AUTOMATIC SWITCHING SYSTEM 455 



mined delay will be closed and the reeds of uU the other relays will be 

 opened. This action is produced by reversing the individual winding 

 and bias magnet of the single relay which is to be operated by the cur- 

 rent through the common stepper winding. The preliminary setting of 

 the stepper to insure correct operation is provided on each origination 

 of a call by the discharge current from the ringer capacitor through the 

 common winding of the stepper. The ringer capacitor is charged from 

 the central office between calls. 



One reed in each of the relays is employed to coiniect successive 

 brushes of the digit selector switch with the line while the other reed in 

 each relay in conjunction with two diode rectifiers per relay winding is 

 employed to control the operation of the stepper. The stepping opera- 

 tion may be explained by reference to Fig. 6 as follows: The stepper is 

 shown with the reeds for the sixth step closed. When the 50-cycle excita- 

 tion current makes the terminal common to the individual stepper coils 

 positive with respect to the terminal common to the stepphig control 

 contacts, current flows through the upper reed contact of the sixth 

 step, a diode rectifier and the winding of the seventh step relay causing 

 its reeds to close. With the seventh set of reeds closed current flows 

 through a diode rectifier and the winding of the sixth step relay causing 

 its reeds to open. The stepper will remain in this position until the re- 

 versal of excitation current a half-cycle later at which time a circuit 

 through an oppositely poled diode rectifier will cause the operation of 

 the relay for the eighth step followed by the release of the relay for the 

 seventh step. The phase of excitation current through the stepper is so 

 adjusted by the shunt phase shifting network that the stepper relays 

 operate and release during the 3.2-millisecond guard interval preceding 

 a start pulse. This prevents mutilation of the signal pulses. The stepping 

 circuit is made reentrant so that the pre-set number will be transmitted 

 repeatedly so long as excitation current is provided. 



With the chosen 50-cycle excitation the complete transmission of 

 eight digits and a two-digit silent interval takes only 0.1 second. This 

 results in a short holding time for the central office receiving circuit 

 and the repetitive signaling feature permits repeated trials in case of 

 signal mutilation as well as direct dialing from the subscriber's tele- 

 phone set to distant offices rather than some form of relayed signaling 

 from registers in the subscriber's own office. 



SIGNAL RECEIVER 



A simplified block diagram of an experimental receiver for the pulse- 

 position signals used in this system is shown in Fig. 8. The receivers 



