DCS vs PLC | 5 IMPORTANT Differences NO ONE Teaches You! // Difference Between PLC and DCS

What are the differences between DCS and PLC? This was found to be one of the most common questions in Industrial control and automation systems A typical answer to this question is that the DCS as its name says is a Decentralized Control System or in other words is a control method in which we have some independent CPUs Each CPU is in charge of controlling individual parts of the process so that if one fails, the other CPUs will continue performing their function without interruption On the other hand, the name of PLC or Programmable Logic Controller does not

imply the concept of Centralization or non-centralization by itself What we see in practice is like PLC mostly used for centralized control meaning that a single PLC or CPU will control the whole process However, this answer sounds to be correct to some extent but this is not the whole story We must be aware of some other major differences between PLC and DCS such as their hardware and software In this video, we will investigate the other important aspects of this question If you want to figure out the answer you can watch the rest of this video and

if you are new to this channel please go ahead and subscribe and click the bell to receive notifications of new videos by Upmation channel The primary goal of the PLCs invention was to replace them with the old control systems which were entirely made of relays and contactors or digital signals in other words DCS or Distributed Control System was made a few years later not to replace the PLCs but to satisfy other needs in the automation industry Therefore, PLC and DCS have coexisted in the industry for many years in different applications and they have their significant

pros and cons Although the PLCs became more advanced and sophisticated these days and are even able to control the PID loops and analog signals they are more appropriate to be used for Discrete Processes automation such as the automobile assembly line in which there are lots of digital signals and a few analog signals DCS is more applicable for Continuous Processes with lots of analog signals and complicated PID control loops such as processes in a power plant or a refinery In Batch or Hybrid Processes automation like some steel-producing industries in which we have to use DCS and

PLC together there would be some overlaps in their functions Meaning that we may use the PLCs to process a limited number of analog signals and PID loops in addition to processing the digital or discrete signals Consider that, as the number of control loops and analog signals assigned to a PLC increases the control reliability of the process decreases dramatically because if the PLC fails then all the loops assigned to that will be out of control and the process fails consequently On the other hand, if we use the DCS to process discrete signals since the reaction time

or Scan Cycle time of the DCS is fairly high in comparison with the PLC the real-time controlling of the process would not be possible The high reaction time or Scan cycle time of the DCS is due to its heavy processing duties For example, the DCS is using high-level programming languages such as CFC or “Continuous Function Chart” to issue some extended and very useful programming monitoring and engineering facilities like lots of predefined functions These high-level languages will generate lots of codes after getting compiled and puts lots of burden on the CPU As a result, the response

of the system to the important inputs will not be fast enough On the contrary the popular Low-Level PLC programming languages such as Ladder Logic Function Block Diagram and particularly STL or Structured Text Language are closer to the machine language and therefore, the generated codes are remarkably fewer than the DCS programming languages It's the reason that the PLC is used for the emergency systems So, to take advantage of the languages like CFC the hardware of the DCS should be more powerful than a PLC system However, with today’s advancements in technology the hardware of the DCS and

PLC are more closed to each other Sometimes it is not possible to judge whether a control system is DCS or PLC, only by looking at its electrical control panel Even in some cases PLC control systems use the same CPU as the ones commonly used in Decentralized Control Systems Finally, it is the DCS and PLC monitoring systems differences In DCS, there is an integrated software package in which there is a fairly complete set of monitoring facilities By calling the control functions into the programming environment their corresponding graphical objects will automatically be added to the HMI pages

with their addresses In this way, we’ll save some precious time! In PLC systems, however usually, there is no relationship between the programming and the monitoring environments and they need their own time to be created and developed And as the last point, the Decentralized Control Systems are more reliable in comparison with the PLC control systems not just for being decentralized but for being usually redundant in different levels From CPUs and Power Supplies to Servers network switches and network cables There are lots of benefits in using both the DCS and PLC As the DCS response time is

more than the PLC the DCS is better to be used for processes with more analog signals and PID loops whereas the PLC systems are better for processes with more discrete and less analog signals In DCS we can benefit from numerous predefined functions and function blocks and its high-level programming languages but in PLCs, we often have to write and define the functions by ourselves We also talked about the benefits of the DCS DCS in designing the monitoring system The network architecture of the DCS is way easier to implement using its integrated software and hardware package However,

DCS software and hardware packages cost a lot more than PLC equivalents That’s it for this video! We hope you have learned something new and if you liked this video please share it with your friends and colleagues You can also watch these videos to take in more interesting and useful skills and information about Industrial automation systems