The Full Form of POU is Program Organization Unit.

he software model for PLCs, as defined in the IEC 61131-3 standard, Programmable Controllers – Part 3: Programming Languages, consists of, at the top level, a configuration. The configuration defines the hardware structure and logic of the PLC system and contains resources (the PLC or PAC, for example), which are able to execute programs. These resources are controlled by tasks, which invoke the execution of software blocks that make up the PLC project. These software blocks are referred to as Program Organization Units.

Program Organizational Units (POUs) are block structures used within PLC programming to create programs and projects.1 POUs can be used to hold user programs written in IEC 61131-3 languages: Structured text, Instruction list, Function block, and Ladder logic.2 They can also provide additional functionality, such as establishing connections between the PLC and other devices using TCON.3

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 4:

• 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.