Teletypewriter Definition

A teletypewriter, also referred to as a teletype machine, is a now largely obsolete electro-mechanical typewriter that was used to communicate typed messages from point to point through a simple electrical communications channel.

A predecessor to the teletypewriter was the stock ticker system, which was developed in the 1870s as a method of transmitting stock market data over long-distance telegraph lines and displaying the data in alphanumeric characters. The system employed a specially-designed telegraph typewriter on one end and tickers, which printed stock quotes on thin strips of paper, on the other end.

Most teletypwriters used the five-bit Baudot code, which was devised around 1875 by a Frenchman named Emile Baudot. The five bits limited the character set to 32 codes, but a FIGS (i.e., figures shift) shift key enabled it to also type numbers and special characters. Specialized versions had FIGS codes for specific applications, such as weather reports. The quality of the printing was poor by current standards.

Many teletype machines were linked to, or had built-in, paper tape punching and reading machines, thus allowing messages to be created and edited off-line as well as stored and retransmitted on other circuits. Complex military and commercial communications networks were constructured which had centers with rows of teletypwriters and large racks for paper tapes awaiting transmission. Skilled operators could determine the priorities of messages directly from the patterns of holes in the tapes and could thus load a priority tape into another machine for retransmission while it was still coming out of the punching unit.

The first general-purpose teletype machine was the Model 12, which was introduced in 1922, and it was followed three years later with the Model 14, of which about 60,000 were produced. The Model 15, which was launched in 1930; it was the mainstay of U.S. military communications during WWII, and approximately 200,000 units were built.

The word Teletype was a trademark of the Teletype Corporation. The Skokie, Illinois-based company was founded in 1906 and became part of AT&T (the former U.S. telecommunications monopoly) in 1930. Operations ceased around 1990.

A global teletype 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 teletypewriter is that Telex machines included a switched routing network, originally based on pulse-telephone dialing. AT&T developed a competing network it called TWX. Telex is still in use for certain applications such as shipping, news, weather reporting and military communications. However, business applications have generally moved to the Internet.

Some of the earliest computers used teletype machines for input and output. This was the origin of the command line interface, i.e., the text-only mode of communicating with a computer. Users typed commands after a prompt character appeared. The punched tape capabilities of the teletypewriters were often used to capture computer output and for the off-line preparation of input.

The most modern form of teletypewriters are completely electronic and utilize an electronic display screen instead of a printer. They are still in use by the deaf for typed communications over the telephone.

In computing, especially in Unix-like operating systems, the legacy of teletypewriters lives on in the designations for serial ports and consoles (i.e., the text-only display mode), which have the prefix tty, such as /dev/tty5 for the fifth virtual console.

Created August 25, 2005.
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