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THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1953 



count time units to cause events to occur in the proper sequence. When 

 these time counters reach a stage where some action is to be per- 

 formed by the operators, "stop-time" signals are produced. These sig- 

 nals lock in and cause the clock to halt. Simultaneously, action signals 

 are displayed to the operators. When each operator completes the in- 

 dicated action she depresses an appropriate key at her position which 

 extinguishes the action signals and removes the stop-time signal. When 

 the last operator has responded, all stop-time signals are removed, the 

 time lock is opened and the clock again advances. Thus the clock, in 

 effect, takes time-out while the operators perform the various manual 

 searching and recording actions necessary to simulate and tabulate the 

 performance of the crossbar system. 



In the throwdown machine, each clock pulse represents one millionth 

 of an hour or 3.6 milliseconds. This quantitization of time is based on two 

 considerations. The first is that it is convenient to represent a particular 

 time during an hour by a six-digit decimal number. The second is that 

 the time represented by one time unit (3.6 milliseconds) is well under 

 the average acting time of the relays and switches used in the system 

 being simulated. Thus the events taking place in the actual system can 

 be reproduced in sufficient detail. Some events of longer time duration 



Fig. 11 — Block diagram of the time system. 



