SCADA System

What is SCADA (Supervisory control and data acquisition)? Basics

SCADA System (Supervisory control and data acquisition):

SCADA is centralized control, computer system to gather and analyze real-time data. SCADA systems are used to monitor and control a plant, any process or equipment in the industry.

A SCADA system received information from field equipment or process, such as a leak on a pipeline, transfers this information back to a central site, notifying the home station that the leak has occurred, carrying out required analysis and control, and displaying the information in a logical and organized manner. SCADA systems can be relatively simple, or complex such as one that monitors environmental conditions of a small office building, or system that monitors all the activity in a thermal power plant.

Basics of SCADA

To Monitor:  SCADA continuously monitors the physical parameters.

To Measure:  It measures all the parameters for processing.

Data Acquisition: It acquires data from RTU.

Data Communication: It helps to communicate a large amount of data between MTU and RTU units

Controlling: Real-time monitoring and controlling of the process

Automation: It allows automation of functionalities.

To perform given functionalities some signals must be acquired by appropriate sensors and transducers from different electrical and electronic devices, installed in the field. This data is processed and analyzed by the SCADA system. This data is used for the supervision of remote processes. Data is processed and analyzed to generate reports which will be transmitted to control the process parameters within certain boundaries.

What is SCADA? Supervisory control and data acquisition

Fig. Example of  SCADA setup

The basic architecture of SCADA system:

What is SCADA? Supervisory control and data acquisition

Fig. Seperation of scada H/W AND S/W

  SCADA systems consist of a combination of hardware and software units The SCADA applications have two basic levels:

The customer level, to performed the human-machine interaction;

The data server level, to control the entire process.

SCADA applications run using a server. Desktop computers and screens act as HMI which are connected to the server. The servers are accountable for the acquisition and management of data for a certain set of parameters.

Is it possible to have dedicated servers for certain tasks.

A SCADA server will directly be connected to the PLC via an Open Platform Communications (OPC) or  RTU, which collects and consolidate data from and from PLC

 OPC and RTU work as a data concentrator. The data is organized using a man-machine interface (HMI) in a suitable format for the operator in order to be able to take any decisions that would optimize the communication between the equipment.

Hardware architecture: Scada contains following basic building blocks.

Supervisory Station

Remote Terminal Unit (RTU)

Programmable Logic Controllers (PLCs)

Communication Network

RTU (Remote terminal unit): Perform the following tasks

The RTU connects with supervised equipment

It Reads the equipment status (such as open/closed position of the valve or relay).

Acquisition of measured signals, such as the pressure, flow, voltage or current.

The control of equipment by sending command signals, such as the closing of a valve or relay or setting the speed of a pump;

Reading the digital or analog signals, and sending the commands using both digital and analog signals.

PLC (Programmable Logic Controller)

PLC Contains a microprocessor used for the automatization of processes.

A PLC is provided with input/output sensors and relays.

PLC is not as expensive compared to old systems which used tens or hundreds of relays and timers.

The PLC is programmed using structured programming languages and elementary logic operations.

Supervisory Station:

Refers to the servers and software responsible for communication with the equipment (such as RTUs, PLCs, etc.) and HMI software which run on master-station in the control room or elsewhere.

The master station may be composed of one PC in the small SCADA system. In large SCADA systems, the master station may include multiple servers, distributed software applications, and disaster recovery strategies.

Master Terminal Unit (MTU)

MTU is the fundamental unit of the SCADA system. It includes a computer, PLC and a network server that assistances MTU to interconnect with the RTUs. MTU begins communication, collects and saves data, helps to interface with operators and to communicate data to other systems.

Communication Network

In general, network means connection. A communication network is defined as a link between RTU in the field to MTU in the central location. The bidirectional wired or wireless communication channel is used for networking purposes. Various other communication mediums like fiber optic cables, twisted pair cables, etc. are also used.

Software Architectures

The SCADA applications are executed on the software architecture. SCADA software architecture has two components

SCADA server application

SCADA client application;

What is SCADA? Supervisory control and data acquisition

Fig. software architecture

SCADA server and SCADA client

SCADA server application is multi-tasking and is being responsible for storing the SCADA client applications in the database.

SCADA client application accepts data through serial port RS232 and MODBUS RTU protocols.

The Web SCADA clients can be a benefit from real-time access (in a manner similar to regular SCADA clients) to the lists of parameters, the lists of events and a list of alarms through a simple web browser.

But, the Web-connected SCADA clients do not have the same level of graphical user interfaces and the same HMI features as the regular SCADA clients have. The regular SCADA clients are inured to running specific applications whereas the Web SCADA clients view the Web pages provided by Web SCADA server.

HMI (Human-machine interface) HMI/SCADA appears as a terminal dedicated for users from the system implemented with PLC units. A PLC is programmed to control a process automatically. Because the PLCs are distributed in a large system, the data

from PLC are stored and then transmitted. HMI/SCADA has the role to gather and combine the data from the PLC using a specific protocol. MI software can be connected to a database in order to realize the charts and diagrams in real-time.


HMI analyse the PLC data.

Use to treat the planned maintenance.

Itdrills the schemes for a particular sensor or machine.

Helps to apply the troubleshooting methods of the system.

The basic SCADA protocol is MODBUS, which is designed to send data to the master station even when the master interrogates the RTU station. MODBUS is based on a master-slave architecture known also as a client-server architecture.

SCADA Systems are widely used in the following:

Pipeline monitoring and control

Oil and Gas

Offshore platforms and onshore wells

Water and Wastewater


Food and Beverage




Remote monitoring and control of production, pumping, and storage locations

Refineries, petro-chemical stations

Water treatment centers and distribution

Wastewater collection and treatment facilities

Electrical power distribution from gas-fired, coal, nuclear

Electrical power transmission and distribution

Agriculture / Irrigation

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