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What are the differences between DCS and PLC+SCADA

Discussion QuestionsWhat are the differences between DCS and PLC+SCADA
Tamunofiniarisa Staff asked 4 years ago

From dhinesh junaa @linkedin

Hai all,

Greetings !! I am a Beginner in Instrumentation field. I have a small Confusion with DCS And PLC+SCADA.

In Furnace Control application, SCADA which collects the data from Furnace where PLC depends on the input states it will energies the Outputs. With these two combinations equals the operations of DCS ?

If not then what will be major difference between DCS and Combination of PLC+SCADA ?


1 Answers
cliff04629727 answered 4 years ago
PLC stands for Programmable Logic Controllers. In the olden days, interlocks and plant sequential operation were implemented using relay logic. The problem with relay logic is that it is difficult to change the logic. If you wanted to change the logic, you had to change the wiring. PLCs came into existence to address this problem of relay logic. PLCs were microprocessor based devices which could be reprogrammed any number of times. If the operation logic changed, only the PLC ladder program had to be changed. The ladder language was very intuitive to the plant technicians and they didn’t need to have any experience in computer/ microprocessor programming. Initially PLCs were meant to handle digital relay logic where as Distributed Control System handled most of the analog control.
Distributed Control System architecture had analog and digital controllers (microprocessor based devices) spread across the plant. These controllers sent data to the control room through proprietary communication networks. The control rooms had VDU (video display unit) where plant data was displayed. DCS systems were proprietary in nature meaning that the complete solution was given by a single vendor. Major DCS system suppliers were Honeywell, Yokogawa, ABB, Foxboro etc. The distributed nature of the technology made it inherently reliable. The feature that made DCS so popular was the fact that the DCS vendor took complete responsibility of the implementation. Evolution of the DCS technology made it extremely reliable over time. DCS technology had fault tolerance and reliability inbuilt and the DCS vendors utilised the expertise of the best designers. DCS systems are still used in the process industry because of the reliability.
SCADA stands for Supervisory Control and Data Acquisition. SCADA technology was used to connect remote sites spread geographically across thousands of kilometres. The remote sites has RTU (Remote Telemetry Units) which transmits data to central control rooms using radio communication. Since there is a lag in data transmission to control room, real time control is done by RTU. Control room does supervisory control by providing set points to the RTUs. To begin with, SCADA vendors supplied complete SCADA solution with RTUs and SCADA software. Now there are many vendors who supply only SCADA software packages (based on Windows platform) with which the user can build his own HMI or control application by using PLC as RTUs.
Now PLCs have advanced to include analog control and many other advanced control functions. So a combination of PLC with SCADA software packages works out to be functionally similar to DCS technology. Such implementation will also be much cheaper than DCS technology. This type of solution will also not have proprietary control by one vendor. But the most important advantage of DCS technology is the reliability.
PLC is controlling device with IO capacity in 1000s with real time process control. It can handle small application with limited IO capacity. PLC is used in small plant or machine.
SCADA is software for control & monitor the process from remote location , it can store the data in scada server.
DCS is distributed control systems with more IO capacity and its more powerful in real time application with redundancy , which is used in big industries like Oil refinery, paper mil & chemical industries.
As for the component of your question regarding SCADA – the role of SCADA is like the role of the cockpit in a aeroplane – it’s the place where all the system information and control is brought together in an easily understood manner. All major control operations can be performed from the SCADA system, and all major system information can be obtained in that one place also.
1.PLC:- It is 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.
2.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.
3.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.
PLC is a controller through which we can control our machineries that how they work . It is totally depends on PLC program (which one we made on PLC software) that when your machinery will on or when your machinery will off.
SCADA is a software through which we can design an outlook about our system or work, like our plant is about bottel filling then we can design an outlook of our plant and see it where ever we are at that time through our PC or lappy that how it works.
DCS is a distributed control system through which we can communicate PLC and SCADA. So when we are not in our plant or near a machine then we can control our machine through SCADA. So totally DCS is a way to communicate SCADA and PLC

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