TELETYPEWRITER EXCHANGE SYSTEM 



521 



during the normal transmission pericxi of the teletypewriter; that is, 

 the code and "break" signals. 



Three types of supervisory signals are required to be sent over the 

 inter-toll trunk. These are (1) the call signal, (2) the recall signal, 

 and (3) the disconnect signal. There is a fundamental difference, 

 however, between the call signal and the others in that it is applied 

 to the trunk only when the stations are not connected. When the 

 stations are connected the apparatus for receiving the call is removed 

 from Ihe trunk. The calling signal can therefore be any type of signal 



I 



Fig. 11 — Selector used for timing supervisory signals. 



with the limitation that it must be such that it will not be produced 

 by ordinary interruptions of the line, or line "hits." 



The three types of supervisory signals chosen are therefore : 



1. The call signal, produced bv sending a spacing signal of 2 seconds. 



2. The recall signal, produced bv sending a spacing signal of 7 seconds. 



3. The disconnect signal, produced by sending a spacing signal of 10 seconds. 



To permit sending these signals, use is made of a mechanism that 

 will measure the length of the signal. The basic apparatus used to 

 measure this time is the selector shown in Fig. 11. By the use of this 

 selector in conjunclion with a ground interrupted 60 times per minute, 



