FB block program is how to be scanned by the PLC - Database & Sql Blog Articles

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In the Siemens S7-400plc, the FB block calls the FB block. When the PLC cyclically scans, how is the called FB block executed, including how the static variables and temporary variables are scanned, and when the static variables are stored. To the background data block, I saw a segment program. The internal input variable of the called FB block has been changing during monitoring, and the external input condition of calling the FB block has been unchanged. Why?
A: The scanning mechanism of the PLC is from top to bottom, scanning from left to right. The content of the interrupt program is executed when there is an interrupt. The interruption is followed by priority sweeping.

No matter how you call FB, (multiple FB calls the same FB or the same FB calls multiple FBs), the PLC's CPU is sequential scan, when the condition you call is satisfied, the call function is executed, this time FB external The corresponding variables filled in the pin, and the internal temporary variables (execution of this FB function, the temp variable is automatically released) and static variables, is the time when the FB is called to satisfy the condition.
The PLC then scans sequentially, and calls the same FB below. Then the variables filled in by the external pins at this time, and the internal temporary variables ((execution of this FB function, temp variable is automatically released) and static variables, are satisfied by this condition. Under the execution of the FB function, then it will overwrite the same FB, external pin variables and static variables.