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Developed for telegraph 'hard copy' communication over the telephone network, a teletype machine was suited to adoption by the rapidly expanding computer market, with its demand for hardware and technology to run it. A ready-made printer and keyboard communication device, it was capable of being attached to the systems by the addition of data link hardware to handle the already established protocol. For this reason teletypes were used both in the computer room to operate the system, and by remote users to communicate with the system, to submit and retrieve information.
A Teletype was a common 'console', used by the operator. They were also used for remote printers connected to the computer by telephone wires via modems. Typically a KSR35 (Keyboard Send and Receive) using a series of characters on pins arranged in a square block. The block was moved to align it in front of a hammer used to strike the appropriate pin and transfer the ink from the ribbon to the paper.
Another common teletype, the ASR33 (Automatic Send and Receive) used a cylindrical block with characters etched on it to align vertically and horizontally clockwise or counter-clockwise, the appropriate character for printing. The operator, or other user, in each case could use the keyboard to input basic commands or data to the computer.
Another console communication device was a modified IBM Selectric or Remington SR-101 electric typewriter. They used a 'golf ball' to type in a similar way to the ASR33. With the addition of changeable 'balls' for a different font set.
These machines were better suited to computer interface as they were much faster than conventional type machines.
VIDEO TERMINALS
Consoles next moved into the 'modern' era when a CRT (Cathode Ray Tube) display was added. At first this was in addition to the to the typewriter device, as a console record was kept on paper to analyse what had happened at various stages of operation (so we know who to blame!).
Consoles have since become data terminals, with keyboard and screen, then mini computers incorporating maintenance and diagnostic capabilities, as well as console function. PCs, at first monochrome, then color, emulating a 'green screen' terminal, then using colors for different purposes, are standard today, as are LCD monitors.
Console records are still maintained on files, either in the PC on hard drive or on the mainframe, depending on the system. (you still can't get away with anything!) Some systems use a control console for each operation, system, network, printers etc. Others incorporate all functions in one control, with auxiliary stations in the print room, network console, etc.
NETWORKS
Networks, originally supported by modems at speeds of 110 baud gradually improved in speed along with modem technology to 9600. Connection methods were either RS232 standard, or some proprietary modified version of it, used by system manufactures to ensure customers used their terminals and printers.
The next stage was the adoption of Ethernet, or other contemporary protocols. Ethernet also increased from 10 to 100 to gigabit transmission rates. Terminals and printers at remote points improved from a basic dot matrix printer to which a monochrome terminal was added.
Currently remote users are using PCs or 'thin clients' (basically a pc with no hard drive, using the network server to download programs) with a laser printer dedicated or shared with a workgroup. A network of several hundred PCs and printers can be managed from a central 'network control', or at any point on the network.
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 will 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.