Line printer

Line printers are mostly associated with unit record equipment and the early days of digital computing, but the technology is still in use.

The wheels, joined to form a large drum (cylinder), spin at high speed.

Because the drum carrying the letterforms (characters) remains in constant motion, the strike-and-retreat action of the hammers has to be very fast.

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.

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

As with the drum printer, as the correct character passes by each column, a hammer is fired from behind the paper.

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.

The hammer bank moves back and forth one character position, increasing the average number of band movements required for each line.)

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).

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.

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.

In later line printers, high-speed servomechanisms usually drove the tractors, allowing very rapid positioning of the paper, both for advancing line-by-line and slewing to the top of the next form.

[7] Prior to that, tabulator operators had to write down totals from counter wheels onto tally sheets.

"[12] The names of the lp and lpr commands in Unix were derived from the term "line printer".

These references served to distinguish formatted final output from normal interactive output from the system, which in many cases in line printer days was also printed on paper (as by a teletype) but not by a line printer.