Difference between PLC and DCS?


Difference between PLC and DCS


A 1)plc is a programmable logic controller which can handle the binary input and output of the logic statement which are stored in its memory.its a digitally operating system which are logic stored in its memory by programming system.
DCS:-Distributed control system its control all types of process variable at a time through this we can control the whole plant,we can see the status of plant,and handling yhe unit.
A2)In PLC the Scan time is less.but the scan time of DCS is high.Because it Control the whole plant. So in safety loop we used PLC
A 3)First let’s take a quick brief history about controllers:
1 - For analog control loops we have a stand alone PID controller.
2 - Then we get a networked PID controllers through a hub and HMI for configuring and monitoring.The PID itself contain a network port.SIEMENS introduced PID 353 & PROCIDIA software for it.
3 - Then we get DDC - DIRECT DIGITAL CONTROL, single CPU, I/O’S for analog instruments only & networked PC is an option.SIEMENS introduced APOGEE, FISHER introduced ROC.
4 - The next phase is DCS - Distributed Control System, and this is first introduced by HONEYWELL & YOKOGAWA.Ability for handling large scale of I/O’S, discrete and analog, complex control loops, PID algorithms calculations and executions.
5 - PLC is basically designed and produced for discrete on/off control, limited I/O’S.
but modern PLC with great progress in h/w technology introduced itself as a less expensive option for PID, with it’s complicated calculations and algorithms.


6.by the time with great achievements in H/W tech. and networking capabilities the single PLC can handle discrete and analog instruments with wide range.
7 - Now the man can make his own DCS with multible networked PLC’S, but it will need a great engineering and effortsto get near from DCS features and functionality.


As the name itself says distributed control system has distributed controls throughout the plant area for handling large complex circuits with difference in geographical locations in industrial processes.
7 Important features of Distributed Control System.
To deal with complicated processes:
PLC- Programming Logic Controller is used for controlling and monitoring high speed process parameters in a factory automation structure but they cannot use complex structures because of the restriction of the number of input devices. But then Distributed Control System (DCS) is mostly suitable for complex systems where more than one input devices is used for example batch process control.
Hence DCS is recommended for complicated management programs with more variety of I/O’s with devoted remotes. These are used in production procedures where developing of several products are in several procedures such as group procedure management.
System redundancy:
DCS helps program accessibility when needed by same feature at every stage. It is always good to
have a resuming steady state system after the outputs rather than using automation control devices. System reliability is increased by maintaining the system operation on a non stop basis even during some fault while system is working. There are various algorithms and application libraries tested before hand along with functions which are predefined for handling large and complex systems. This saves a lot of time for programming and control for various applications.

Powerful Programming Languages:
Ladder, function, block, and sequence are provided by this language for developing custom programming based on user interest.
More innovative HMI:
Just like the SCADA program, DCS can also observe and manage through HMI’s (Human Machine Interface) and it provides data which is sufficient to the owner to charge over various procedures and it functions as heart of the program. But this type of industrial control system that includes vast regional areas whereas DCS covers only a small area.
Just like a PC window, DCS takes the entire process plant as a control room. Logging, graphical representation, and new trends of the HMI’s give effective customer interface. For quick response of operators to the plant conditions, a powerful alarming system of DCS is used.
Scalable platform:
By adding more clients to the communication system or by adding lots of input output devices in distributed controllers, DCS can be based on scaling various input/outputs from a range of small to large server.
System security:
For plant safety access to various control processes are used. DCS offers help for better safety automation control as it offers better security system for handling the automation control. Protection is also offered at different levels such as entrepreneur level, engineer level, user stage, etc.


PLC - Programmable logic controllers have the capabilities once found only in distributed control systems DCSs, while a DCS can handle many functions previously thought more appropriate for PLCs.


The main difference is the DCS architecture that employs a single, central server/database that both the controller(s) and the HMI software jointly access, unlike PLCs with HMIs where there are two databases that are maintained separately.

The DCS tight integration of the HMI to the controller(s) is inherent in a DCS, whereas the integration is much looser in the PLC world. Tag databases can be exported and imported, but they’re still two different databases.


Programmable Logic controller is a digital computer used for industrial process automation. A PLC could be standalone or networked.

DCS or Distributed Control System is a network of Highly Powerful PLCs and Data Server.

PLC is usually used for controlling medium or large scale applications, whereas DCS is used for controlling entire plant.