Introduction to SCADA

What is SCADA?

SCADA stands for Supervisory Control and Data Acquisition. SCADA is a control system which is used in industrial process and infrastructural process. The SCADA is not a specific technology, but a type of application, it is a pure software package that is positioned on top of hardware to which it is interfaced, in general via a Programmable logic controller (PLCs), or other commercial hardware modules. SCADA enables a user to collect data from one or more distant facilities and send back the control action.

A SCADA system allows an operator in a location central to a widely distributed process, such as oil and gas plant, pipeline system, hydroelectric complex systems, to produce control action and to monitor the measured variable.


What is Data acquisition?

Data acquisition is the process of retrieving control information from the equipment which is out of order or may lead to some problem or when decisions need to be taken according to the situation in the equipment. So this
acquisition is done by continuous monitoring of the equipment to which it is employed. The data accessed are then forwarded to a telemetry system ready for transfer to the different sites.


Elements of a SCADA system:

  • HMI
  • RTU
  • MTU
  • Data transmission


HMI unit:

A Human – Machine Interface (HMI) is the apparatus which presents process data to a human operator, and through this, the human operator monitor and control the process. HMI offers real-time monitor of data about the process and through which an operator can send commands to the controller. Input devices such as the keyboard, mouse, trackballs are available in this section.


MTU (master control unit), which is the system controller. Some industries use the term “host computer” instead of MTU.

MTU communicates with the RTU that is located away from the central location. There can be many RTUs in the field, MTU can monitor and control the field using the scheduled program even when the operator is not present. Changes can be done in the process from the MTU end, can read some process parameter.


Remote terminal unit (RTU) connection to sensors in the process and converting sensors signals to digital data and sending digital data to the supervisory system. RTU communicate with the MTU using modulated signal. RTU receives the modulated data from MTU and connection can be through cable or radio.

RTU field device connection is through cables. RTU supplies both electrical power and actuator signal to the field device.


SCADA tags:

SCADA systems typically implement a distributed database, commonly referred to as a tag database, which contains data elements called tags or points. A point represents a single input or output value monitored or controlled by the system. Tags are two types:

  • System-defined Tags It represents the system value in the SCADA it is represented by dollar sing $
  • User-defined tags:  This tag is defined by the user or programmer. Except (A, S) are not defined

Classification of tags:

  • Direct tags
  • Indirect tags

Direct tags:
These tags are directly used by the programmer to represent the value of plant or variable

Indirect tags:
These tags are used by another tags to show or represent the value of plant or variable
Direct tags Memory tags: these tags having no connection with the process the plant.

Advantages of SCADA:

  • Long distance monitoring
  • Long distance training
  • Protection against terrorism/vandalism-alarm
  • Data management (engineering and operations)
  • Automated operations with real-time control
  • Reliability and Robustness (very large installed base, mission-critical processes)
  • Rich functionality (50 – 100 p-yrs investment)



Instrumentation Engineer

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