PLC Vs DCS | Difference | Comparison

The main difference between a Programming logic controller(PLC) and Distributed control system(DCS) is widely utilized in industrial automation to control and automate machines and processes. Both the PLC and the DCS monitor the input and field signals and make decisions depending on the programming. So let's debate PLC Vs DCS to understand more about it.

What is PLC?

PLC stands for programming logic controller. The PLC is a microcontroller device used in industry to control and automate a machine or process. PLCs are primarily intended to replace hardwired relay logic controllers in order to improve industrial processes. 

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What is DCS?

DCS stands for Distributed control system. DCS refers to a control system for processes or facilities such as chemical plants, power plants, water management systems, and so on. DCS does this by incorporating a variety of autonomous controllers distributed throughout a network.  Although there is no central supervisory control in this distributed network, high-speed network connectivity allows for the linking of numerous controllers.

PLC VS DCS | Difference between PLC and DCS:

  • The main difference between a PLC and a DCS is that a PLC is a single entity designed to manage processes. DCS is a full system that is deployed across a broad network and is used to control the entire system.
  • PLC is a replacement for electromechanical relay while DCS replaced single loop controller by including multiple control components.
  • The speed with which logic operates also makes a significant difference between the two. As PLC is designed to perform high-speed operations, DCS operations are not expected to be quicker than it is intended to perform bulk processes.
  • The architecture of PLC is quite simple than that of DCS.
  • The response time of the programmable logic controller is one-tenth of a second but that of the distributed control system is approximately 30ms.
  • The PLC is a centralized controller that is typically found within the RTU. Due to the distributed nature of the network, there is no centralized controller in the case of DCS.
  • The programmable logic controller (PLC) is intended for managing machines such as lathe machines, with the PLC handling the machine's operation. Unlike a power system, where multiple activities are handled at different sites, distributed control systems control the entire process.
  • Because of their low scalability, PLCs aid in the reliability of repetitive activities. The increased scalability of DCS enables its usage in process enhancement and data integration.
  • PLC processes a centralized controller which is generally present within the RTU. However, there is no centralized controller present in the case of DCS due to distributed network.
  • The programmable logic controller is designed for controlling machines such as a lathe machine in which the PLC handles the operation of that particular machine. Whereas distributed control systems control the complete process like a power system where multiple operations are to be handled at various locations.
  • PLC due to low scalability helps to perform repetitive tasks with reliability. The higher scalability of DCS supports their use in process enhancement and data integration.
  • The programming of PLC is an application oriented whereas there is a set of built control functions with DCS which is configured according to its various uses.
  • The scan time provided by PLC is approximately 10 milliseconds, which is significantly smaller than the scan time provided by DCS, which is 100 to 500 milliseconds.
  • In general, a PLC supports less than 1000 I/O ports. While DCS can accommodate hundreds of I/O ports and are hence more scalable than PLC.
  • PLC offers more difficulty in troubleshooting than DCS to the reason that issues are more clearly visible in a distributed network than within complex machinery.
  • PLC involves minimum human interoperation whereases there is comparatively more than interaction in DCS.
  • PLCs are built in such a way that they can be used for certain processes. whereas DCS's design promotes its use in complex processes that necessitate regular alterations.
  • PLCs are less adaptable than DCS in terms of operator accessibility. The more flexible structure of DCS, on the other hand, raises the likelihood of unwanted engagement by multiple operators, culminating in an incorrect shutdown.
  • PLCs use discrete control. DCS entails regulatory oversight.
  • For human-machine interfaces, PLC employs simple visuals. DCS's UI features stunning graphics.
  • In the case of PLC, the application data are stored on several databases. The application data of DCS are stored on a single database. 
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