1100 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMRER 1952 



from translation of the three office code digits. The connector for making 

 the momentary connection of the large number of leads reciuired between 

 the senders and decoders presented new problems which were solved by 

 the development of new relay preference and lockout circuits to permit 

 as many simultaneous connections between senders and decoders as there 

 were decoders and to permit an even distribution of calls to decoders. 

 Decoder circuits were completely self-checking for trouble, provided for 

 second trial in another decoder when trouble was discovered, and re- 

 corded troubles on a lamp bank trouble indicator. 



In the early 1930's, encouraged by the success of decoders, the Bell 

 System started development of the No. 1 crossbar system with markers 

 in both originating and terminating equipments and with improved 

 features over the coordinate system which it resembled in many respects. 

 Self-checking circuits, second trials and trouble indicators which had 

 proven highly successful in the decoder type panel system were important 

 features of No. 1 crossbar. Automatic alternate routing and the ability 

 to operate with non-consecutive PBX assignments were major new 

 features introduced in this system for the first time. 



The subsequently developed No. 5 crossbar system included a number 

 of improvements, the chief of which from a common-control standpoint 

 was the use of common markers for originating and terminating business 

 and the use of the call back feature in setting up the connection. In this 

 system the common equipment records the calling line identification as 

 well as the called number, and after setting up to the called line or 

 outgoing trunk, breaks down the connection to the common equipment 



ORIGINATING EQUIPMENT 



I TERMINATING EQUIPMENT 



CALLED 

 TELEPHONE 



DISTRICT 

 JUNCTOR 



rv 



PRI SEC 

 LINE LINK FRAME 

 (used FOR BOTH 



originating and 



terminating 



calls) 



LINK / 

 / 

 OUTGOING^/ 

 TRUNK 



ORIGINATING 

 SENDER 



sender 



LINK 



ORIGINATING 

 MARKER 



Fig. 4 — No. 1 crossbar. 



INCOMING 

 TRUNK 



TERMINATING 

 SENDER 



SENDER 

 LINK 



TERMIN- 

 ATING 

 MARKER 



