282 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1956 



reset pulses to the concentrator so that, as described in Section 7a, 

 only the VF counter continues to step once each 0.010 sec. So long as 

 the line continues to request service this service request pulse is gated 

 to reset the RL bi-stable circuit within the same time slot that it was 

 set. If, however, a request for service is abandoned the RL bi-stable cir- 

 cuit of Fig. 26 will remain on and permit a TP pulse from the scanner 

 pulse generator to reset the HGT bi-stable circuit which initiated the 

 service request action. 



Whenever the RH bi-stable circuit of Fig. 26 is energized it closes a 

 gate over lead SRS for each concentrator to prevent any further service 

 request pulses from being recognized until the originating call which 

 has been registered is served. The resetting of the RH bi-stable circuit 

 occurs once the call has been served. When more than one line concen- 

 trator is being served it is possible that the HGT bi-stable circuit of 

 more than one concentrator will be set simultaneously as a result of 

 coincidence in service requests from correspondingly numbered lines in 

 these concentrators. The decision as to which concentrator is to be 

 served is left to the marker, as it would normally decide which horizontal 

 group to serve. 



c. Line Selection 



On all calls, originating and terminating, the marker transmits to the 

 frame circuits the complete identity of the line which it will serve. In 

 the case of originating calls it has received this information in the manner 

 described in Section 8b. In either case, it operates wire spring relays 

 VGO-U and VFO-4, which enable gates so that the information may be 

 stored in the 5- and 12-stage counters of the line selection circuit shown " 

 in Fig. 28. 



The process of reading into the line selection counters starts when 

 selection information has been received by the actuation of the HGS 

 bi-stable circuit in the concentrator control trunk circuit of Fig. 27. 

 This action stops the regular transmission of scanner pulses if they 

 have not been stopped as a result of a call origination. At the same time 

 it enables gates for transmission of information from the line selection 

 circuit. Fig. 28. 



The ST bi-stable circuit of the line selection circuit is also enabled 

 to start the process of setting the line selection counters. The next TP 

 pulse sets the Rl bi-stable circuit. This bi-stable circuit enables a gate 

 which permits the next TP pulse to set the counters and transmit a re- 

 set pulse to the concentrator through pulse amplifier RIA. At the same 

 time bi-stable circuit ST is reset to prevent the further read-in cr reset 



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