SUPERVISORY CONTROL AND DATA ACQUISITION (SCADA) – It is an integrated system that is used to control and monitor the working of the individual parts of the plant. It helps in management of automation of large and complex processes. They are used to perform data collection and control at supervisory level.

The SCADA system consists of following components: –

  1. Supervisory computers– This is the heart of the SCADA system, which collects the data from the ongoing process and sending control commands to the field connected devices.
  1. Remote terminal units– Also known as (RTUs), these are connected to sensors and actuators in the process, and are networked to the supervisory computer system.
  1. Programmable logic controllers (PLC)- These are connected to supervisory computer system in the same way as RTUs but they are often use than RTUs as they are economical, versatile, flexible and configurable.
  1. Communication infrastructure– It connects the supervisory computer system to RTUs and PLC. During an error in communication system like in case of power failure the process of plant didn’t hamper and on resumption of power a person can continue the process.


  1. Human machine interface-HMI is the graphical user interface for the operator, collects all data from external devices, creates reports, performs alarming, sends notifications, etc.

PROGRAMMABLE LOGIC CONTROLLER (PLC)- It is an industrial computer control system which is used to control machines and processes. It continuously monitors the state of input devices (proximity sensors, limit switches etc.) and processes it, in order to control the state of output devices (valves, actuators, solenoids etc).

Following are the key features of PLC: –

  1. As the program is predefined, PLC generally work only in automated mode. If there is any kind of error during the operation, the operation cannot proceed till it rectifies or corrected.
  2. The set point predefined in PLC cannot be altered and needs a licensed program in order to make changes.
  3. In PLC system, there is no display unit due to which we cannot monitor what is going on during the execution of a process.

HUMAN MACHINE INTERFACE (HMI): – It is a component of a device that allows interaction between human operators and machine. By using this component, operator controls or operate the process by interacting with supervisory system. Process can be graphically visualized by using it.

Following are the key features of HMI: –

  1. HMI work in both auto and manual mode. If there is any error during a process, it can be turned to manual mode to ensure the continuity of the operations.
  2. Unlike PLC, the set points for HMI are not predefined. Therefore, they can be changed as per requirement.
  3. HMI always consists of display unit with function buttons to control or operate a process which makes user able to monitor or control the undergoing process.
  4. HMI is generally a touch screen to the PLC.


Reference links:-