Earlier PLCs use as only controlling device. you make the program, download the program in PLC and it will run for years.

Nowadays the automation system is a combination of PLCs and SCADA software.

If you use PLC & SCADA combination the advantages are that you have better monitoring and control of the plant and also you have access to the information the way you want.

SCADA enables a different operator to view and interact with the workings of entire operations through a graphical representation of their production process

SCADA installed on PC.and run on PC. SCADA is communicating or connected with plc or other peripheral devices.

It enables you to generate applications for the most demanding requirements of different operator tailored precisely to the needs of each plant.

SCADA constantly fetch data from the plant in real-time, data stores on a database, processes it in the database generates report, evaluates and generates alarms, show information on display plant operators to plant operators, supervisors, and managers.

SCADA systems are majorly used in industry for Supervisory Control and Data Acquisition of industrial processes.

SCADA stands for Supervisory Control And Data Acquisition.

As the name indicates, that is not a full control system, but that focuses on the supervisory level.

As such, that is a unique software package that is connected with Programmable Logic Controllers (PLCs) or other commercial hardware modules.

One difference two basic layers in a SCADA system: the “client layer” which refers to the man-machine interaction and the “data server layer” which handles most of the process data control activities.

The data servers communicate with devices in the field using PLC. Process controllers, e.g. PLCs, are connected to the data servers either directly or via networks or field buses that are or non-proprietary (e.g. Profibus).

Data servers are connected to each other and to client stations via an Ethernet LAN or some other Communication media.

The software multi-tasking and are based upon a real-time database (RTDB) located in one or more servers.

Servers are responsible for data acquisition and handling (e.g. polling controllers, alarm checking, calculations, logging and archiving) on a set of parameters, typically those they are connected to.

Scada system architecture

SCADA Communications :


  • Internal Communication Server-client and server-server communication is in uses a TCP/IP protocol
  • i.e., a client application access to a parameter which is ‘owned’ by a particular server application and only changes to that parameter(if consider ethernet than i/p address, if consider Modbus than baud rate, device id, parity, com port) are then communicated to the client application. (Like web client functionality, Viewpoint functionality)

Features of SCADA:


  • Dynamic process graphics
  • Alarm summary.History
  • Real-time trends, Historical trends
  • Application security
  • Database connectivity, Device connectivity
  • Recipe management
  • Scripts

There are five components to the SCADA system, as listed below

  • Field Devices and Signals
  • Programmable Process Controller
  • SCADA Server Computer
  • SCADA Operations User Workstation
  • Communication Network

Field device in terms of signal.these may be an analog signal or digital signal. this signal are input from plc and output from plc when designing SCADA system in terms of information is to be monitored and controlled

Field device different type :


Signal Transmitters-analog,pressure transmitters-analog,valve-open/close,motor-run/stop,Discrete input signal:-limit switches-open/close,photocell sensors,push button,selector switch,Discrete output signal,motor control relays,solenoid valves

Scada Operations and workstation:


Scada Workstations provide the process graphic displays with controlling and monitoring information or equipment, the software involves the creations of graphic display, historical trend, report display, alarm and event summary, process data recording

Communication Network:


PLC and SCADA Operations and workstation(SOW) via connected ethernet or high-speed communication network, the different type of system architecture or network topology used for SCADA system

Scada server computer:


Most of the SCADA system working with Data server computer. Historical data maintained on the server computer in the databases.and all field controllers data is also maintained in databases on the server computer server computer perform communication with plc and Field device. Today a SCADA system working with (Relational Data Base Management System (RDBMS) to store, retrieve and report information. RDBMS also use with SQL-compliant application

Initial SCADA Project Specifications:


  • Specifications for field controllers plc and SCADA operator workstations (SOWs)
  • Identification of input and output signals required for each controller
  • Process flow chart and flow diagram– initial version of Process Control Logic Descriptions
  • Process & Instrumentation Diagrams (P&IDs)
  • Details on the construction of the works.

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