thumb|Teletype machines in World War II thumb|A Teletype Model 33 ASR teleprinter, with punched tape reader and punch, usable as a computer terminal

A teleprinter (teletypewriter, Teletype or TTY) is a electromechanical typewriter that can be used to communicate typed messages from point to point and point to multipoint over a variety of communication channels that range from a simple electrical connection, such as a pair of wires, to the use of radio and microwave as the transmission medium. They could also serve as a command line interface to early mainframe computers and minicomputers, sending typed data to the computer with or without printed output, and printing the response from the computer.

Teleprinters are now largely obsolete, though they are still widely used in the aviation industry (AFTN and airline teletype system), and variations called Telecommunications Devices for the Deaf (TDDs) are still used by the hearing impaired for typed communications over ordinary telephone lines. In computing teleprinters have been replaced by fully electronic computer terminals which usually use a display screen instead of a printer, though the term "TTY" is still occasionally used to refer to them, such as in Unix systems.

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.

Ways in which teleprinters were used

There were at least five major types of teleprinter networks:

Exchange systems such as Telex and TWX. These created a real-time circuit between two machines, so that anything typed on one machine appeared at the other end immediately. US and UK systems had actual telephone dials; German systems did "dialing" via the keyboard. Typed "chat" was possible, but because billing was by connect time, it was common to prepare messages on paper tape and transmit them without pauses for typing.

Leased line and radioteletype networks arranged in point-to-point and / or multipoint configurations to support data processing applications for government and industry such as integrating the accounting, billing, management, production, purchasing, sales, shipping and receiving departments within an organization to speed internal communications.

Message switching systems. This was an early form of E-mail, done with electromechanical gear. See Telegram, Western Union, Plan 55-A. Military organizations had similar but separate systems. See Autodin.

Broadcast systems such as weather information distribution and "news wires". See Associated Press, National Weather Service, Reuters, and United Press (later UPI).

"Loop" systems, where anything typed on any machine on the loop printed on all the machines. Police departments used such systems to interconnect precincts.

Teleprinter operation

Most teleprinters used the 5-bit Baudot code (also known as ITA2). This limited the character set to 32 codes (2⁵ = 32). One had to use a "FIGS" shift key to type numbers and special characters. Special versions of teleprinters had FIGS characters for specific applications, such as weather symbols for 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 synchronize mechanically). Other codes, such as ASCII, Fieldata and Flexowriter, were introduced but never became as popular as Baudot.

''Mark'' and ''space'' are terms describing logic levels in teleprinter circuits. The native mode of communication for a teleprinter 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 (current is flowing), the spacing condition is when the circuit is open (no current is flowing). The "idle" condition of the circuit is a continuous marking state, with the start of a character signalled by a "start bit", which is always a space. Following the start bit, the character is represented by a fixed number of bits, such as 5 bits in the Baudot code, each either a mark or a space to denote the specific character or machine function. After the character's bits, the sending machine sends one or more stop bits. The stop bits are marking, so as to be distinct from the subsequent start bit. If the sender has nothing more to send, the line simply remains in the marking state (as if a continuing series of stop bits) until a later space denotes the start of the next character. The time between characters need not be an integral multiple of a bit time, but it must be at least the minimum number of stop bits required by the receiving machine.

When the line is broken, the continuous spacing (open circuit, no current flowing) causes a receiving teleprinter to cycle continuously, even in the absence of stop bits. It prints nothing because the characters received are all zeros, the Baudot blank (or ASCII) null character.

Teleprinter circuits were generally leased from a communications common carrier and consisted of twisted pair copper wires through ordinary telephone cables that extended from the teleprinter located at the customer location to the common carrier central office. These teleprinter circuits were connected to switching equipment at the central office for Telex and TWX service. Private line teleprinter circuits were not directly connected to switching equipment. Instead, these private line circuits were connected to network hubs and repeaters configured to provide point to point or point to multipoint service. More than two teleprinters could be connected to the same wire circuit by means of a current loop.

Earlier Teletype machines had three 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 U.S. Federal Communications Commission (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.

Both wire-service and private teleprinters 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 10 bells was a FLASH, used only for very important news.

The teleprinter circuit was often linked to a 5-bit 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 code 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 saved in machine-readable form and edited off-line.

Communication by radio, RTTY, was also common. Amateur radio operators continue to use this mode of communication today.

