause minor misalignment of the resulting printed characters. For drum or typebar printers, this appeared as vertical misalignment, with characters being printed slightly above or below the rest of the line. In chain or bar printers, the misalignment was horizontal, with printed characters being crowded closer together or farther apart. This was much less noticeable to human vision than vertical misalignment, where characters seemed to bounce up and down in the line, so they were considered as higher quality print.

Line printers are the fastest of all impact printers and are used for bulk printing in large computer centres. A line printer can print at 1100 lines per minute or faster, frequently printing pages more rapidly than many current laser printers. On the other hand, the mechanical components of line printers operate with tight tolerances and require regular preventive maintenance (PM) to produce a top quality print. They are virtually never used with personal computers and have now been replaced by high-speed laser printers. The legacy of line printers lives on in many operating systems, which use the abbreviations "lp", "lpr", or "LPT" to refer to printers.
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