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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