Understanding the different types of OBs (Organization Blocks) and how to use them in Siemens S7 PLCs is essential for effective programming. Here's a detailed overview:
1. Free Loop Organization Block OB1: This is the main cyclic block that runs continuously after the CPU starts. It can call functions like FB, FC, etc., and has the lowest priority among most OBs. If the execution time exceeds the set scan time (default 150ms), OB80 is triggered, which may lead to a stop mode if not handled properly.
2. Date Interrupt OBs (OB10~OB17): These are triggered based on specific dates or intervals. You can set them via the CPU properties or through system functions like SFC28 and SFC30, allowing flexible runtime adjustments.
3. Time Delay Interrupt OBs (OB20~OB23): These are activated using SFC32 with a specified delay. They offer higher precision than standard timers and are ideal for precise timing applications.
4. Cyclic Interrupt OBs (OB30~OB38): These run at fixed intervals and are used for tasks requiring regular processing, such as PID control. The interval must be longer than the program execution time to avoid errors.
5. Hardware Interrupt OBs (OB40~OB47): Triggered by external signals from modules like FM or CP. These allow fast response times, bypassing normal input scanning and image updates.
6. DPV1 Interrupt OBs (OB55~OB57): Used for PROFIBUS-DP V1 communication, these handle interrupt events from slave devices.
7. Multi-Processor Interrupt OB (OB60): Enables synchronization across multiple CPUs in S7-400 systems, ensuring coordinated execution.
8. Clock Synchronization OBs (OB61~OB64): Required for precise synchronization in PROFIBUS-DP V1 systems, ensuring consistent data transfer and processing.
9. Process Synchronization OB (OB65): Handles motion control tasks in T-CPU systems, ensuring synchronized execution of technology blocks.
10. Redundant Fault OBs (OB70, OB72): Triggered in redundant S7-400H systems when I/O or CPU redundancy fails. These help maintain system stability during faults.
11. Asynchronous Fault OBs (OB80~OB87): Handle various error conditions, such as timeouts, power supply issues, and diagnostic alerts. Each OB addresses specific fault types, ensuring proper error handling.
12. Processing OB (OB88): Deals with program structure issues, such as stack overflow or invalid memory access, preventing system crashes.
13. Background Loop OB (OB90): Has the lowest priority and ensures minimal scan time, often used during program download or deletion.
14. Start-up OBs (OB100~OB102): Triggered during CPU startup, used for initialization tasks like resetting data or loading configurations.
15. Synchronization Error OBs (OB121, OB122): Handle programming and I/O access errors, helping identify and address software or hardware issues quickly.
When working with OBs, it’s important to remember that they respond to events but do not resolve them. Diagnostics like OB82 or OB86 indicate hardware problems, which require physical intervention. Also, frequent OB calls can affect overall performance, especially in high-speed processes. Avoiding uncontrolled OB activation is crucial to maintaining system stability and safety.
For simple programs, only OB1 may be needed. For complex systems involving DP communication, additional OBs are necessary to ensure reliable operation. For example, OB86 is vital for diagnosing DP network issues, while OB82 helps detect analog module faults. Proper configuration of OBs ensures smooth and safe operation of the PLC system.
Organization blocks serve as the bridge between the operating system and user code. They are automatically called by the OS, contain temporary variables, and are categorized into startup, cyclic, and interrupt types. Each OB has a 20-byte local variable area with start-up information provided by the OS. The priority of an OB determines its execution order, and some OBs in S7-400 allow priority adjustments via STEP 7.
Key takeaways include: OB1 handles the main loop, OB30~OB38 are used for timed tasks, and OB100~OB102 are for initial setup. Other OBs are typically reserved for specific fault handling or advanced features. Understanding their roles and interactions is essential for efficient and robust PLC programming.
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