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Teletype machines in World War II
A teleprinter (teletypewriter, Teletype or TTY for TeleTYpe/TeleTYpewriter) 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 (Telecommunications Devices for the Deaf) or TTY (although TTY, as indicated in the previous paragraph, refers to teleprinters in general).

The teleprinter evolved through a series of inventions by a number of engineers, including Royal Earl House, David E. Hughes, Edward Kleinschmidt, Charles Krum, Emile Baudot and Frederick G. Creed. 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 asynchronously 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.

Mark and space are terms describing logic levels in Teletype circuits. The native mode of communication for a Teletype is a simple series DC circuit that is interrupted, much as a rotary dial interrupts a telephone signal. The marking condition is when the circuit is closed, the spacing condition is when the circuit is open. The start of a character is signalled by a space. The stop bits are marking. When the line is broken, a Teletype cycles continuously but prints nothing because it is receiving all zeros, the ASCII (or Baudot) NUL character. Each Teletype circuit was leased from AT&T and consisted of twisted pair copper wires through ordinary telephone cables. These Teletype circuits were wired in series but were not connected to telephone switches.

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 edited off-line.

More than two teleprinters could be connected to the same wire circuit by means of a current loop. Communication by radio, RTTY, was also common. Amateur radio operators still use this communications mode.

Teletype machines

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Teletype model ASR-33
Teletype was a trademark of the Teletype Corporation of Skokie, Illinois, USA. The predecessor Morkrum Company was founded in 1906 by Charles Krum and Mr. Joy Morton (of Morton Salt). They made their first commercial installation of a printing telegraph with the Postal Telegraph Company in Boston and New York in 1910. It became popular with railroads, and the Associated Press adopted it in 1914 for their wire service. Morkrum merged with their competitor E.E. Kleinschmidt to become Morkrum-Kleinschmidt Corporation shortly before being renamed the Teletype Corporation. The company became part of AT&T in 1930.

The two parts kept their own peculiar design styles. "Teletype" machines tended to be large, heavy, and extremely robust-- capable of running non-stop for months at a time. In particular the Model 15 and Model 28 lines had very strong cast-iron frames, heavy-duty mechanisms, and heavy sound-proofed cases. The "Kleinschmidt" line tended to be somewhat more typewriter-like-- lighter, quieter-- more aluminum and less iron.

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
  • 20 - 1950s - upper/lower case printer machine with four rows of keys, using a six-bit code for TeleTypeSetter (TTS) use
  • 28 - 1950s - regarded as the most rugged machine Teletype ever built
  • 29 - 1950s - eight-bit machine using an IBM BCD code
  • 32/33 - 1961 - a low-cost, all-mechanical design. The 32 was Baudot, the 33 ASCII, but still upper case only. The 33 ASR was ubiquitous as a console device in the early minicomputer era
  • 35 - 1961 - an ASCII version of the model 28
  • 38 - 1970s - upper/lower case, wide carriage version of the model 33
  • Dataspeed 40 - late 1970s, used for Switching Control Center System and similar purposes
  • 42/43 - 1979 - an electronic, dot-matrix printer design, 42 being Baudot and 43 ASCII
Earlier Teletype machines had 3 rows of keys and only supported upper case letters. They used the 5 bit baudot code and generally worked at 60 words per minute. Teletypes with ASCII code were an innovation that came into widespread use in the same period as computers began to become widely available.

Speed, intended to be roughly comparable to words per minute, was the standard designation introduced by Western Union for a mechanical teleprinter data transmission rate using the 5-bit baudot code that was popular in the 1940s and for several decades thereafter. Such a machine would send 1 start bit, 5 data bits, and 1.42 stop bits. This unusual stop bit time was actually a rest period to allow the mechanical printing mechanism to recycle. Since modern computer equipment cannot easily generate 1.42 bits for the stop period, common practice is to either approximate this with 1.5 bits, or to send 2.0 bits while accepting 1.0 bits receiving.

For example, a 60 speed machine is geared at 45.5 baud (22.0 ms per bit), a 66 speed machine is geared at 50.0 baud (20.0 ms per bit), a 75 speed machine is geared at 56.9 baud (17.5 ms per bit), a 100 speed machine is geared at 74.2 baud (13.5 ms per bit), and a 133 speed machine is geared at 100.0 baud (10.0 ms per bit). Only 66 speed was in common use commercially for news agency wires and similar services, with migration to 100 speed as more reliable devices were introduced. Military users tended to operate at 60 speed, and the widespread availability of military surplus equipment during the 1960s made this the de facto standard for amateur radio RTTY operation. Most Teletype sound effects in existence today are at 60 speed, and mostly of the Model 15.

