Program Organization Units

Program Organizational Units (POUs) are block structures used within PLC programming to create programs and projects. POUs can be used to hold user programs written in IEC 61131-3 languages: Structured text, Instruction list, Function block, and Ladder logic. Application - 201203 They can also provide additional functionality, such as establishing connections between the PLC and other devices using TCON. Stuxnet uses a simple code-prepending infection technique to infect Organization Blocks (OB). For example, the following sequence of actions is performed when OB1 is infected: *Increase the size of the original block. *Write malicious code to the beginning of the block. *Insert the original OB1 code after the malicious code.