Step7 OB organization block introduction and use skills - Database & Sql Blog Articles

Understanding the different types of OB blocks and their usage in Siemens S7 PLCs is essential for efficient program development and system stability. Here's a detailed breakdown of each organization block (OB) and its purpose:

1. Free Loop Organization Block (OB1): This is the main cyclic block that runs continuously after the CPU starts. It can call other functions like FB, FC, etc., and has the lowest priority. If it exceeds the set scan time (default 150ms), OB80 is triggered, potentially stopping the CPU if not handled.

2. Date-Time Interrupt Blocks (OB10–OB17): These are triggered based on specific date or time settings. They can be configured to run once or periodically. The SFC28 function allows dynamic adjustments during runtime, making them flexible for scheduling tasks.

3. Time-Delay Interrupt Blocks (OB20–OB23): These allow triggering an OB after a specified delay. The SFC32 function is used to activate them, with delays ranging from 1 to 60,000 ms. This provides better accuracy than standard timers.

4. Cyclic Interrupt Blocks (OB30–OB38): These execute at fixed intervals, typically used for real-time applications like PID control. The minimum interval is 55 ms, and the execution time must be shorter than the interval to avoid errors. They can be enabled/disabled via SFC39–SFC42.

5. Hardware Interrupt Blocks (OB40–OB47): Triggered by external events such as digital inputs or modules. These are ideal for fast response applications where normal input scanning isn't sufficient. They can be configured to trigger on rising/falling edges or both.

6. DPV1 Interrupt Blocks (OB55–OB57): Used for PROFIBUS-DP V1 slave communication. These blocks handle interrupts generated by the DP slaves and are crucial for synchronization in distributed systems.

7. Multi-Processor Interrupt Block (OB60): Designed for multi-CPU systems (e.g., S7-400). It allows all CPUs to execute the same OB simultaneously, useful for complex parallel processing tasks.

8. Clock Synchronization Interrupt Blocks (OB61–OB64): Ensures synchronization across multiple PROFIBUS-DP stations. These blocks help maintain timing consistency, especially in high-speed automation environments.

9. Process Synchronization Block (OB65): Used for motion control applications. It ensures synchronized execution of technology blocks and start programs in T-CPU systems.

10. Redundant Fault Blocks (OB70, OB72): For S7-400H redundant systems, these blocks handle I/O and CPU redundancy faults. They prevent the CPU from entering stop mode if properly implemented.

11. Asynchronous Fault Blocks (OB80–OB87): These handle various fault conditions like timeout errors, power issues, diagnostics, and communication failures. Each block addresses specific error types and can cause the CPU to stop if not managed.

12. Processing Error Block (OB88): Triggers when there are memory allocation or program nesting issues. It ensures the system remains in a safe state even during critical errors.

13. Background Loop Block (OB90): Has the lowest priority and is used to ensure minimal scan time. It’s triggered during program download or deletion and is exclusive to S7-400 CPUs.

14. Start-up Blocks (OB100–OB102): These are executed during different startup modes (cold, warm, hot). They’re ideal for initialization routines and resetting variables upon boot.

15. Synchronization Error Blocks (OB121, OB122): Handle programming and I/O access errors. Proper handling of these blocks prevents unexpected system behavior and ensures stable operation.

When using OB blocks, keep the following in mind:

• OBs respond to faults but do not resolve them. They allow the CPU to continue running with some faults.
• Diagnostic OBs like OB82 or OB122 indicate hardware issues, which must be addressed manually.
• Excessive use of diagnostic OBs can slow down the system, especially in high-speed applications.
• Never ignore faults—implement safety measures to bring the system into a controlled state quickly.

For simple logic-based programs, only OB1 may be needed. However, for more complex systems involving DP communication or analog modules, additional OBs are necessary to ensure smooth operation and error handling.

Each OB plays a unique role in the system, and understanding their priorities and behaviors is key to developing robust and reliable PLC programs. Always refer to the CPU manual for specific configurations and capabilities.