Control characters

A typewriter or electromechanical printer can print characters on paper, and execute operations such as move the carriage back to the left margin of the same line (carriage return), advance to the same column of the next line (line feed), and so on. Commands to control non-printing operations were transmitted in exactly the same way as printable characters by sending control characters with defined functions (e.g., the ''line feed'' character forced the carriage to move to the same position on the next line) to teleprinters. In modern computing and communications a few control characters, such as carriage return and line feed, have retained their original functions (although they are often implemented in software rather than activating electromechanical mechanisms to move a physical printer carriage) but many others are no longer required and are used for other purposes.

"Here is" key

Some teleprinters had a "Here is" key, which transmitted a fixed sequence 20 or 22 characters, programmable by breaking tabs off a drum. This sequence could also be transmitted automatically upon receipt of an ENQ (control E) signal, if enabled. This was commonly used to identify a station; the operator could press the key to send the station identifier to the other end, or the remote station could trigger its transmission by sending the ENQ character, essentially asking "who are you?"

Manufacturers and models

Morkrum

Morkrum 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 Kleinschmidt Electric Company to become Morkrum-Kleinschmidt Corporation shortly before being renamed the Teletype Corporation.

Teletype

Teletype was a trademark of the Teletype Corporation of Skokie, Illinois, and was the successor to the Morkrum-Kleinschmidt Company. Despite its longlasting trademark status, the word went into common generic usage in the news and telecommunications industries. Records of the United States Patent and Trademark Office indicate the trademark has expired and is considered dead.

Teletype and Kleinschmidt competed for many decades following, each concentrating on their strengths. "Teletype" machines tended to be large, heavy, and extremely robust, capable of running non-stop for months at a time if properly lubricated. In particular the Model 15 and Model 28 lines had very strong frames (cast iron in the Model 15; resilient sheet metal "plates" in the Model 28), 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. While Teletype Corp. developed a strong civilian customer base in addition to their military products, Kleinschmidt tended to be satisfied with the United States Signal Corps as their primary customer.

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 suffixed the configuration to the model number, e.g., "Model 33 ASR" (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". The U.S. military had their own system of identifying the various models, often identifying various improvements, included options / features, etc. The TT-47/UG was the first Model 28 KSR, and while Teletype's designation for the basic machine remained the same over the next 20+ years, the TT-47/UG took on suffixes to identify the specific version. The last TT-47/UG was the TT-47L/UG. The U.S. Navy also assigned some "set" designations using the standard Army/Navy system, such as the AN/UGC-5, a Teletype Model 28 ASR which has a keyboard, printer, tape punch and reader facilities all in one cabinet. Major models and their dates:

12 (1922) – First general purpose page teleprinter. Baudot code. (Morkrum) Based on an Underwood typewriter mechanism.

14 (1925) – A family of devices, printing, reading or punching narrow tapes; Baudot code. About 60,000 were built.

15 (1930) – Baudot code page printer; the mainstay of U.S. military communications in WWII. About 200,000 were built; a reliable, heavy-duty machine with a cast frame.

20 (1950s) – Model 15 variant; upper/lower case printer machine with four rows of keys, using a six-bit code for TeleTypeSetter (TTS) use

26 (1946) – Baudot code page printer; a lower-cost machine using a typewheel. The platen and paper moved while typing, like a manual typewriter.

28 (1950s) – Baudot code page printer; regarded as the most rugged machine Teletype ever built. Used a moving type block for printing.

29 (1950s) – Eight-bit machine using an IBM BCD code

32/33 (1963) – A low-cost, all-mechanical design; used a type cylinder for printing. Many plastic parts. The Model 32 was Baudot, the Model 33 was ASCII, but still upper-case only. The Teletype Model 33 ASR was ubiquitous as a console device in the early minicomputer era. About 100,000 were made.

35 (1963) – 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

The Model 15 stands out as one of a few machines that remained in production for many years. 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. 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 the Model 15. The Model 15, in its RO (Receive Only, no keyboard) version was the classic "news Teletype" for decades.

The last vestiges of what had been Teletype Corporation ceased in 1990, bringing to a close the dedicated teleprinter business.

Creed & Company

Creed & Company, a British company, built teleprinters for the GPO's teleprinter service.

Creed model 7B

Creed model 7

Creed model 7/RP (teleprinter reperforator)

Creed model 54

Creed model 75

Creed model 85 (reperforator)

Creed model 86 (reperforator)

Creed model 444 (GPO type 15)

Kleinschmidt

In 1931 Edward Kleinschmidt formed Kleinschmidt Labs to pursue a different type design of Teletype. In 1944 Kleinschmidt demonstrated their lightweight unit to the Signal Corps and in 1949 their design was adopted for the Army's portable needs. In 1956 Kleinschmidt Labs merged with Smith-Corona, which then merged with Marchant Calculators, forming the SCM Corporation. By 1979 the Kleinschmidt division was branching off into Electronic Data Interchange, a business in which they became very successful, and replaced the mechanical products, including teleprinters.

