SIMATIC S7-1500 PLC Error Handling Organization Block Description

Error Handling Organization Block

(1) Error handling overview

SIMATIC S7-1500 PLC has strong error (or fault) detection and handling capabilities, which refers to functional errors or programming errors within the PLC, rather than failures of external devices. After the CPU detects an error, the operating system calls the corresponding organization block, and the user can program in the organization block to take appropriate measures for the error that occurred. For most errors, if the organization block is not programmed, the CPU will enter STOP mode when an error occurs.

(2) Incorrect classification

There are two basic types of errors that are detected by the S7 CPU and can be handled by the user via organization blocks.

① Asynchronous errors are errors that are closely related to the PLC hardware or operating system, have nothing to do with program execution, and have serious consequences. Asynchronous error OBs have the highest priority and other OBs cannot interrupt them. If multiple asynchronous error OBs of the same priority appear at the same time, they will be processed in the order they appear.

The system program can detect the following errors: incorrect CPU functions, errors in the execution of the system program, errors in the user program, and errors in the I/O. Depending on the type of error, the CPU is set to enter STOP mode or call an error handling organization block (OB).

When the CPU detects an error, it calls the appropriate organization block, as shown in Figure 1. If there is no corresponding error handling OB, the CPU will enter STOP mode. The user can write a program in the error handling OB to handle this error in order to reduce or eliminate the impact of the error.

Figure 1

To prevent the CPU from shutting down when a certain error occurs, you can create a corresponding empty organization block in the CPU. Users can use the information provided by the variable declaration table in the OB to determine the type of error.

② Synchronous errors (OB121 and OB122) are errors related to program execution. The priority of their OBs is the same as the priority of the block that was interrupted when the error occurred, that is, the program in the synchronous error OB can access the contents of the accumulator and status register when the block was interrupted. After processing the error, the processing result can be returned to the interrupted block.