314 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1954 



Grouping and Individual Relays for Outgoing Trunks 



In the controller the outgoing trunks are divided into groups of ten 

 circuits. This is done to keep the length of chains on the control relays 

 short (ten rather than forty relays long) and to reduce current drain 

 during light periods since a possible maximum of nine relays will be 

 locked up instead of 39. For each outgoing trunk one out trunk (ot-) 

 relay (shown in Fig. 10) is provided in the controller and for each ten 

 of these a group relay (ciP-) is provided (shown in Fig. 11). When the 

 connection of an incoming trunk to an outgoing trunk is completed the 

 controller operates the ot relay, corresponding to the outgoing trunk, 

 which advances the selection path to the next idle trunk in the group 

 and locks up under control of the gp relay for that group. When the last 

 idle trunk in the group has been selected the group relay is released ad- 

 vancing the selection path to the next group and releasing all ot relays 

 in its own group which are associated with idle trunks. These trunks are 

 then locked out of service until all trunks above or below it (depending 

 upon the direction of selection) have been selected. Two chains are car- 

 ried through contacts of the group relays and through the ot relays of 

 each group for controlling the operation of the switch select magnets, 

 one for each direction of selection. These chains are shown in Fig. 7. 



Units, Unit Gates and End Relays 



Ten Unit relays designated uo to U9 are provided to identify the indi- 

 vidual incoming trunks in a group of ten which have calls waiting to be 

 served. These relays, together with the unit gates and lock unit 

 relays are shown in Fig. 6. 



When the units connect relay (uc-) associated with a group of 

 trunks operates as previously described, it connects these ten u- relays 

 to the US-leads from the incoming trunks. Those of the u relays which 

 find ground on these leads, indicating calls waiting to be served, operate 

 and lock. Any u relay operated operates one of two end relays ei or E2. 

 The END relay in turn operates one of two unit gate relays (ugi or 

 UG2) which opens the operating, but not the locking, path of all u relays 

 thereby preventing a late arrival from stealing preference from an earlier 

 originated call. One end and one units gate relay are provided for each 

 direction of selection. The hold magnets associated with the incoming 

 trunks in the group being served are connected through contacts on the 

 HOLD CONTROL (hc-) relay to two chains of transfer contacts on the 

 UNIT relays (1 chain for each direction of selection). As the connection 

 of the incoming trunk is completed to the outgoing trunk the unit relay 



