Telegram Typewriter

[Ptolomeo Shop]

Initiallyfrom 1887 at the earliest, teleprinters were used in telegraphy.[1] Electrical telegraphy had been developed decades earlier in the late 1830s and 1840s,[2] then using simpler Morse key equipment and telegraph operators. The introduction of teleprinters automated much of this work and eventually largely replaced skilled operators versed in Morse code with typists and machines communicating faster via Baudot code.

With the development of early computers in the 1950s,[3] teleprinters were adapted to allow typed data to be sent to a computer, and responses printed. Some teleprinter models could also be used to create punched tape for data storage (either from typed input or from data received from a remote source) and to read back such tape for local printing or transmission. A teleprinter attached to a modem could also communicate through telephone lines. This latter configuration was often used to connect teleprinters to remote computers, particularly in time-sharing environments.

Teleprinters have largely been replaced by fully electronic computer terminals which typically have a computer monitor instead of a printer (though the term "TTY" is still occasionally used to refer to them, such as in Unix systems). Teleprinters are still widely used in the aviation industry (see AFTN and airline teletype system),[4] and variants called Telecommunications Devices for the Deaf (TDDs) are used by the hearing impaired for typed communications over ordinary telephone lines.

The teleprinter evolved through a series of inventions by a number of engineers, including Samuel Morse, Alexander Bain, Royal Earl House, David Edward Hughes, Emile Baudot, Donald Murray, Charles L. Krum, Edward Kleinschmidt and Frederick G. Creed. Teleprinters were invented in order to send and receive messages without the need for operators trained in the use of Morse code. A system of two teleprinters, with one operator trained to use a keyboard, replaced two trained Morse code operators. The teleprinter system improved message speed and delivery time, making it possible for messages to be flashed across a country with little manual intervention.[5]

There were a number of parallel developments on both sides of the Atlantic Ocean. In 1835 Samuel Morse devised a recording telegraph, and Morse code was born.[6] Morse's instrument used a current to displace the armature of an electromagnet, which moved a marker, therefore recording the breaks in the current. Cooke & Wheatstone received a British patent covering telegraphy in 1837 and a second one in 1840 which described a type-printing telegraph with steel type fixed at the tips of petals of a rotating brass daisy-wheel, struck by an "electric hammer" to print Roman letters through carbon paper onto a moving paper tape.[7] In 1841 Alexander Bain devised an electromagnetic printing telegraph machine. It used pulses of electricity created by rotating a dial over contact points to release and stop a type-wheel turned by weight-driven clockwork; a second clockwork mechanism rotated a drum covered with a sheet of paper and moved it slowly upwards so that the type-wheel printed its signals in a spiral. The critical issue was to have the sending and receiving elements working synchronously. Bain attempted to achieve this using centrifugal governors to closely regulate the speed of the clockwork. It was patented, along with other devices, on April 21, 1841.[8]

By 1846, the Morse telegraph service was operational between Washington, D.C., and New York. Royal Earl House patented his printing telegraph that same year. He linked two 28-key piano-style keyboards by wire. Each piano key represented a letter of the alphabet and when pressed caused the corresponding letter to print at the receiving end. A "shift" key gave each main key two optional values. A 56-character typewheel at the sending end was synchronised to coincide with a similar wheel at the receiving end. If the key corresponding to a particular character was pressed at the home station, it actuated the typewheel at the distant station just as the same character moved into the printing position, in a way similar to the (much later) daisy wheel printer. It was thus an example of a synchronous data transmission system. House's equipment could transmit around 40 instantly readable words per minute, but was difficult to manufacture in bulk. The printer could copy and print out up to 2,000 words per hour. This invention was first put in operation and exhibited at the Mechanics Institute in New York in 1844.

In France, mile Baudot designed in 1874 a system using a five-unit code, which began to be used extensively in that country from 1877. The British Post Office adopted the Baudot system for use on a simplex circuit between London and Paris in 1897, and subsequently made considerable use of duplex Baudot systems on their Inland Telegraph Services.[11]

In 1908, a working teleprinter was produced by the Morkrum Company (formed between Joy Morton and Charles Krum), called the Morkrum Printing Telegraph, which was field tested with the Alton Railroad. In 1910, the Morkrum Company designed and installed the first commercial teletypewriter system on Postal Telegraph Company lines between Boston and New York City using the "Blue Code Version" of the Morkrum Printing Telegraph.[16][17]

In 1916, Edward Kleinschmidt filed a patent application for a typebar page printer.[18] In 1919, shortly after the Morkrum company obtained their patent for a start-stop synchronizing method for code telegraph systems, which made possible the practical teleprinter, Kleinschmidt filed an application titled "Method of and Apparatus for Operating Printing Telegraphs"[19] which included an improved start-stop method.[20] The basic start-stop procedure, however, is much older than the Kleinschmidt and Morkrum inventions. It was already proposed by D'Arlincourt in 1870.[21]

Instead of wasting time and money in patent disputes on the start-stop method, Kleinschmidt and the Morkrum Company decided to merge and form the Morkrum-Kleinschmidt Company in 1924. The new company combined the best features of both their machines into a new typewheel printer for which Kleinschmidt, Howard Krum, and Sterling Morton jointly obtained a patent.[20]

A teleprinter system was installed in the Bureau of Lighthouses, Airways Division, Flight Service Station Airway Radio Stations system in 1928, carrying administrative messages, flight information and weather reports.[22] By 1938, the teleprinter network, handling weather traffic, extended over 20,000 miles, covering all 48 states except Maine, New Hampshire, and South Dakota.[23]

Teleprinters could use a variety of different communication channels. These included a simple pair of wires, public switched telephone networks, dedicated non-switched telephone circuits (leased lines), switched networks that operated similarly to the public telephone network (telex), and radio and microwave links (telex-on-radio, or TOR).

Before the computer revolution (and information processing performance improvements thanks to Moore's law) made it possible to securely encrypt voice and video calls, teleprinters were long used in combination with electromechanical or electronic cryptographic devices to provide secure communication channels. Being limited to text only was an acceptable trade-off for security.

Most teleprinters used the 5-bit International Telegraph Alphabet No. 2 (ITA2). This was limited to 32 codes (25 = 32). One had to use "FIGS" (for "figures") and "LTRS" (for "letters") keys to shift state, for a combined character set sufficient to type both letters and numbers, as well as some special characters. (The letters were uppercase only.) 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 ITA2 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 FIELDATA and Flexowriter, were introduced but never became as popular as ITA2.

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 ITA2 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 ITA2 blank (or ASCII) null character.

Teleprinter circuits were generally leased from a communications common carrier and consisted of 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.

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