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The traditional system, monitoring processes across multiple locations, involved the collection of data to a single location. This is what SCADA does, basically the "DA(data acquisition)" portion of SCADA. Some control was usually present but not a significant amount. The modern "DCS", or distributed control system, could be described by taking each word of the acronym out and explaining it. 

First, 'Distributed' implies that the functions are all distributed at various locations. These functions refer to the data collection, processing, controlling outputs, alarming, collecting historical data, etc. In a typical DCS system, you will find physical boxes that handle each of the above functions (well, almost). There is additionally an implication about geographical distribution, or that the functions can be distributed in different physical locations. This is a benefit in that prevents failure in one part of the system from affecting another part.

The word 'Control' implies the ability of the system to close the loop between an input measurement and a manipulated handle in the process. This means that the DCS shall have the capability for basic and advanced control algorithms.

The word 'System' implies that all of the above is connected as one contiguous system i.e. in simple words, one comprehensive system with systems and physical parts spread over potentially a wide area carrying out monitoring and control functions.

A control system may have one or more PLCs (programmable Logic Controllers). The PLC is a term applied in different ways. In large process facilities, there is a requirement that there be two systems-one for control while another for safeguarding. The plant-wide safeguarding portion is typically done by a safety-certified PLC. However, smaller PLCs could be used in smaller single loop control and safeguarding applications as well.

PLC Simplified with limited logic control for like a machine or Simple process with limited interlock logic - progamme can be modified easily.This will give only indication of failure.No recording the event or process parameter ,with limited memory.Cost wise it is very cheap.

DCS concern Huge multiple logic controlls with  greater than 1000 of inter lock for process of power plant or refineries or any process Industries it will give all types of alarm in soft copy & hard copy about process , failure & record all the process input & out put parameters & efficiency if required .(Boiler,Turbine, Generator controlls Switch gear cotrolls ie power generation to Transmission.) Very Large memory back up, failuer rate very less,process parameter all are fixed cannot be changed by Individual. Troubleshooting very easy . Production will not be affected at all. 

 Fully integrated DCS System:   


 
  
              


The ADVANTAGES were greatly reduced wiring costs, much more limited failure and less cost to add more points.   The DISADVANTAGES were that wiring costs were that wiring costs were still significant and there was lack of interoperability among controllers of various manufacturers due to the proprietary protocols. Hence the user was locked into a single vendor.
Popular DCS Systems Available:
  • Yokogawa – Centum Excel  , CS 3000, CS 5000
  • Fisher - Rosemant - Delta V
  •  Honeywell - TDC 3000, GUS
  • ABB - Freelance 2000
  • Moore -  APACS
  • Fox boro - I/A series             
 

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