Omron C200HW-PRM21 Operation Manuals page 115

Actual application
Once the actual application program in the PLC has been completed, and the DM settings have been
optimised to transfer only the data required for the application,
•
Read the actual value of the PLC scan time from AR27 with the PLC in Run/Monitor mode.
•
Calculate the C200HW-PRM21's I/O refresh time as shown in paragraph 3-2-2, 'PLC cycle time'.
•
By subtracting these two figures, calculate the time required for the PLC program plus the overhead for
servicing additional units and peripherals.
Only if the PROFIBUS-DP cycle is several times smaller than the time taken by the PLC program + overhead,
choosing the asynchronous data exchange method will result in a consistent reduction of remote I/O response
time.
Asynchronous data transfer mode with PLC program + overhead ≈ 2 * PROFIBUS cycle.
This will result in irregular data transfer, but data is updated at least once per PLC cycle.
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