S7-300 OB: Siemens PLC Programming Organization Blocks

The S7-300 OB, also known as an organization block, is a programming element in the Siemens S7-300 series programmable logic controllers (PLCs). It is used to structure and organize the program code into logical sections for better readability and modularity.

The OBs in the S7-300 series are numbered from OB1 to OB255, with OB1 being the main organization block. Each OB serves a specific purpose and is executed under different conditions or events during the PLC's operation.

OB1 is the default and mandatory organization block, which is executed cyclically and contains the main control program logic. OB1 scans the entire program in a loop, executing the program instructions in a specific order.

Other OBs, such as OB2 to OB99, are used for specific tasks or events, such as interrupt handling, error handling, startup and shutdown sequences, communication handling, and more. These OBs are executed when certain conditions or events occur, allowing for more specialized control and functionality.

The S7-300 OBs are programmed using the Step 7 programming software, which provides a graphical interface for creating and configuring the OBs. Each OB can be assigned a specific event or condition for execution, and the program code within the OB can be written using a high-level programming language, such as ladder logic or structured text.

Overall, the S7-300 OBs provide a structured and modular approach to programming the PLC, allowing for better organization, control, and flexibility in the control system.