Sequential control systems traditionally have been designed using ladder diagrams. This is a result of the extensive use of electro-mechanical relays in these systems in the past and the need to develop a standardized technique for specifying relay logic circuits. In fact, many commercial programmable logic controllers have facilities for programming with ladder diagrams. Ladder dia- grams entered by the programmer are translated into equivalent Boolean operations by the programmable logic controller so that the required logic functions can be performed.