Teleprinter

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 E. Hughes, Edward Kleinschmidt, 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 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 like 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.

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

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 to Teletype. The company became part of AT&T 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
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 29
38 - 1970s - upper/lower case, wide carriage version of the model 33
42/43 - 1979 - an electronic, dot-matrix printer design

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 60.0 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.

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.

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. AT&T developed a competing network called TWX. 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 and TWX networks, see telegraphy.

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.

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, 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.

External links

Patents

"Telegraph printer" (Type 12 Teletype), filed June 1924, issued April 1928
"Telegraph receiver" (Type 14 Teletype), filed December 1924, issued February 1930
"Signalling system and apparatus therefor" (Type 15 Teletype) - filed July 1930, issued April 1933

Telecommunications history | Typewriters

Gourt Home::Gourt :: Teleprinter