Everything You Need to Know About Distributed Control System

Distributed Control System is most popular which is specially designed with redundancy and diagnostic capabilities to improve control reliability and performance. It gives greater flexibility to control distributed discrete field devices and its operating stations

In this era of revolution technology, industrial automation system deals with advanced automation control technologies to have better control performance over complex processes.

Distributed Control System
Distributed Control System

To increase reliability, productivity and quality while minimizing the production cost, process control industries must be driven by integrated controllers with high distributed control capability.

What is Distributed Control System

Distributed Control System is a specially designed control system used to control complex, large and geographically distributed applications in industrial processes. In this, controllers are distributed throughout the entire plant area.

These distributed controllers are connected to both field devices and operating PCs through high speed communication networks as shown in figure.

Discrete Field devices such as sensors and actuators are directly connected to input and output controller modules through communication bus. These field devices or smart instruments are capable of communicating with PLC’s or other controllers while interacting with real world parameters like temperature, pressure, etc.

DCS Architecture
DCS Architecture

Controllers are distributed geographically in various section of control area and are connected to operating and engineering stations which are used for data monitoring, data logging, alarming and controlling purpose via another high speed communication bus.

These communication protocols are of different types such as foundation filed bus, HART, Profibus, Modbus, etc. DCS provides information to multiple displays for user interface.

4 Basic Elements of Distributed Control System

Distributed Control System continuously interacts with the processes in process control applications ones it gets instruction from the operator. It also facilitates to variable set points and opening and closing of valves for manual control by the operator. Its human machine interface (HMI), face plates and trend display gives the effective monitoring of industrial processes.

Elements of DCS
Elements of DCS

Engineering PC or controller

This controller is the supervisory controller over all the distributed processing controllers. Control algorithms and configuration of various devices are executed in this controller. Network communication between processing and engineering PC can be implemented by simplex or redundant configurations.

Distributed controller or Local control unit

It can be placed near to field devices (sensors and actuators) or certain location where these field devices are connected via communication link. It receives the instructions from the engineering station like set point and other parameters and directly controls field devices.

It can sense and control both analog and digital inputs / outputs by analog and digital I/O modules. These modules are extendable according to the number of inputs and outputs. It collects the information from discrete field devices and sends this information to operating and engineering stations.

In above figure AC 700F and AC 800Fcontrollers acts as communication interface between field devices and engineering station. Most of the cases these act as local control for field instruments.

Operating station or HMI

It is used to monitor entire plant parameters graphically and to log the data in plant database systems. Trend display of various process parameters provides the effective display and easy monitoring.

These operating stations are of different types such as some operating stations (PC’s) used to monitor only parameters, some for only trend display, some for data logging and alarming requirements. These can also be configured to have control capabilities.

Communication media and protocol

Communication media consists of transmission cables to transmit the data such as coaxial cables, copper wires, fiber optic cables and sometimes it might be wireless. Communication protocols selected depends on the number of devices to be connected to this network.

For example, RS232 supports only for 2 devices and Profibus for 126 devices or nodes. Some of these protocols include Ethernet, DeviceNet, foundation filed bus, modbus, CAN, etc.

In DCS, two or more communication protocols are used in between two or more areas such as between field control devices and distributed controllers and other one between distributed controllers and supervisory control stations such as operating and engineering stations.

7 Important features of DCS

• To handle complex processes:

In factory automation structure, PLC-Programming Logic Controller is used to control and monitor the process parameters at high speed requirements. However due to limitation of number of I/O devices, PLC’s cannot handle complex structure.

Handling Complex Processes
Handling Complex Processes

Hence DCS is preferred for complex control applications with more number of I/O’s with dedicated controllers. These are used in manufacturing processes where designing of multiple products are in multiple procedures such as batch process control.

System redundancy:

System Redundancy
System Redundancy

DCS facilitates system availability when needed by redundant feature at every level.

Resuming of the steady state operation after any outages, whether planned or unplanned is somewhat better compared to other automation control devices.

Redundancy raises the system reliability by maintaining system operation continuously even in some abnormalities while system is in operation.

Lot of Predefined function blocks:

Predefined Function block
Predefined Function block

DCS offers many algorithms, more standard application libraries, pre-tested and pre-defined functions to deal with large complex systems. This makes programming to control various applications being easy and consuming less time to program and control.

Powerful programming languages:

It provides more number of programming languages like ladder, function block, sequential, etc for creating the custom programming based on user interest.

More sophisticated HMI:

Similar to the SCADA system, DCS can also monitor and control through HMI’s (Human Machine Interface) which provides sufficient data to the operator to charge over various processes and it acts as heart of the system. But this type of industrial control system covers large geographical areas whereas DCS covers confined area.

Sophisticated HMI
Sophisticated HMI

DCS completely takes the entire process plant to control room as a PC window. Trending, logging and graphical representation of the HMI’s give effective user interface. Powerful alarming system of DCS helps operators to respond more quickly to the plant conditions

Scalable platform:

Structure of DCS can be scalable based on the number I/O’s from small to large server system by adding more number of clients and servers in communication system and also by adding more I/O modules in distributed controllers.

System security:

Access to control various processes leads to plant safety. DCS design offers perfect secured system to handle system functions for better factory automation control. Security is also provided at different levels such as engineer level, entrepreneur level, operator level, etc.

Application of Distributed Control System

DCS system can be implemented in a simple application like load management using network of microcontrollers.

Application of DCS
Application of DCS

Here the input is given from a keypad to a microcontroller, which communicates with the other two microcontrollers.  One of the microcontrollers is used to display the status of the process as well as the loads, while the other microcontroller controls the relay driver. The relay driver in turn drives the relay to operate the load.

Hope you have understood the concept of distributed control system and its importance. Here is a basic question for you – Give any application of a DCS, you are aware of.

Please share your ideas and suggestions about this article in the comment section below.

Photo Credits:

Distributed Control System (DCS) by ferret
DCS architecture by plantservices
Elements of DCS by automation
Industrial complex structure by congatec
System redundancy by kepware
Predefined Functioning Block by tracemode
Sophesticated HMI by tradeindia


  1. Information is very useful please send me some Dcs programming example logic diagram

  2. Good content. please add good quality images.

  3. What is the difference between PLC and DCS?
    When the PLC should be use and when the DCS is to use?

    1. Hi Sujeeth
      PLC is used where no of IO are less or we can say in hundreds. But in process industries, no of IO are in thousands so plc is not able to handle such a large control system or PLC is not able to scan data faster.
      So Distributed Control System used as Master where so many plc and controller with a different protocol and complex networking are controlled by DCS with maximum redundancy and optimization.
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  4. Fat'hi Shamma says:

    Please could we have the latest technology of DCS for LPG trucks loading station.
    Best Regards.

  5. Dear sir,

    The information is very useful and informative

    Please indicate by Mail the latest developments of DCS

    Automation of Cryogenic Air Separation plants



    B.E( Mech): M.tech(IIT,Cryogenics). Aiche:FIE: CE: FOV

    Industrial gases,Cryogenics Consultant( a International)


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