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The Baudot Code
In digital telegraphy (teletype, telex) a standard 5-bit code is commonly used to represent a character (letter, number or punctuation mark). This is often called the Baudot code, although this name is actually wrong. The official name for the latest telegraphy standard is ITA2 (International Telegraph Alphabet No 2). It has been in use until it was replaced by ASCII in 1963. The most common Baudot code is also known as the Murray code.
 
The ITA2 Standard (CCITT-2)
Normal text consists of over 50 different characters (26 letters, 10 numbers, 10 punctuation marks and some control codes). In the ITA2 standard, 5 bits are used to represent a character, which means that we can create only 32 different codes (25). As this would not be sufficient to create normal text, most codes are used twice (i.e. have two different meanings) and special codes are used to 'shift' between the two code sets (letters and figures, or LTRS and FIGS).

The table below shows the interpretation of the commonly used ITA2 standard. Although this code has officially been superceeded by ASCII, it is still in use on some old telex networks and by Radio Amateurs. Some of the cipher machines described on this website, use 5-bit digital telegraphy and most of these use the ITA2 standard.

LTRS-shift is represented by 11111 (5 holes), so that it can be used to wipe part of a paper tape, without affecting the rest. Normally, a paper tape would start with two LTRS characters, to ensure that the teletype is in Letter-mode.
 
Ltr Letters (A-Z)
Fig Figures (Numbers and punctuation marks)
Ctrl Control characters
Hex Hexadecimal code
Bin Binary, Positions of the holes in the paper tape

 
# Ltr Fig Hex Bin Description
0 NUL 00 00000 NULL, Nothing (blank tape)
1 E 3 01 10000  
2 LF 02 01000 Line Feed (new line)
3 A - 03 11000  
4 SP 04 00100 Space
5 S ' 05 10100  
6 I 8 06 01100  
7 U 7 07 11100  
8 CR 08 00010 Cariage Return
9 D ENC 09 10010 Enquiry (Who are you?, WRU)
10 R 4 0A 01010  
11 J BEL 0B 11010 BELL (ring bell at the other end)
12 N , 0C 00110  
13 F ! 0D 10110  
14 C : 0E 01110  
15 K ( 0F 11110  
16 T 5 10 00001  
17 Z + 11 10001  
18 L ) 12 01001  
19 W 2 13 11001  
20 H $ 14 00101 Currency symbol (can also be £)
21 Y 6 15 10101  
22 P 0 16 01101  
23 Q 1 17 11101  
24 O 9 18 00011  
25 B ? 19 10011  
26 G & 1A 01011  
27 FIGS 1B 11011 Figures (Shift on)
28 M . 1C 00111  
29 X / 1D 10111  
30 V ; 1E 01111  
31 LTRS 1F 11111 Letters (Shift off)

 
Cipher machines that use ITA2

Other standards

Further information

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Copyright 2009-2013, Paul Reuvers & Marc Simons. Last changed: Sunday, 23 June 2013 - 15:52 CET
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