92 BELL SYSTEM TECHNICAL JOURNAL 



channel through the switch frames, and finally operate the proper 

 selecting and holding magnets of the crossbar switches to establish 

 the connections from the subscriber line to the trunk circuit. 



The "terminating markers" perform similar functions in the 

 terminating end of the office to set up the connection from the incoming 

 trunk circuit to the called subscriber line. They have access to all of 

 the subscriber lines terminating in the office, and to all crossbar 

 switch frames used for connecting to subscriber lines. 'They test the 

 called line to determine whether it is idle, and also test for and find an 

 idle channel through the switch frames and finally operate the proper 

 magnets of the crossbar switches and establish the connection to the 

 called subscriber line. 



In addition there are common "control" circuits associated with 

 the "line link" and the "sender link" frames for controlling the 

 operation of the switches on these frames. There are also the common 

 "connector" circuits, consisting mainly of multi-contact relays, which 

 are used for connecting the markers to the senders, to the switch 

 frames and to the test terminals of the called subscriber lines. 



It should be noted that the line link frames, although shown sepa- 

 rately, are used for both originating and terminating traffic. 



After the talking connection has been established between two 

 subscribers, all of the common control units, such as the senders, 

 markers, connectors, line link control circuit, and the sender link 

 frames and their associated control circuits, will have been released, 

 and the talking connection will be maintained in this condition by 

 the holding magnets of the crossbar switches used on the line link, 

 district, office and incoming link switch frames. These switch 

 magnets are held operated under control of the supervisory relays in 

 the district junctor and the incoming trunk circuits and are released 

 when the subscribers replace the receivers. 



Trunkinc; Arrangements 



The fundamental method of using the crossbar switch for setting 

 up connections is illustrated in Fig. 8. This figure shows a 200-point 

 crossbar switch with twenty vertical units each wired to a subscriber 

 line and ten trunks strapped horizontally across the switch. With 

 such an arrangement, any one of the twenty lines may be connected 

 to any one of the ten trunks. The number of lines which can be 

 connected to the same ten trunks may be increased to forty by adding 

 a second 200-point crossbar switch with twenty difi^erent lines con- 

 nected to its verticals and by wiring the horizontal contact multiple 

 of this second switch to the horizontal multiple of the switch shown 



