SCADA
A SCADA (or supervisory control and data acquisition) system. It consists of many remote terminals units for collection of data (field),that is being connected with master station through any communication system, having main task of collection of accurate data and controlling of process for smooth operation.DCS
It stands for distributed Control System, controlling is performed by embedded system (Microcontroller based or Microprocessor based controlling unit for device or instruments from which data is to be collect. It provides very intelligent analog control capability. It is very sensitive for HMI (Human machine Interface) for easy and smooth control of process.PLC
It stands for Programmable Logic controller, having get this name from the fact that it replace the relay logic at the initial stage then it get the capability for analog channels also for display then it get the ability for close loop control and after some time it has the ability for redundant operation, and also its HMI having the ability for Indicatiion,controlling,data logging ,Alarming and backup data facility. It is also defined as below:“A digitally operating electronic apparatus which uses a programmable memory for the internal storage of instructions for implementing specific functions, such as logic, sequencing, timing, counting and arithmetic, to control through digital or analog input/output, various types of machines or process.”
National Electrical Manufacturers Association (NEMA), 1978