Siemens SIMATIC ET 200AL, System Manual

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Functions
5.7 Isochronous mode
PROFINET with STEP 7
Function Manual, 05/2021, A5E03444486-AL
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Access to isochronous I/O through call of instructions
You access the isochronous I/O via a process image partition. That is, the addresses of the
isochronous modules must be within one process image partition.
You program access to the isochronous IO in isochronous mode interrupt OB Synchronous
Cycle (OB 6x) with the SYNC_PI and SYNC_PO instructions.
The isochronous I/O is updated by the call of the "SYNC_PI" and "SYNC_PO" instructions and
therefore in the corresponding process image partition.
Note
Recommendation: To prevent inconsistent data from being returned to OB 6x, do not use the
"DPRD_DAT" and "DPWR_DAT" instructions (direct data access) in the isochronous mode
interrupt OB.
The "SYNC_PI" and "SYNC_PO" instructions update the process image partition only within the
permitted execution window. The execution window stretches from the end of the cyclic data
exchange to the point in time before the end of T_DC at which the outputs can still be copied
in time. The data exchange must be started within this time window. If the execution
window is violated by the processing of the "SYNC_PI" and "SYNC_PO" instructions, the
instructions indicate a corresponding error message.
Program execution models
Depending on the order of the "SYNC_PI" and "SYNC_PO" instruction calls in OB 6x, there are
two basic models for the program execution:
•
IPO model (read I nputs - P rocessing - write O utputs)
• OIP model (write O utputs - read I nputs - P rocessing)
5.7.5.2 Program execution according to the IPO model
If the execution time of the isochronous mode interrupt OB is significantly shorter than one
data cycle T_DC, use the IPO model. In the IPO model, you do not reduce the data cycle. That
is, the application cycle of the isochronous mode interrupt OB is equal to data cycle T_DC.
The IPO model enables the shortest response times.