A PLC platform-independent structural analysis on FBD programs for digital reactor protection systems☆

•FBD has been widely used to implement safety-critical software for PLC-based systems.•The safety-critical software should be developed strictly with safety programming guidelines.•There are no argued rules that have specific links to higher guidelines NUREG/CR-6463 PLC platform-independently.•This paper proposes a set of rules on the structure of FBD programs with providing specific links to higher guidelines.•This paper also provides CASE tool ‘FBD Checker’ for analyzing the structure of FBD.