A SCADA system is divided into process areas. Each of the process area are defined in operations to be performed.

For example, a pumping station may use two or three pumps, operating in a lead/lag/standby mode; the automation program in the controller is configured to operate the pumps based upon operator-entered setpoints and duty assignments. Each process area identified within a SCADA system will require its own Programmable Process Controller.

The PLC is programmed in order to monitor all signals sensors from the production shop floor, and to effect control over the process equipment, based upon the design of the program.The software designer must allocate the various field I/O signals, which collects the data from sensors and the PLC module control the inputs like:

• special inputs (Motion, regulation, speed counting)
• discrete input
• discrete output
• analog input and analog output

From there, the necessary input/output modules can be determined and configured for the Programmable Process Controller. Depending upon the number of signals, a PLC may consist of multiple racks or chassis of modules. From the perspective of the application program, however, the software recognizes all modules as though they all reside in a single long rack.Each of the programmable controllers requires programming in one or more forms.

PLC is an acronym for Programmable Logic Control, this is a device that was invented to replace the necessary sequence of relay circuits for a control of machines. The PLC works observing its inputs and depending on its states, the outputs change from on to off. Users can program this device usually by a language called ladder program logic, which resembles the electrical control circuitry used before the PLC came into being. Today, most PLCs allow for a variety of programming languages to meet the application requirements; for example:

• Function Block Diagram
• Structure Text (High-Level Programming)
• Sequential Function Chart
• Instruction or Statement List (Low-Level Assembly Programming)