Another measure of the speed of a Teletype machine was in total operations per minute (OPM). For example, 60 speed was usually 368 OPM, 66 speed was 404 OPM, 75 speed was 460 OPM, and 100 speed was 600 OPM. Western Union Telexes were usually set at 390 OPM, with 7.0 total bits instead of the customary 7.42 bits.

A major difference should be noted between the majority of "teletypes" and the Model 26, 32, 33 and 38 series. All the older teletypes were built for heavy-duty 24-hour continuous use with only occasional oiling and cleaning, and an eventual lifetime of tens of thousands of hours until completely worn out. By contrast, the Model 26, 32, 33, and 38's were designed for light-duty use, just a few hours a day, and wearing out beyond economical refurbishment in just a few thousand hours. The older models were built with long-lasting parts: steel levers, ball-bearings, cast-iron frames. The 26, 32,33, and 38 were mostly "monkey-metal" castings, thin levers, plastic gears, and plastic cases.

The Model 15 stands out as one of a few machines that remained in production for many many decades. It was introduced in 1935 and remained in production until 1963, a total of 28 years of continuous production. Very few complex machines can match that record. To be fair, the production run was stretched somewhat by World War II-- the Model 28 was scheduled to replace the Model 15 in the mid-1940's, but Teletype built so many factories to produce the Model 15 during World War II, it was more economical to continue mass production of Model 15's for another couple of decades.

There were about 100,000 33-ASR Teletypes made in total. Now any personal computer equipped with a serial port can emulate the functionality of a Teletype. About the only feature that was required by Teletypes that has been generally abandoned is that a real Teletype required two stop bits to work reliably, so that each ASCII character (7 bits plus one parity bit) took 11 bit times. This is why 100 word per minute Teletypes transmitted at 110 baud. Today, most asynchronous serial data connections use one stop bit.


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, which in the United States was provided by Western Union. AT&T developed a competing network called TWX which initially also used rotary dialing and Baudot code, carried to the customer premises as pulses of DC on a metallic copper pair. TWX later became an ASCII-based service using Bell 103 type modems served over lines whose physical interface was identical to regular telephone lines. In many cases, TWX was a special class of service provided by the same telephone central office that handled voice calls. Telex is still in use in some countries for certain applications such as shipping, news, weather reporting and military command. Many business applications have moved to the Internet as most countries have discontinued telex/TWX services.
Further information: telegraphy


In addition to the 5-bit Baudot code and the much later eight-bit ASCII code, there was a six-bit code known as the TTS code (TeleTypeSetter) used by news wire services. A Model 20 Teletype machine with a punch ("reperforator") was installed at subscriber newspaper sites. Originally these machines would simply punch paper tapes and these tapes could be fed directly to a suitably equipped Linotype machine. In later years the incoming 6-bit current loop signal was coupled directly into a minicomputer or mainframe for editing and eventual feed to a phototypesetting machine.

Teleprinters in computing

Some of the earliest computers (for example, 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 text terminal and 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 text terminal, like an external console device, a user dialing in to the system on a modem on a serial port device, a printing or graphical computer terminal on a computer's serial port or the RS-232 port on a USB-to-RS-232 converter attached to a computer's USB port, or even a terminal emulator application in the window system using a pseudo terminal device. Such devices have the prefix tty, such as /dev/tty13, or pty (for pseudo-tty), such as /dev/ptya0.

Teleprinters in popular culture

  • The characteristic rhythmic "chunking" sound of a teleprinter in operation has long been audio shorthand for news, and countless television news themes have been based on musical emulations of the staccato teleprinter sound.
  • A song on Radiohead's album The Bends is entitled Planet Telex
  • A teleprinter-related malfunction is a crucial plot point in Michael Crichton's The Andromeda Strain.
  • A misprint caused by a bug falling into a teleprinter sets into motion the plot of Terry Gilliam's Brazil.
  • The before-its-time voice-recognition dictation machine the dying Ben Rand uses in 1979's Being There is a Teletype Model 40 VDT. (The sound of its printer performing a form feed--a comic effect in the film--is authentic.)
  • Teleprinters are visible on the sets of Murphy Brown, Mary Tyler Moore and WKRP in Cincinnati, among others.
  • Howard 100 News currently (2006-2007) uses the sound of teleprinters in the background during their news broadcasts.

