FUNDAMENTALS OF TELETYPEWRITERS 



621 



several dots and dashes for some of the less frequently used letters or 

 numerals. 



For machine operation it has thus far appeared desirable in order to 

 obtain simplest mechanisms and to obtain maximum operating speeds 

 with low line signaling frequencies, to have the signals for the different 

 characters of uniform length, that is, each contain the same number of 

 time units. This condition is met by the five-unit code where each 

 character is identified by the impulses in five units of time, and this is 

 the code normally employed in Bell System teletypewriters. Each of 

 the five units of this code may be either positive or negative, current 

 or no current, or either of two values of current, and the permutations 

 provided are 2'^ or 32. These are sufficient for the 26 letters of the 

 alphabet, a space, carriage return and paper feed signals as well as 

 case shifting signals to bring another set of characters into action so as 

 to include numerals and punctuation marks. A chart of this code as 

 used in Teletypewriter P^xchange Service (TWX), is shown below. 



Fig. 1 — Chart of five-unit TWX code. 

 The keyboard used for sending this code is shown below. 



Fig. 2 — Chart of TWX keyboard. 



It will be noted that this keyboard is similar to the ordinary type- 

 writer keyboard except that there are only three rows of keys instead 

 of four as in the typewriter. In the typewriter keyboard, the lower 

 three rows of keys are used ordinarily for small letters but when a shift 



