Baudot code - Details

A code for transmitting verbal messages in telegraph systems, devised by Jean-Maurice-Emile Baudot in 1874, and originally known as the International Telegraph Code 1. Unlike Morse code, which uses short dots and long dashes, the Baudot code uses equal length electrical pulses, either on or off. Every letter, number or figure can be represented by a five-bit combination of these pulses. Two sets of 32 combinations were devised - one to represent letters and the other containing figures and punctuation. It is still in use today for international wire communications and for press and weather systems. Modern versions of the Baudot code usually operate with groups of seven or eight ‘on’ and ‘off’ signals. Groups of seven allow transmission of 128 characters. With groups of eight, one element may be used for error correction or other functions. A later system (multiplexing) allowed several messages to be sent along the same wire simultaneously. In multiplexing, each operator is connected to the wire for the exact amount of time it takes to transmit one letter of the Baudot code. By synchronizing the outgoing signals, it is possible to separate them and correctly read them when they are received. With the advent of digital computers in the 1960s, a new coding scheme, the American Standards Code for Information Interchange (ASCII) was developed and came to be widely used.

The Baudot code, named after its inventor Émile Baudot, is a character set predating EBCDIC and ASCII and used originally and primarily on teleprinters.

Baudot's original code, developed around 1874 is known as International Telegraph Alphabet No 1, and is no longer used.

Around 1901 Baudot's code was modified by Donald Murray (1865-1945) by re-ordering the characters, adding extra characters and shift codes.

A further modification of Murray's code mostly by Western Union consisted of dropping some characters.

Details

NOTE: This table presumes the space called "1" by Baudot and Murray is rightmost, and least significant.

In ITA2, characters are expressed using five bits. 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.

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.

US American implementations of Baudot code may differ in the use of ENQ, +, and f,g,h on the FIGS layer.

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.