INTERTOLL TRUNK CONCENTRATING EQUIPMENT 321 



cessfuUy completed except as stated above where a continuity check 

 has failed. When the ta relays release, the ot relay associated with the 

 trunk selected, or passed by, is operated and advances the selection to 

 the next idle trunk. Thc^ operated ot relay locks to its own group 

 relay gp until the last ot relay in that gi'oup has operated. When the 

 last trunk in that group is selected the gp rela}'- of that group is re- 

 leased, advancing the selection path into the next group. The ot re- 

 lays in the first group are then controlled only from the individual out- 

 going trunk with \\hich they are associated and will be released when 

 the trunk becomes idle. The trunks in that group cannot be selected 

 again until the group relay has been reoperated which can not occur 

 until all of the outgoing trunks on the concentrator have been selected 

 in tvu"n or until the controller times out. In both of these cases the con- 

 troller will go through end or cycle operation and reoperate the group 

 relays as later described. With the above arrangement the traffic is 

 spread evenly over the whole group of outgoing trunks. 



End of Cycle and Guard Timing 



When a connection through the concentrating equipment is released 

 1)3' an operator, the outgoing trunk sends a disconnect signal to the other 

 end, to release the equipment there. It may take approximately 0.75 

 seconds for the distant equipment to release after it has received the 

 disconnect signal. If this equipment should be reseized before it is re- 

 leased the new call would be connected to the same subscriber. A guard 

 time could be incorporated in every outgoing trunk to prevent the trans- 

 mission of a seizure signal for this 0.75 seconds, but this would be rela- 

 tively expensive. To avoid this procedure the end of cycle and guard 

 timing feature has been incorporated in the controller. This feature, 

 shown in Fig. 11, together with the outgoing trunk lock out feature 

 insures that no outgoing trunk can be seized for at least 0.8 of a second 

 after it has been released from a previous call. 



When the last available idle outgoing trunk on the concentrator has 

 l)een selected the last operated group relay gp releases, and two end of 

 cycle relays eci, ec2 which are normally operated also release. Either 

 one of these relays released operates both the group restore relay gr 

 and an all busy relay ab. Both the gr and ab relays cause the idle 

 indicating lamp associated with trunks to the concentrator at all origi- 

 nating switchboards to remain dark as an indication that all trunks are 

 busy. If no trunk outgoing from the concentrator is idle at this time 

 nothing else will happen in the controller. When any outgoing trunk 

 in any group becomes idle the group relays associated with such groups 



