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Scada is basically a combination of PLC's.A concept of Master and Slave PLC's can be seen in here. It is not neccessary that a wireless/remote unit is called SCADA. Scada generally means a centralized system which can be controlled or monitored centrally.
If remote Controlling is done, then Remote Terminal unit (RTU's) are introduced in here.They are used to link up the system with wifi/booster.
SCADA is remote unit for monitoring and controlling. PLC is connected with SCADA using different protocols like Modbus . Profibus and etc. SCADA can combine different and multiple PLCs into a signle screen and HMI .
SCADA is mostly Plc's dependant but for expanding the overall system Rtus can be implemented but Plc's play avital role in SCADA as well as HMI controls
SCADA or DCS are called controllers of controllers. Both have levels/heirarchies, PLC is used to control a small machine, but DCS and SCADA are famous for process control. But due to advances in automation battles, every company is struggling for integration of every automation control its controller. Today PLC and RTU (Remote Terminal Units) are commonly manufactured as single device, which will used to communicate with higher controllers (DCS/SCADA). Google it, you will find further helpful advance approaches.
