Editing Teleprinter (section)

==Teleprinter operation==
[[Image:Baudotkeyboard.png|right|thumb|upright=1.85|[[Alphanumeric keyboard|Keyboard]] of a [[Baudot code|Baudot teleprinter]], with 32 keys, counting the space bar (only partially visible in this picture; the key left blank in this layout is not the space bar<ref>cf. page C-8 / Fig. C-3; PDF page 75; "[https://books.google.com/books?id=AuIOu97MduQC Principles of Telegraphy (Teletypewriter)]"</ref>)]]
[[File:International Telegraph Alphabet 2 brightened.jpg|thumb|upright=1.85|[[International Telegraph Alphabet No. 2]] development of the Baudot–Murray code]]

Most teleprinters used the 5-[[bit]] [[International Telegraph Alphabet No. 2]] (ITA2). This was limited to 32 codes (2<sup>5</sup> = 32), which was insufficient to encode all the alphabet as well as numbers and other characters. Therefore, the keyboard used the "FIGS" (for "figures") and "LTRS" (for "letters") keys to allow two encoding [[state (computer science)|state]]s, with a total of 60 codes by sending one of two reserved characters to indicate the state of the following encoded stream (the letters were single case only). Special versions of teleprinters had FIGS characters for specific applications, such as weather symbols for weather reports. The ITA2 code was used [[asynchronous start-stop|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|''Mark'' and ''space'']] are terms describing [[logic level]]s in teleprinter circuits. The native mode of communication for a teleprinter is a simple series [[direct current circuit|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 cable]]s that extended from the teleprinter located at the customer location to the common carrier [[telephone exchange|central office]]. These teleprinter circuits were connected to switching equipment at the central office for [[Telex]] and [[Telegraphy#TWX|TWX]] service. [[Private line]] teleprinter circuits were not directly connected to switching equipment. Instead, these private line circuits were connected to [[network hub]]s and [[repeater]]s 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 teleprinters had three rows of keys and only supported upper case letters. They used the 5 bit ITA2 code and generally worked at 60 to 100 words per minute. Later teleprinters, specifically the [[Teletype Model 33]], used ASCII code, an innovation that came into widespread use in the 1960s as computers became more widely available.

"Speed", intended to be roughly comparable to [[words per minute]], is the standard term introduced by [[Western Union]] for a mechanical teleprinter data transmission rate using the 5-bit ITA2 code that was popular in the 1940s and several decades thereafter. Such a machine would send 1 start bit, 5 data bits, and 1.42 stop bits. This unusual stop bit time is actually a rest period to allow the mechanical printing mechanism to synchronize in the event that a garbled signal is received.<ref>{{cite web |website=Sam's Telecomms Documents Repository |url=http://www.samhallas.co.uk/repository/telegraph/introduction_to_RTTY.pdf |title=Introduction to RTTY}}<!-- http://www.samhallas.co.uk/repository/telegraphy.htm --></ref> This is true especially on [[high frequency]] radio circuits, where selective fading is present. Selective fading causes the mark signal amplitude to be randomly different from the space signal amplitude. Selective fading, or [[Rayleigh fading]] can cause two carriers to randomly and independently fade to different depths.<ref>{{Cite web |url=http://w7ay.net/site/Technical/RTTY%20Demodulators/index.html |title=RTTY Demodulators}}</ref> 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 [[millisecond|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]] [[radioteletype|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 audio recordings in existence today are of teleprinters operating at 60 words per minute, and mostly of the Teletype Model 15.

Another measure of the speed of a teletypewriter 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 [[punched tape|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, known as ''[[radioteletype]]'' or ''RTTY'' (pronounced ''ritty''), was also common, especially among military users. Ships, command posts (mobile, stationary, and even airborne) and logistics units took advantage of the ability of operators to send reliable and accurate information with a minimum of training. [[Amateur radio]] operators continue to use this mode of communication today, though most use computer-interface sound generators, rather than legacy hardware teleprinter equipment.  Numerous modes are in use within the "ham radio" community, from the original ITA2 format to more modern, faster modes, which include error-checking of characters.

===Control characters===
{{Main|Control character}}
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.

===Answer back mechanism===
Some teleprinters had a "Here is" key, which transmitted a fixed sequence of 20 or 22 characters, programmable by breaking tabs off a drum. This sequence could also be transmitted automatically upon receipt of an [[Enquiry character|ENQ]] (control E) signal, if enabled.<ref name="Hereis">{{Cite web |url=http://www.pdp8online.com/asr33/pics/main_back.shtml |title=ASR 33 Teletype Rear View of Main Assembly |website=www.pdp8online.com}}</ref><ref name="AnotherHereis">{{Cite web |url=http://www.k7tty.com/development/teletype/model-32/index.html |title=TELETYPE MODEL 32ASR. |website=www.k7tty.com}}</ref> 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?"