SYSTEM AND WORKSTATION PRINTERS

The line printer used on mainframes was variously a drum printer, where all the characters are etched on a spinning drum, a band printer with an oscillating or circulating print band, or a shuttle printer, all of these using print hammers to transfer ink from a ribbon to continuous sprocket feed paper. A typical maximum speed for a line printer was 2000 lines per minute, though at that speed the maintenance overhead was quite heavy, mechanical components wore out more rapidly. These mechanical printers were known as 'impact' printers.

A large scale laser printer was possible in the late 1980s, it was the size of your kitchen, and controlled by a mini-computer. It used standard sheet feed line printer paper and could print 200 pages a minute. Its mode of operation was essentially similar to the laser printer on your desk.

LASER PRINTER

The laser printer grew to wide use in the 1980s and produced a vastly superior print quality to the contemporary dot matrix printer, (see below). Initially problems like messy toner and streaks and marks on the prints were disadvantages, but were overcome. The laser printer works by transferring a complete page image to a photosensitive drum or band, scanning the data from a buffer with a laser beam as the drum turns. At the same time a high voltage is applied to transfer toner to the drum and thence to the paper, also moving under the drum. The paper passes through a fuser where a heated roller fuses the toner to the paper.

This system allows graphics, as well as text, to be printed in high quality, one complete page at a time. Color is also possible by using four separate sets of drums and toners. Different paper sizes are available, and the printer will also make overhead slides and other special types of printing.

INKJET PRINTER

Also used, mainly for its color printing capacity before color laser printers were available, the inkjet printer became fairly common. These work by putting tiny, measured droplets of black or colored ink onto the paper in a matrix accordance with data in a buffer. Commercial versions of these printers then heat the paper to make the ink more permanent. A print is formed one line at a time, from the matrix, for a complete sheet of paper. They wil

l also print graphics and are capable of high quality, comparable to photographic prints. Maintenance costs are high for ink replacement cartridges, and the printers are not always robust enough to take continuous use in sometimes 'unfriendly' environments.

DOT MATRIX PRINTERS

Early small printers for use remotely from the central computer usually accompanying video terminals, but initially on their own, were mostly Dot Matrix printers. Some of these have survived today, as they are of the 'impact' type. These being the only type which can print multi-part forms. A laser or inkjet printer (a NIP or Non Impact Printer) must print duplicate pages to achieve the same result. Remember the warning on multi-part forms -'Press Hard You Are Writing 6 Pages!'?

A dot matrix printer uses small pins, usually 9, but can be more or less depending on the quality required. These pins are arranged vertically and print by being struck against the ribbon and paper on a platen, much like a typewriter. The print head moves sideways across the page so that a single character is formed by sequential strikes on the appropriate pins. It can print left to right, or right to left.

The effect is similar to the segments of a digital clock. This type of printer can produce graphics depending on data sent from the computer. Color is also possible with a four color ribbon, the ribbon carrier being moved vertically to change color.

In Part 3 we look at the development of consoles and networking.

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