Abstract: Aiming at the PLC sequential control system of thermal power plants, the basic conception and design method of the sequential logic were expounded. Two sequential logic design ideas were listed, and the advantages and disadvantages of the two design ideas were compared. The practical use shows that to adopt the combined method of the step sequence status words with the device control words to complete PLC sequential control system is a more simple and clear sequential logic design method with easy extensibility.

Key words: sequential logic, step sequence, programmable logic controller (PLC), ladder diagram