Organization Blocks (OBs) are the interface between the operating system of the CPU and the user program. OBs are used to execute specific program sections:
- At the startup of the CPU
- In a cyclic or clocked execution
- Whenever errors occur
- Whenever hardware interrupts occur.
Organization blocks are executed according to the priority they are allocated.
In the following link by Siemens support, you will find the complete table of the different OBs available in Siemens PLCs: https://sie.ag/2ZvTp4z