Hi,

I found that in the manual of Siemens:

"...
Synchronous errors occur during the execution of a particular instruction. When these errors occur, the operating system makes an entry in the I stack and starts the OB for synchronous errors.
The error OBs called as a result of synchronous errors are executed as part of the program in the same priority class as the block that was being executed when the error was detected. OB121 and OB122 can therefore access the values in the accumulators and other registers as they were at the time when the interrupt occurred. You can use the values to react to the error condition and then to return to processing your program (for example, if an access error occurs on an analog input module, you can specify a substitute value in OB122 using SFC44 RPL_VAL). The local data of the error OBs, do, however, take up additional space in the L stack of this priority class.

With S7-400 CPUs, one synchronous error OB can start a further synchronous error OB. This is not possible with S7-300 CPUs
..."

Well, as I had mentioned, I had a problem ofreading errorof DB in the past, and I corrected this problem using L DB1.DBW 0instead OPN DB1 L DBW0. So, in this case,I had many OB's errors call, inclusive OB121and OB122.

My OBs 121 and 122 were clean, but suppose that this OBs opened the DB2, I would not go to overwrite the DB register?

I don't really know as it happened, but the DB register was overwrite, I don't have doubts.

As I stated before, each task switch saves the CPU registers bank.

By task switch I meant halt the current task execution and execute a task with higher priority.

On S7300 stand alone tasks are OB1, OB10, OB20, OB35 and OB40. Each one has a predefined execution priority and is scheduled based on the task role (OB35 every 100ms, OB40 on HW Interrupt etc).

Execution of a synchronous interrupt (OB121, 122) occurswithin the current task scope and priority and uses the current taskregisters bank. Execution of OB121, 122 are implemented like try..catch..throw exception handling in C++.

Dec

Hi Dec,

As I stated before, I don't have doubts that the DB register was overwrite.

Well, don't forget, that in the try/catch block, you can create new variables and overwrite local variables that was created in function thatthrow the exception. If you don't catch the exception,the nextfunction inthe call stackcan catch the exception (OB121/OB122).