An Application of Boolean Algebra to Switching Circuit Design

The demands made upon telephone switching systems in regard to improvements in handling capacity, speed, flexibility and economy are continually increasing. In order to meet design objectives enabling the fulfillment of these demands, switching circuits have of necessity become more and more complex and intricate. As certain types of relay switching circuits increase in complexity, the problem of control and output contact network design becomes more and more laborious and time consuming. This is especially true in those circuits in which an attempt has been made to achieve the ultimate in efficiency and economy in that the number of relays used therein approaches the absolute minimum necessary to provide the required number of distinct output combinations. In this type of near-minimum combinational or sequential relay circuit there are numerous parallel control and output contact paths which thread through the same relays repeatedly, thereby causing the individual relay contact loads to become relatively large. Thus the designer's problem becomes that of first developing a workable control and output contact network and then manipulating and minimizing contacts within that network so that the maximum number of contacts used on any one relay is within that permissible on any commercially available relay having the necessary speed characteristics. Even in those combinational and sequential relay circuits which are not near-minimum and therefore probably have fairly light individual relay contact loads, there are, of course, advantages to be gained by using the least number of contacts possible.