Considering the Hot Stand-By system, the scan time mainly comes from the addition of three parts:
Part 1: User application logic execution time, with an additional data transfer time from Primary to Stand-By communication load RIO management.
Part 2: Depending on the communication load on Modbus, there will be around 1 or 2 ms needed to answer to Modbus read/write requests coming from Modbus network.
Part 3: The scan time needed for RIO management is 2.5 ms
For 100kb data to transfer from Primary to Stand-By and user logic execution time being less than 6ms, then the scan time of Part 1 is 15 ms (using last CPU firmware version v2.50 or v2.60). Thus ultimately the average scan time will be roughly 19ms for the given example.