There is a huge difference between PLC and RTU.
PLCs are microprocessor-based controllers designed to complete a process. Therefore, the program in the next I/O control will be after a loop. This also happens in a certain time. Signal changes that occur until the next I/O control cannot be detected by the PLC for this reason.
RTUs, on the other hand, are designed based on parameters. Therefore, they detect all signals at the right time and record them chronologically together with the time information. They do not cause signal loss. For example, the late detection of a power outage in a factory by the PLC causes the backup power supplies to be activated late, which is a big deal for that factory. It may cause a loss. In places where such precise control is required, RTU is used instead of PLC.
PLCs also have modules developed to capture signals instantly, but they are very costly.
In addition, RTU is used for scada systems in power plants. Because the minimum induction protection of RTUs is 2 kV. RTUs detect induction by measuring the signals coming to their inputs for a certain period of time. Thus, more accurate signal detection is provided.