282 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1952 



"Algebra of Logic" to switching circuits and is just such a means. It is 

 a too] which can be used to investigate the complex combinational and 

 sequential networks to determine satisfactory contact arrangements or 

 reject unsatisfactory ones with a minimum of time and effort. It should 

 be emphasized, however, that as with any tool, satisfactory results de- 

 pend upon the judgment, ingenuity and logical reasoning of the user. 

 Furthermore, as A\dll be evident from the following development, switch- 

 ing (Boolean) algebra in its present state is not to be considered entirely 

 selfsufhcient but, for the most beneficial results, should be applied, when 

 warranted, in conjunction with inspection techniques so that the latter 

 may fill in any limitations in the algebra techniques which have not been 

 completely systematized as yet due to the newness of this field. 



The problem of solving the contact requirements of a minimum relay 

 dial pulse counting and translating circuit recently developed as a com- 

 ponent of the originating register of the No. 5 Crossbar System will be 

 used as a means of illustrating the practical use now" being made of 

 switching algebra and of indicating exactly where the application of the 

 algebra enters the design problem. 



BASIC DIAL PULSE COUNTER REQUIREMENTS 



The primary function of the originating register is to receive pulse 

 signals representing digits from a telephone dial or similar calling device 

 and to store a record of the digits in a form suitable for use by an external 

 circuit. The dial pulse counting and translating circuit, an integral part 

 of the originating register, is oriented with respect to other parts of the 

 register by the block diagram of Fig. 1. The L relay is the pulse detecting 

 relay. When the subscriber's switchhook contact is closed due to the 

 lifting of the phone, the originating register is connected to the line and 

 the L relay is operated. Thereafter it follows the breaks and makes of 

 the subscriber's dial and feeds these repeated dial pulses into the counter. 

 After the pulses are counted they are translated to a new code. In switch- 

 ing systems it is advantageous to translate from the basic dial ten pulse 

 decimal code to a "two out of five" self-checking code. In this latter code 

 any single error within the circuit will result in either one or three relays 

 operated in the associated storage circuit rather than two and thus an 

 error can readily be detected. The output of the translator is fed via a 

 steering circuit to the register or storage circuit. The slow release RA 

 relay is the pulse train detecting relay which holds between the indi- 

 vidual pulses of a digit and releases only at the end of the pulse train. 

 When it releases it activates the translating circuit and thereby transfers 

 the translated code information to the storage circuit. The RAl relay 



