Learn PLC

currently searching for learning PLC program which is the PLC can interface with scada to communicate with the sensor or device that we use. In generally, the scada system is to monitor and control anything. between the scada and the device that we use such as sensor we need one platform to interface the analog signal from the sensor and the coding that we produce at scada system. that why i need to know PLC also. here are the link that i can study plc : http://www.plcs.net/

below is the block diagram for PLC