284 



THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1956 



producing, during its transition, the last VF pulse for transmission to 

 the concentrator. 



d. Trunk Selection and. Identification 



The process of selecting an idle concentrator trunk to which the line 

 has access utilizes familar relay circuit techniques.^^ This circuit, in 

 Fig. 29, will not be described in detail. One trunk selection relay, TS, is 

 operated indicating the preferred idle trunk serving a line in the particu- 

 lar vertical group being selected as indicated by the VG relay which 

 has been operated by the marker. 



The TS4 and TS5 relays select trunks 8 and 9 which are available to 

 each line while the 4 trunks available to only half of the lines are selected 

 by relays TS0-TS3. The busy or idle condition of each trunk is indicated 

 by a contact on the hold magnet associated with each trunk through 



TRUNK 

 SELECTION 

 COMPLETE T 



VFLD 



_l 

 O 



cr 



I- 

 z 

 Oh 



^5 

 tr u 

 O cr 



Z : 



LU : 



^! 



o 

 o 



o 



t- 



VFP 



VGP_ 

 RS 



VFLI 



VFL 2 



0-1 



5-STAGE COUNTER 



VF4X -2V 



VGO 



y^ 



12-STAGE COUNTER 



0-0 



ST 



OPA 



R1A 



ST 



FROM SCANNER 

 PULSE GENERATOR I 



TP 



Fig. 28 — Line selection circuit. 



