314 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1953 



connections are also placed in proper time sequence by the operators 

 according to written records. 



The actions of the operators are checked electrically in many cases. 

 An improper setting of certain switches, which is inconsistent with the 

 state of the system at the time will in some cases give an alarm and in 

 others, block the progress of the machine until the error is corrected. 

 Certain actions performed by the operators involve the insertion of plugs 

 into jacks. Where alternative actions are possible in response to a given 

 signal, separate groups of jacks correspond to the several alternatives. 

 Insertion of a plug into a particular group will signal the circuits as to 

 which alternative is taken. The circuits make one of several keys effective 

 and the operator must then depress this key (corresponding to the action 

 she has taken) before the circuits will advance. 



Where several operators must cooperate to perform a given action, 

 signals are passed between the operators by means of keys and all opera- 

 tors must respond before the circuits can advance. Wherever possible, 

 overlap operation is employed. Signals are presented to several operators 

 simultaneously and each operator starts the indicated action, signaling 

 the circuits w^hen it is completed. When the signal from the last opera- 

 tor is received, the circuit advances. In some cases an operator is allowed 

 to start a particular action before the stage has been reached at w^hich 

 this action is required. For example the information necessary to choose 

 links and determine a suitable path through the interconnecting network 

 of the No. 5 crossbar system is available before it is necessary to establish 

 the connection. Since this search is time consuming, the operator is 

 allowed to start as soon as the information is available. At the proper 

 time she is given a signal to complete the record of this connection, or if 

 the call has been blocked before reaching this stage, she is given a ''back 

 out" signal instructing her to restore her records to their previous con- 

 dition. Since this is a rare condition occurring less than one time in 100 

 tries, little useless work is done and the overall action is speeded up. 



A block diagram showing the relations between the operators and 

 the various major components of the machine is shown in Fig. 8 The 

 CLOCK controls the flow of time in the machine and gives an indication 

 of simulated present time in the traffic run being tested. The time de- 

 tectors, of which three are used, provide a means for the operators to 

 indicate a future time at which some action must be taken and be sig- 

 naled when this time arrives. The block labeled control circuits, which 

 present action signals to the operators, represents the main body of 

 circuits which maintain a current record of the states of the various 



