line printer - Paper (forms) handling

A type of printer, usually associated with larger computer systems, which prints a complete line of information at a time. It is very fast, especially when compared with printers that print one character at a time. Their principal advantage is speed, and they tend to be economic only in commercial situations where there is a heavy printing load.

The line printer is a form of high speed impact printer in which a line of type is printed at a time.

Three principle 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. This avoided the need to fire all of the hammers simultaneously when, for example, a complete line of dashes ("----") needed to be printed. 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.

Bar printers were similar to chain printers but were slower and less expensive. 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. 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.

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

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. 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. The fastest line printers also used "stackers" to re-fold and stack the fan-fold forms as they emerged from the printer. Line printers frequently used a variety of discharge brushes and active (corona-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. 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.

Laser printers became popular when word processing replaced typewriters. In high volume printing, continuous form laser printers have become popular. 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.