Difference between PLC and DCS

Both PLC and DCS are extensively used in industrial automation to control and automate a machine or a process. Both PLC and DCS monitor the input and field signals and take the decisions based on how they are programmed.

There are many differences between PLC and DCS which we will highlight in this article. Let's start with some basics of PLC and DCS so that it becomes easier to understand the differences between them.

What is a PLC?

PLC stands for Programmable Logic Controller. The PLC is a microcontroller device that is used to control and automate a machine or a process in the industry. The PLCs are mainly designed to replace the hard wired relay logic controllers so that the industrial processes can be improved.

A typical PLC consists of five major parts namely a processing unit, a memory unit, an input-output interface, a power supply and a communication interface. Every PLC required to program depending on the application for which it is used. There are many types of programming languages developed to program a PLC such as ladder logic, structured text, functional block diagram, instruction list and structured functional blocks.

What is a DCS?

DCS stands for Distributed Control System. A DCS is a computerized control system which has a collection of multiple control components distributed throughout a process or a plant. A DCS does not have central operator supervisory control.

The DCS system makes the process more reliable and minimizes the installation cost by localizing the control functions near the plant or the process, with remote supervision and monitoring.

The DCS is highly scalable and is used to control complex industrial processes which need large interaction between multiple controllers. The major parts of a typical distributed control system (DCS) are: multiple controllers, IO (input-output) modules, operator stations, engineering stations, and storage and communication system.

The most extensive applications of DCS are in petro-chemical industries, paper industries, power plants, pharmaceutical industries, refineries and many other manufacturing and processing industries and plants.

Difference between PLC and DCS

Both PLC and DCS are used in industrial automation to control and automate the processes and the plants. However, there are several differences between them that are given in the following table −

Basis of Difference	PLC	DCS
Full form	PLC is an acronym for Programmable Logic Controller.	DCS is an acronym for Distributed Control System.
Definition	A PLC is a microcontroller designed to control and automate the processes in industries.	A DCS is a computerized control system consists of multiple controllers that are distributed throughout the system for control and automation of a plant.
Major parts	A typical PLC consists of five major parts namely a processing unit, a memory unit, an inputoutput interface, a power supply and a communication interface.	The major parts of a typical distributed control system (DCS) are: multiple controllers, IO (input-output) modules, operator stations, engineering stations, and storage and communication system.
Objective	PLCs are mainly designed to replace the hard wire relay logic controllers that consist electromagnetic relays.	DCS is primarily designed to replace the pneumatic or single loop controllers (SLCs).
Type of control	PLC involves discrete control.	DCS involves regulatory control.
Operator interface	PLC uses simple graphics human machine interface.	The interface of DCS has sophisticated graphics.
Development of logics	PLCs are programmed based on the application.	DCS has in-built control functions that are configured according to application.
Programming	For PLCs, various high level programming languages are available for creating custom logic.	For DCS, the existing function blocks are used to create custom logic.
Redundancy	In case of PLCs, the redundancy may not be cost justified. It is because the additional hardware are used to make PLCs redundant which makes them costlier.	DCS has redundancy as its default feature
Architecture	PLC has comparatively simple and flexible architecture.	The architecture of a DCS system is relatively complex and less flexible.
IO (Input - Output) capacity	The IO handling capacity of a PLC is less, around few hundred IOs.	The IO handling capacity of a DCS is more than that of a PLC. It is around thousands of IOs.
Response speed	The response speed of a PLC is comparatively high, i.e. a PLC can typically respond to a change within 1/10th of a second.	The response speed of a DCS is relatively slower. For a DCS, the typical response time is around 30 ms.
Storage of application data	In case of PLCs, the application data are stored on several databases.	The application data of DCS are stored on a single database.
Number of controls	PLC has a limited number of controls.	DCS can handle many controls.
Processes	PLC performs batch processing with low level of process interactions.	DCS involves inter related continuous complex processes.
Predictable	The processes of PLCs are not completely predictable.	The processes of DCS are predictable.
Alert	PLCs diagnostic to inform you when something is broken.	DCS alters you to what might break before it does.
Cost	PLCs are cheaper than DCS hardware but their engineering is expensive.	DCS hardware is expensive but the engineering involved is relatively cheaper.
Size	PLCs are compact in size.	DCS is larger in size.
Applications	PLCs are mainly used in discrete production industries, such as steel industry, automobile industry, chemical industry, etc.	DCS is mainly used in process industries such as refineries, paper mills, power plants, water management and treatment plants, boiler controls, etc.

Conclusion

The most significant difference between PLC and DCS is that PLC is primarily focused in process industry, whereas DCS is in discrete production industry. However, both are nearly same in their function, i.e., control and automation.