The Baudot code, named after its inventor Émile Baudot, is a character set predating EBCDIC and ASCII, and the root predecessor to International Telegraph Alphabet No 2 (ITA2), the teleprinter code in use until the advent of ASCII. Each character in the alphabet is represented by a series of bits, sent over a communication channel such as a telegraph wire or a radio signal. Baudot's original code, developed around 1874, is known as International Telegraph Alphabet No 1, and is no longer used. It was sent using a five-key keyboard where each key represented one bit of the five state signal. A mechanical wiper would scan the keyboard state and unlock the keys allowing the operator to enter the next character. Around 1901 Baudot's code was modified by Donald Murray (1865-1945) by re-ordering the characters, adding extra characters and shift codes. Murray's re-ordering of the characters was prompted by his development of a typewriter-like keyboard. Since the layout of the bits was now disassociated from the keypress of the operator, Murray could arrange his code-set so that the most frequently used characters in English (E,T,A, etc) resulted in the fewest state transitions, minimizing wear on the equipment. A further modification of Murray's code mostly by Western Union consisted of dropping some characters. This final modification is what is generally known as the 'Baudot code', also known as the International Telegraph Alphabet No 2 (ITA2). ITA2 is still used in TDDs and some amateur radio applications, such as radioteletype ("RTTY"). (Baudot's code was never used in any teleprinter equipment; nearly all 20th century teleprinter equipment used Western Union's code, ITA2, or variants thereof. Radio amateurs casually call ITA2 and variants "baudot" incorrectly, and even the American Radio Relay League's Amateur Radio Handbook does so, though in more recent editions the tables of codes correctly identifies it as ITA2.)
Details
NOTE: This table presumes the space called "1" by Baudot and Murray is rightmost, and least significant. The actual order of transmission varied by manufacturer.
In ITA2, characters are expressed using five bits. ITA2 uses two code sub-sets, the "letter shift" (LTRS), and the "figure shift" (FIGS). The FIGS character (11011) signals that the following code is to be interpreted as being in the FIGS set, until this is reset by the LTRS (11111) character. "ENQuiry" will trigger the other machine's answerback. It means "Who are you?" CR is carriage return, LF is line feed, BEL is the bell character which rang a small bell (often used to alert operators to an incoming message), SP is space, and NUL is the null character (blank tape). Note: the binary conversions of the codepoints are often shown in reverse order, depending on (presumably) from which side you are viewing the papertape. Note further that the "control" characters were chosen so that they were either symmetric or in useful pairs so that inserting a tape "upside down" did not result in problems for the equipment and the resulting printout could be deciphered. Thus FIGS (11011), LTRS (11111) and space (00100) are invariant, while CR (01000) and LF (00010), generally used as a pair, result in the same output when the tape is reversed. LTRS could also be used to overpunch characters to be deleted on a paper tape (much like DEL in 7-bit ASCII). The sequence RYRYRY... is often used in test messages, and at the start of every transmission. Since R is 01010 and Y is 10101, the sequence exercises much of a teleprinter's mechanical components at maximum stress. Also, at one time, fine-tuning of the receiver was done using two coloured lights (one for each tone). 'RYRYRY...' produced 0101010101..., which made the lights glow with equal brightness when the tuning was correct. On a QWERTY keyboard the top row will also be the numbers 1234567890. US implementations of Baudot code may differ in the use of ENQ, +, and f,g,h on the FIGS layer. The above table represents the official ITA2 code. The Russian version of Baudot code (MTK-2) used three shift modes; the Cyrillic letter mode was activated by the character (00000), unused in original ITA2. Because of the larger number of characters in the Cyrillic alphabet, the characters !, &, £, and BEL were omitted and replaced by Cyrillics.
References
- Five-unit codes
- An examination of Baudot code in respect to other character codes by Tom Jennings
- MTK-2 code table
See also
This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.
