AUTOMATIC SWITCHING SYSTEM 457 



the blank interval is a start pulse ami eight start i)ulses at luiitorin 0.01- 

 second time-interval spacing occiu- between l)laiik intervals. One and 

 only one stop pulse occurs between start pulses. The total number of 

 signal pulses between blank intei'vals is sixteen. The cluM-k (;ircuit uti- 

 lizes one or more of these properties to insure Ihat no signal pulses have 

 been lost during transmission and that no extraneous pulses have been 

 detected. If the actions of the check circuit indicate that an error in 

 transmission has occurred, the receiver circuits are completely reset for 

 another trial. 



THE SWITCHING NETWORK 



To meet the objective of a single common control circuit for the opera- 

 tion of the switching network, wliich provides the selectable paths 

 between any subscriber and any trunk, it was necessary to have the 

 switches in the network considerably faster than any of present com- 

 mercial design. The laboratory model of the switching network and its 

 associated path selecting equipment employing cold cathode gas tubes 

 and dry-reed relays was developed by E. Bruce and S. T. Brewer. In 

 addition to liigh operating speed this switching arrangement has certain 

 other desirable properties: The idle path testing and selection functions 

 are incorporated in the internal controls of the network. Busy sections 

 of the network are automatically isolated from the sections tested for 

 subsequent calls. Selection of a trunk within a trunk group, as well as 

 path selection through the network, may be accomplished by the internal 

 controls of the network if the trunks of a group are assigned one trunk 

 per frame. Selection of an idle trunk and an idle switch path in com- 

 bination reduces blocking. These internal selection controls eliminate 

 many of the connector contacts that would otherwise be required be- 

 tween the switches and external common control circuits. 



The switching network consists of line frames and trunk frames with 

 each frame divided into primary and secondary switches. Each primary 

 line and trunk switch has a number of vertical input columns across the 

 switch to which are connected line or trunk circuits respectively and a 

 number of horizontal output rows across the switch. At the intersection 

 of each row and column of a switch is a relay consisting of an operating 

 coil and three dry-reed make contacts. By analogy to the crossbar sys- 

 tem which employs a somewhat similar rectangular array of rows and 

 columns per switch and a similar primary-secondary path distribution, 

 a switch intersection is called a crosspoint and a switch relay is called 

 a crosspoint relay. In the crosspoint relay two of the contacts are used 



