Baudot-Code

The Baudot code (also telex telex code or code ), named after Jean -Maurice -Émile Baudot ( 1845-1903 ) is a digital, originally synchronous 5-bit character code and was later standardized as CCITT -1. The Baudot code is the ancestor of today's seven-bit ASCII code.

Today is known as Baudot code commonly by Donald Murray (1865-1945) for use with an alphanumeric keypad adapted version (CCITT -2). This took place in the telegraph and telex operation use around the world and is also known as Baudot - Murray - Murray code or code.

• 3.1 Entering Characters
• 3.2 coding
• 3.3 Application

Prehistory

After development of the electric telegraph and of Morse code in the first half of the 19th century sought ways to enable a simpler, more direct transmission of text, without having to leave the text by the operator in the telegraph stations to transmit convert into a code. Engineers and inventors have sought to develop methods to make this code transformation, carried out by the telegraph devices themselves, so automatically. The efforts were aimed to transfer a character (or any character ) at once by telegraph lines.

Example, it was trying to convey the information about the respective characters with different voltages on a telegraph line. Other methods have been devised that use 26 lines simultaneously - one for each letter. Also codes have been developed to reduce the number of required voltage values ​​and lines. Here presented particularly 5 - and dar. 6 -bit codes as a favorable compromise

Émile Baudot succeeded - based on the already -made developments - to develop a device that was using a 5 -bit codes not only able at the receiver side to print the message text directly on a piece of paper, but also several telegrams in a multiplex system could transmit over a single telegraph line simultaneously.

The early Baudot code ( CCITT 1)

Operation

The original Baudot code ( International Telegraph Alphabet No. later. 1 ( ITA1 ), CCITT -1 ) was designed by Émile Baudot in 1870 for a system developed by his telegraph machine. The code has been entered directly via a piano-like keyboard with five keys, the pressing or not pressing a button corresponding to the setting or not setting a bit in the to be transmitted 5 -bit word ( the character code ). For this, the keyboard has been operated with the index and middle fingers of the left and with the index, middle and ring fingers of the right hand. The characters to be sent to the corresponding keys had to be pressed simultaneously and locked one for a moment, until the combination sent from the device as a sequence of current pulses and the keyboard for the next character were released. In this way speeds of 180 characters per minute were obtained.

The fact that, unlike the Morse code, all characters are represented by a code of the same length, was a heavy decryption significantly easier to implement and feasible with the resources available. Thus, a daisy wheel was placed depending on the received code by an electromechanical device in a corresponding position to print the associated character on a piece of paper in Baudots telegraph receiver.

Encoding

Since there are five buttons, each of which can be either pressed or not pressed, only 32 (= 25 ) different keyboard shortcuts (31 if you do not count the rest position of the keyboard), would not be coded once all 26 letters plus 10 digits, when Baudot would not have introduced an escape code, the double occupancy of almost all combinations allowed: he defined two spaces. When one was sent, the next character to a table should be interpreted with letters, by one, a table with numbers and symbols should not be used.

The Baudot code has been designed ergonomically for ease of learning, it remains, for example, for the vowels of the part code of the left hand equal and consecutive characters often follow a certain stitch pattern. In the table, "·" for a non- compressed, and "•" for a depressed key or a 0 bit and a 1 bit. The numbering of the bits was chosen according to the finger: 5 4 3 2 1

More history

Baudots telegraph system was adopted in 1875 by the French administration. After a first test operation between Paris and Bordeaux was successfully carried out in November 1877, the system was held in the following years in Europe and later also outside of Western and Central Europe, in countries such as Russia and Argentina dissemination. Since the available characters are not satisfied the needs of other telecommunications administrations, there were different national character assignments. The CCITT International Alphabet No later than. 1 standard version differs in the assignment of the special character of Baudots version, the position of the 26 letters and the numbers 0 to 9, however, always remained the same.

Baudots code was the first bit-oriented character code that represented each character with the same number of bits, and thus the precursor of modern computer codes such as EBCDIC, ASCII and Unicode. After the Baudot baud unit is named. The baud rate is a measure for the speed step.

The Baudot - Murray Code (CCITT -2)

Character input

To help you type characters, Donald Murray developed around 1901 a keyboard similar to a typewriter, the punched by pressing a single button, the associated string of five bits in a paper tape. This tape immediately ran into a station that read the code punched and sent out. Murray's devices were very fast, so a speed of 1260 characters per minute was achieved in a test operation with the UK Post 1908. This device was the forerunner of modern telegraph, working with an alphanumeric keypad and an alphanumeric printing unit.

Encoding

Murray maintained the level switch Baudots, but put the order of the characters in the code so that frequently used letters, such as " E" and " T" code positions were, for the transmission and reception, the mechanics in the devices shorter and less often be moved had. Thus, the wear and the need for maintenance is reduced. Murray also added a new-line character, and to subdivide telegrams can.

How Baudots was often changed over time in order to satisfy local needs and Murray's code in the area outside of letters and numbers. An important change was that spaces and letters / numbers change and were separated from each other into separate characters. To chart recorders (devices that write row by row on a sheet, rather than endlessly on a strip ) be able to control, control characters for carriage return and line feed were added. Then there was a code to trigger an automatic namesake on the remote machine in order to make sure even with unmanned remote stations can, with whom they were connected ( Who's there? ), And the ability to control a bell to attract the attention of operating personnel.

This so modified code was standardized in 1932 by the CCITT as International Telegraph Alphabet No. 2 (short CCITT -2 or ITA2 ). Here some code positions were reserved for national extensions. In the table, " 0" for a pause step, hibernation and "1 " represents a current step or a mechanical movement.

Standardized representation of telex characters in CCITT -2 and MTK -2. The perforation as well as the dispatch of telex characters are: Start- 1.FsZ - 2.FsZ - ( sprocket holes ) 3.FsZ. - 4.FsZ. -5- FsZ. -1, 5 stop steps. .

In the left table, the order in accordance with the CCITT -2 is shown in alphabetical order. The right table shows the teletype characters in order of severity, that is, letter E has the hole 1-0 -. -0 -0-0 corresponds to the first decimal

Application

The CCITT - 2 code has become a standard code in telex networks. It was the synchronous data transmission originally used, which would imply that the sending and the receiving device always ran exactly the same timing, replaced by an asynchronous. To this were added before and after each character start and stop bits to allow the receiving device to synchronize again with each incoming characters. Telegraph in telex networks were operated with fixed -network speeds of about 400 to 600 characters per minute.

Also, the CCITT -2 code has been adapted and extended. In the USSR, 1963 МТК -2 was introduced ( among other things with Cyrillic letters). In particular, in the United States by the CCITT - 2 code slightly different character assignments were used in different networks. So it was, for example, special character sets for a weather network and an exchange network. Elsewhere, the code has been extended. In order to represent more characters, a third level was introduced for the representation of Greek or Cyrillic characters, for use in the pleading even more. The ability to use the free code positions held for the German umlauts, was not perceived in the German telex network.