THROWDOWN MACHINE FOR TRAFFIC STUDIES 325 



are cross-connected to the time arc of the sequence switch to fix the work 

 time preceding the event represented by each sequence switch position. 



In the general operating scheme, the sequence switch stands at a posi- 

 tion representing the next event to take place. The timer switch is started 

 from normal and counts clock pulses until it reaches the terminal cross- 

 connected to the position on which the sequence switch stands. This 

 initiates signals which stop the clock and cause action signals to be dis- 

 played. When the operators respond, the sequence switch advances to 

 the terminal for the next event in the call, the time switch returns to 

 normal and the clock restarts. The time switch then counts time units 

 leading to the next event. Since several markers may be in use at the same 

 time in different stages of their calls, two markers can reach an action 

 point during the same clock pulse. A lockout circuit insures that only 

 one marker at a time displays its action signal. At any time that a 

 marker stops the clock, the timer switches of all other markers halt but 

 continue their count when the clock is restarted. Thus, relative time 

 relations are maintained while a true count of time consumed by an 

 operating system is obtained. 



The circuit action can be illustrated by a discussion of the events in a 

 dial tone call. This call represents an attempt by a marker to establish 

 a connection from an originating subscriber line to a dial register. The 

 possible sequences of events are diagramed in Fig. 13. As indicated, this 

 class of call employs eight sequence switch positions in addition to the 

 normal position. 



The call starts when the gate circuit assigns an idle marker to a call 

 which has been originated at the proper time and placed in the gate. The 

 assigned marker is prepared to process this type of call by the operation 

 of an associated "class" key which selects and advances the sequence 

 switch which carries the events of a dial tone call. Advance of the switch 

 is from normal to Position 1 to control, at the proper time, signals for 

 the first event, namely, seizure of a trunk link frame and selection of a 

 register. The timer switch is set at zero and in a condition to step one 

 terminal at a time in response to clock pulses. Terminal 1 of the sequence 

 switch time arc is cross-connected to a terminal of the timer switch 

 representing the marker work time between the time the marker is first 

 seized and the time it attempts to seize a trunk Hnk frame. 



The operator in control of the marker now operates a start key and 

 the clock 'starts pulsing, each pulse causing the marker timer switch to 

 advance one step. When the specified work time has elapsed the timer 

 switch reaches the terminal connected to Position 1 of the sequence 

 switch. This passes a signal from the timer switch through the sequence 



