322 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1953 



is continuously driven by the clock. This counter records simulated 

 present time throughout the entire running of a throwdown test. 



The master counter does not, in itself, generate stop time or action 

 signals. Its primary purpose is to furnish correct present time to the 

 time comparator circuit where such signals are generated. It also con- 

 trols a pulse divider which furnishes pulses at one tenth and at one 

 hundredth the basic pulse rate. 



Since failure of the master time counter to advance on each pulse 

 would introduce present time errors which are cumulative over the 

 throwdown run, the integrity of this circuit is rigidly guarded. Checking 

 circuits verify that each basic clock pulse advances the master time 

 counter. The checking circuits, not shown on Fig. 11, upon detecting a 

 failure to advance, produce a stop-time signal and an alarm signal which 

 can be released only after the counter is brought to correct time. The 

 master time counter consists of pulse driven rotary switches arranged so 

 that each switch represents one decimal digit of time. To count one 

 hour of simulated time, as provided in the master time counter, six 

 switches are necessary. These switches record 000000 to 999999 units 

 of 0.0036 seconds. 



Means are provided for presetting such items as subscriber originating 

 times, incoming call originating times, and hold release times by the 

 time detectors. When a time so set coincides with simulated time, the 

 clock is stopped by a stop-time signal and an action signal indicates that 

 a call is to be originated or a held item released. 



The time detectors consist of sets of ten-position manually controlled 

 switches with each switch representing a decimal time digit. Since 

 simulated time is divided into millionths of an hour, these switches are 

 preset to the millionth interval, say 003162, in which an event is to 

 occur. Information from each switch is transmitted to the time compara- 

 tor, Fig. 12. Also transmitted to the time comparator, from the master 

 time counter, is the simulated machine time, say at present, 003159. 

 When the master time counter advances to a time 003162 which coin- 

 cides with the detector time, the time comparator generates a stop- 

 time signal to stop the clock. An action signal, associated with this 

 particular time counter in coincidence is also lighted. The operator, after 

 taking the appropriate action, resets the time counter to a future time — 

 the time of the next waiting item in the category — and depresses the 

 start time key. Checking circuits prevent advancement of the clock 

 should the time detector accidentally be set to a time value which has 

 already passed in the throwdown run, in the example, to a value less 

 than 003162. 