Kleinschmidt machines, with the military as their primary customer, used standard military designations for their machines. The teleprinter was identified with designations such as a TT-4/FG, while communication "sets" to which a teleprinter might be a part generally used the standard Army/Navy designation system such as AN/FGC-25. This includes Kleinschmidt teleprinter TT-117/FG and tape reperforator TT-179/FG.

Siemens & Halske

Siemens & Halske, a German company, founded in 1897. Teleprinter Model 100 Ser 1 (early 1960s) – Used for Telex service

Teleprinter Model 100 Ser. 11 – Later version with minor changes

Olivetti

Italian office equipment maker Olivetti (est. 1908) started to manufacture teleprinters in order to provide Italian post offices with modern equipment to send and receive telegrams. The first models typed on a paper ribbon, which was then cut and glued into telegram forms.

Olivetti T1 (1938 - 1948)

Olivetti T2 (1948 - 1968)

Olivetti Te300 (1968 - 1975)

Olivetti Te400 (1975 - 1991)

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.

Teletypesetter

In addition to the 5-bit Baudot code and the much later seven-bit ASCII code, there was a six-bit code known as the TTS code (Teletypesetter) used by news wire services. Through the use of "shift in" and "shift out" codes, this six-bit code could represent a full set of upper and lower case characters, digits, symbols commonly used in newspapers, and limited typesetting instructions such as "flush left" or "center." 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 read by a tape reader attached to a Linotype machine, creating type for printing in newspapers and magazines. In later years the incoming 6-bit current loop signal was coupled directly into a minicomputer or mainframe for storage, editing, and eventual feed to a phototypesetting machine.

==Teleprinters in computing== Computers used teleprinters for input and output from the early days of computing. Punched card readers and fast printers replaced teleprinters for most purposes, but teleprinters continued to be used as interactive time-sharing terminals until video displays became widely available in the late 1970s.

Users typed commands after a prompt character was printed. Printing was unidirectional; if the user wanted to delete what had been typed, further characters were printed to indicate that previous text had been cancelled. When video displays first became available the user interface was initially exactly the same as for an electromechanical printer; expensive and scarce video terminals could be used interchangeably with teleprinters. This was the origin of the text terminal and the command line interface.

Paper tape was sometimes used to prepare input for the computer session off line and to capture computer output. The popular ASR-33 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 early operating systems such as Digital's RT-11, serial communication lines were often connected to teleprinters and were given device names starting with tt. This and similar conventions were adopted by many other operating systems. Unix and Unix-like operating systems use the prefix tty, for example /dev/tty13, or pty (for pseudo-tty), such as /dev/ptya0. In many computing contexts, "TTY" has become the name for any text terminal, such as 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.

Teleprinters were also used to record fault printout and other information in some TXE telephone exchanges.

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.

In the 1980s, packet radio became the most common form of digital communications used in amateur radio. Soon, advanced multimode electronic interfaces such as the AEA PK-232 were developed, which could send and receive not only packet, but various other modulation types including Baudot. This made it possible for a home or laptop computer to replace teleprinters, saving money, complexity, space and the massive amount of paper which mechanical machines used.

As a result, by the mid-1990s, amateur use of actual Teletype machines had waned, though a core of "purists" still operate on equipment originally manufactured in the 1940s, 1950s, 1960s and 1970s, a testament to the workmanship and durability of this equipment.

See also

Letter-quality printer

Plan 55-A, a message switching system for telegrams.

Radioteletype

Siemens and Halske T52 - the ''Geheimfernschreiber'' (''secrets teleprinter'')

References

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 "Frequency-Shift Teletypewriter" - filed August 1966, issued April 1970

Further reading

"Teletype Messages Sent Through Switch Board", April 1932, Popular Mechanics AT&T; offering two way service through switchboards

External links

A first-hand report of Teletype Corporation's early years

A Gallery of Teletype Images

History of Teletypewriter Development by R.A. Nelson

"Some Notes on Teletype Corporation"

Mass.gov: TTY explanation and government best practices for TTY use

Category:History of telecommunications Category:Telegraphy Category:Typewriters Category:Impact printing Category:Deaf culture in the United States

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