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THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1956 



breaking down shunts the local holding circuit of the crosspoint causing 

 it to release. The — 130-volt disconnect signal is applied during the 

 release time of the slow release relay which is long enough to insure the 

 release of the crosspoint relay at the concentrator. 



The release circuit is individual to the trunk and independent of the 

 signal sent over the control pairs. 



g. Pulse Signalling Circuits 



To control the concentrator four distinct pulse signals are transmitted 

 from the central office. Two of these at times must be transmitted 

 simultaneously, bvit these and the other two are transmitted mutually 

 exclusively. In addition, service request and line busy signals are trans- 

 mitted from the concentrator to the central office. The two way trans- 

 mission of information is accomplished on each pair by sending signals in 

 each direction at different times and inhibiting the receipt of signals 

 when others are being transmitted. 



To transmit four signals over two such pairs, both positive and nega- 



CONTROL 

 PAIR NO. 1 



VF 



M 



LB 



D 



SR 



-16V 



VG 



CONTROL 

 PAIR NO. 2 



16 V 



M 



CONCENTRATOR 

 AMPLIFIERS 



I 



CENTRAL OFFICE 



AMPLIFIERS 



PER CONCENTRATOR 



Fig. 15. — Signal transmission circuit. 



