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THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1953 



switches. One of these, the sequence switch, determines the sequence in 

 which events take place, while the other, the timer switch, measures 

 work time between events. In the throwdown machine each possible 

 event which may occur in setting up a call is represented by a position 

 on the sequence switch. Circuits through the switch cause action signals 

 for the event represented by its position to be displayed to the operators. 

 All of the events for one type of call are arranged in order on the switch 

 terminals so that by omitting certain positions (events) all of the vari- 

 ations of this type of call can be represented. At the conclusion of an 

 event the switch is directed to the position corresponding to the next 

 appropriate event according to the setting of ''memory" relays located 

 in each marker unit. These relays operate at various stages of the call 

 in response to key signals from the operators indicating the conditions 

 they have encountered in attempting to respond to action signals. Thus, 

 significant events are recorded in order to control the future progress of 

 the call. 



To provide for all types of calls it was found necessary to furnish three 

 sequence switches for each marker. These switches have 22 positions and 

 six arcs. Five arcs are used for displaying signals and for control while 

 the sixth arc is used in conjunction with the timer switch to control the 

 time at which signals are displayed. A timer switch is individual to each 

 marker. It is a 22-position, six-arc switch arranged with auxiliary relays 

 to count a maximum of 105 clock pulses. Terminals of the timer switch 



--o 



O TIME SIGNAL 



ARC ARCS 



SEQUENCE SWITCH 



Fig. 12 — Block diagram of marker progress circuits. 



