Teleprinter

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Teletype machines in World War II
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).

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[edit] History

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.

[edit] 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, so as to be distinct from the subsequent start bit. When the line is broken, a Teletype cycles continuously but prints nothing because it is receiving all zeros, the ASCII (or Baudot) null 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 (or "reperforator") 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 mode of communication.

[edit] Teletype machines

Teletype model ASR-33
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 last vestiges of what had been Teletype Corporation ceased in 1990. [1]

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.

Teletype machines were given a model number, often modified 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)

Teletype Corporation documents invariably prefixed the configuration to the model number, e.g. "M33ASR" (Model 33 Automatic Send and Receive). In contrast, some customers and users tended to place the configuration before the model number, e.g. "ASR-33".

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; used a type block for printing
  • 29 - 1950s - eight-bit machine using an IBM BCD code
  • 32/33 - 1961 - a low-cost, all-mechanical design; used a type wheel for printing. 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
  • 37 - 1973 - upper/lower case, 150 baud version of the model 35
  • 38 - 1973 - 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). 60 speed became the de facto standard for amateur radio RTTY operation because of the widespread availability of equipment at that speed and the FCC restrictions to only 60 speed from 1953 to 1972. Telex, news agency wires and similar services commonly used 66 speed services. There was some migration to 75 and 100 speed as more reliable devices were introduced. However, the limitations of HF transmission such as excessive error rates due to multipath distortion and the nature of ionospheric propagation kept many users at 60 and 66 speed. 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 38s 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 mazak ("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-1940s, 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 15s 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.

Both wire-service and private teletypes had bells to signal important incoming messages and could ring 24/7 while the power was turned on. For example, ringing 4 bells on UPI wire-service machines meant an "Urgent" message; 5 bells was a "Bulletin"; and 11 bells was a FLASH, used only for very important news.

[edit] 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, 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 added a second ASCII-based service using Bell 103 type modems served over lines whose physical interface was identical to regular telephone lines. In many cases, the TWX service was provided by the same telephone central office that handled voice calls, using class of service to prevent POTS customers from connecting to TWX customers. 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

[edit] TeleTypeSetter

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.

[edit] 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.

[edit] Obsolescence of Teleprinters

Although printing news, messages, and other text at a distance is still universal, the dedicated teleprinter tied to a pair of leased copper wires was made functionally obsolete by the Fax, personal computer, inkjet printer, broadband, and the Internet.

[edit] Teleprinters in popular culture

  • The HTML element <tt> is referred to as "Teletype" and renders text in the browser's default monospaced font (usually Lucida Console or Courier).
  • The local and remote console client in UNIX system has the name of TTY console terminals.
  • 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 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.)
  • Howard 100 News currently (2006-2007) uses the sound of teleprinters in the background during their news broadcasts.
  • New York City radio station WINS uses the Teleprinter sound effect which plays in the background during news updates.

[edit] See also

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