In this paper, we discuss the use of digital circuits in decision support systems. For this, it is first necessary to translate the descriptions of situations and conclusions which are discovered step by step into a suitable Predicate Calculus. Then, a digital implementation for the constructs of Predicate Calculus must be found. So, the main issue is the realization of logically correct inference rules. In this paper , the following inference rules have been studied in a detailed fashion: Contrapository rule, the Modus Ponens and Modus Tollens. Keywords. Decision-making support systems, formulas for Predicate Calculus, inference steps and inference rules, the realization of correct inference steps by digital circuits.