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Question: Is the CPU’s I/O update time different in centralized mode and distributed mode? Answer: The CPU varies in I/O update time. The differences arise from the different times at which I/O is refreshed. In cluster mode, the CPU can directly access I/O via the backplane bus ； In the distributed mode, whether the PROFIBUS interface module is integrated into the CPU or a CP is used with the I/O modules separated from the central rack, the I/O status will be provided by the PROFIBUS interface module. In distributed mode, the duration of I/O updates depends primarily on three factors: the time it takes for the PROFIBUS slave interface to refresh its own I/O, the time it takes for the PROFIBUS master to read the I/O data from all slaves, and the time it takes for the CPU to refresh the I/O areas. Usually, these processing steps occur in parallel, so the time required by the CPU to refresh I/O varies from time to time. This can cause problems in closed-loop control (such as position control). By synchronizing the three processes, the CPU can obtain the most accurate value for this cycle. But this means a loss of time, namely the waiting time required during synchronization. Therefore, these data are usually not up to date. To synchronize time, clock synchronization can be used—by directly connecting the user program to PROFIBUS. Here, the clock synchronization alarm (OB61-64) triggers synchronization and interrupts the normal user program in order to handle the program sections that require synchronized access to the slave devices. Synchronization between user programs and peripherals applies only to distributed peripherals.