From OpenSCADAWiki
Modern systems of Automatic Process Control (APCS) are quite complex. Conventionally, the hierarchy of PCS can be divided in two levels: the lower and upper levels. The lower level of the PCS contains a field of equipment (sensors and executive mechanisms), as well as programmable logical controllers (PLC). The upper level consists of a system of operational visualization and monitoring of the process — SCADA system. PLC is the responsible part of the APCS, which performs function of the data acquisition from the field of equipment, calculation and making the regulatory, blocking and other actions on the regulating parts of the field of equipment.