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A teleprinter (teletypewriter, teletype or TTY) is a now largely obsolete electro-mechanical typewriter which can be used to communicate typed messages from point to point through a simple electrical communications channel, often just a pair of wires.
The most modern form of these devices are fully electronic and use a screen instead of a printer. These teletypewriters are still in use by the deaf for typed communications over the telephone, usually called a TDD or TTY. For more information, please see Telecommunications devices for the deaf.
The teleprinter evolved through a series of inventions by a number of engineers, including Royal E. House, David Edwin Hughes, Charles Krum and Emile Baudot. A predecessor to the teleprinter, the stock ticker machine, was used as early as the 1870s as a method of displaying text transmitted over wires. A specially-designed telegraph typewriter was used to send stock exchange information over telegraph wires to the ticker machines.
Teleprinter operation
Most teleprinters used the 5-bit Baudot code (also known as ITA2). This limited the character set to 32 codes. One had to use a "FIGS" shift key to type numbers and special characters. Special versions had FIGS codes for specific applications like weather reports. Print quality was poor by modern standards. The Baudot code was used asychronously with start and stop bits: the asynchronous code design was intimately linked with the start-stop electro-mechanical design of teleprinters. (Early systems had used synchronous codes, but were hard to synchronise mechanically). Other codes, such as Fieldata and Flexowriter, were introduced but never became as popular as Baudot.
The teletype circuit was often linked to a paper tape punch and reader, allowing messages received to be resent on another circuit. Complex military and commercial communications networks were built using this technology. Message centers had rows of teleprinters and large racks for paper tapes awaiting transmission. Skilled operators could read the priority from the hole pattern and might even feed a "FLASH PRIORITY" tape into a reader while it was still coming out of the punch. Routine traffic often had to wait hours for relay. Many teleprinters had built-in paper tape readers and punches, allowing messages to be created and, to a limited extent, edited off-line.
More than two teleprinters could be connect to the same wire circuit by means of a current loop (a possible source of the phrase "being in the loop"). Communication by radio, RTTY, was also common. Amateur radio operators still use this communications mode.
Teletype machines
Teletype was a trademark of the Teletype Corporation of Skokie, Illinois, USA. Founded in 1906, it became part of ATT in 1930. Operations ceased around 1990. Teletype machines were given a model number, often followed by letters indicating the configuration:
- RO - Receive only
- KSR - Keyboard send and receive
- ASR - Automatic send and receive (i.e. built-in paper tape reader and punch)
Major models and their dates:
- 12 - 1922 - the first general purpose teletype
- 14 - 1925 - about 60,000 were built
- 15 - 1930 - the mainstay of U.S. military communications in WWII. About 200,000 were built
- 28 - 1950s - regarded as the most rugged machine Teletype ever built
- 32/33 - 1961 - a low-cost, all-mechanical design. The 32 was Baudot, the 33 ASCII. The 33 ASR was ubiquitous as a console device in the early minicomputer era
- 35 - 1961 - an ASCII version of the model 28
- 42/43 - 1979 - an electronic, dot-matrix printer design
Telex
A global teleprinter network, called the Telex network, was established in the 1920s, and was used through most of the 20th century for business communications. The main difference from a standard teleprinter is that telex includes a switched routing network, originally based on pulse-telephone dialing. Telex is still in use for certain applications such as shipping, news, weather reporting and military command. Many business applications have moved to the Internet.
For information on the development of telegraphy, including the Telex network, see telegraphy.
Teleprinters in computing
Some of the earliest computers (e.g. the LGP-30) used teleprinters for input and output. Teleprinters were also used as the first interactive computer terminals. They had no video display. Users typed commands after a prompt character appeared. This was the origin of the command line interface. The paper tape function was sometimes used to prepare input for the computer session offline, or to capture computer output. The popular ASR33 teletype used 7-bit ASCII code (with an eighth parity bit) instead of Baudot. The common modem communications settings, Start/Stop Bits and Parity, stem from the teletype era.
In computing, especially under Unix and Unix-like operating systems, teletypewriter has become the name for any external console device, like a user dialing in to the system, or a modem on a serial port. Such devices have the prefix tty, such as /dev/tty13.
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