See also

External links


typewriter is a mechanical, electromechanical, or electronic device with a set of "keys" that, when pressed, cause characters to be printed on a document, usually paper.
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A telecommunications device for the deaf (TDD) is an electronic device for text communication via a telephone line, used when one or more of the parties has hearing or speech difficulties.
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Royal Earl House (September 9, 1814 - February 25, 1895) was the inventor of the first printing telegraph, which is now kept in the Smithsonian Institution. His nephew Henry Alonzo House is also a noted early American inventor.
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David Edward Hughes (16 May 1831 – 22 January 1900) was an accomplished musician and a professor of music as well as chair of natural philosophy at a ladies' seminary in Bardstown, Kentucky.
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Charles L. Krum was a key figure in the development of the Teletype, a machine which played a key role in the history of telegraphy and computing.

In 1902, electrical engineer Mr. Frank Pearne approached Mr.
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Jean-Maurice-Émile Baudot, (September 11 1845–March 28 1903), French telegraph engineer and inventor of the Baudot code, was one of the pioneers of telecommunications.
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Frederick George Creed (1871–1957) was a Canadian inventor, who worked in the field of telecommunications, and played an early role in the development of SWATH vessels, The CCGS Frederick G. Creed, a SWATH vessel, is named after him.
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Ticker tape was used by ticker tape machines, the Ticker tape timer, stock ticker machines, or just stock tickers.


The origin of the term ticker tape came from the sound made by the machine as it printed, and tape simply refers to the
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Telegraphy (from the Greek words (τηλη) = far and (γραφειν) = write) is the long-distance transmission of written messages without physical transport of letters, originally by changing something that could
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A stock exchange, share market or bourse is a corporation or mutual organization which provides facilities for stock brokers and traders, to trade company stocks and other securities.
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The Baudot code, named after its inventor Émile Baudot, is a character set predating EBCDIC and ASCII, and the root predecessor to International Telegraph Alphabet No 2 (ITA2), the teleprinter code in use until the advent of ASCII.
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The Baudot code, named after its inventor Émile Baudot, is a character set predating EBCDIC and ASCII, and the root predecessor to International Telegraph Alphabet No 2 (ITA2), the teleprinter code in use until the advent of ASCII.
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Asynchronous serial communication describes an asynchronous transmission protocol in which a start signal is sent prior to each byte, character or code word and a stop signal is sent after each code word.
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Fieldata was a pioneering computer project run by the US Army Signal Corps in the late 1950s that intended to create a single standard for collecting and distributing battlefield information.
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The Friden Flexowriter, or flexowriter as on its nameplate, was a teleprinter based on a 1940s IBM product that was spun off as an independent company and later sold to the Friden Corp. It could punch and read 6-bit paper tape.
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rotary dial is a device mounted on or in a telephone or switchboard that is designed to send interrupted electrical pulses, known as pulse dialing, corresponding to the number dialed. The early form of the rotary dial used lugs on a finger plate instead of holes.
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American Standard Code for Information Interchange (ASCII), generally pronounced ask-ee IPA: /ˈæski/ ( [1] ), is a character encoding based on the English alphabet.
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The Baudot code, named after its inventor Émile Baudot, is a character set predating EBCDIC and ASCII, and the root predecessor to International Telegraph Alphabet No 2 (ITA2), the teleprinter code in use until the advent of ASCII.
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Punched tape or paper tape is a largely obsolete form of data storage, consisting of a long strip of paper in which holes are punched to store data. It was widely used during much of the twentieth century for teleprinter communication, and later as a storage medium for
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on-line and off-line have specific meanings with respect to computer technology and telecommunication. The concepts have however been extended from their computing and telecommunication meanings into the area of human interaction and conversation.
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A current loop describes two different electrical signalling schemes.


For digital serial communications a current loop is a communication interface that uses current instead of voltage for signaling.
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Radioteletype (RTTY) is a telecommunications system consisting of two or more teleprinters using radio as the transmission medium.

Early RTTY operators used equipment built for Bell System wire-linked Teletype(tm) systems.
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Amateur radio, often called ham radio, is both a hobby and a service that uses various types of radio communications equipment to communicate with other radio amateurs for public service, recreation and self-training.
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Skokie (formerly Niles Center) is a village in Cook County, Illinois, United States. It is a suburb of Chicago, located on the northwest border of Chicago.
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Morton Salt is a United States company specializing in the production of salt for food, water conditioning, industrial, agricultural, and road/highway use.

The company began in Chicago, Illinois in 1848 as a small sales agency.
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Nickname: Beantown, The Hub (of the Universe), The Cradle of Liberty, City on the Hill, Athens of America
Location in Suffolk County in Massachusetts, USA
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