320 



THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1954 



To insure that continuity exists through the switch a check is made by 

 making the advance of the controller dependent upon receiving a signal 

 from the distant end. The os relay, Fig. 9, in the outgoing trunk is 

 operated directly by a ground on one of the contacts of the crosspoint, 

 and opens the operating path of the select magnets and ss relay (shown 

 in Fig. 7). The release of the ss relay causes the operation of the cp 

 relay shown in Fig. 8. The operation of the cp relay changes the potential 

 connected to the winding of the hs relay from ground to about —2.5 

 volts. The operation of the os relay is the outgoing trunk sends a seizure 

 signal over the trunk to the distant end which then sends a stop signal 

 back to the incoming trunk of the concentrator causing the operation 

 of the SUPERVISORY RELAY (sv) and the cut through relay (ct) of the 

 incoming trunk which locks under control of the operator and connects 

 ground to the hold magnets of the switch. This ground causes the current 

 through the hs relay to be reversed, operating the relay, indicating a 

 successful continuity check. A failure to make a successful check will 

 cause the controller to time out, lock out the selected outgoing trunk 

 and proceed to the next idle outgoing trunk. 



Outgoing Trunk Lock Out and Advance 



A TRUNK ADVANCE (ta) relay shown in Fig. 10 is provided for each 

 group of outgoing trunks. The (ta) relays operate in multiple on ev- 

 ery call. They are operated when the select magnets are energized and 

 are released, under normal conditions, when a successful continuity 

 check is made, or in the event of an unsuccessful check, when the con- 

 troller times out. Their function is to prevent the operation of the ot re- 

 lay associated with the outgoing trunk until a connection has been suc- 



OUTGOING 

 TRUNK 



1 I S02 



jHili-^ 



^ 



11-^ 



S02 



± 



TO OTHER 

 TA RELAYS 



± 



^"tJ 



TO ALL OT 



RELAYS IN 



SAME GROUP 



1 



E 



Fig. 10 — Outgoing trunk lockout and advance. 



