What is the Difference Between PLC and DCS?

What is the Difference Between PLC and DCS?


Before we get into today’s video, if you love our videos, be sure
to click the like button below. and make sure to click subscribe and the bell to receive notifications
of new RealPars videos. This way you never miss another one! In a nutshell, a PLC or
programmable logic controller is a ruggedized computer used
for automating processes. A DCS or distributed control
system is similar to a PLC in that it has rugged computer controllers however the DCS contains
multiple autonomous controllers that are distributed throughout a system, also used for automating processes. You may have read that and said
“So, what’s the difference?” For the answer to that question, we have to go back more than 40 years. After several years in a
corner designing office, this guy named Dick Morley quit his job after asking his employer to allow him
to work on Saturdays instead of Fridays, which they declined. You see, Mr. Morley loved to ski but found that
the weekends were too crowded for his liking. Due to financial obligations and such, Mr. Morley and a friend had formed
the company Bedford Associates where they were writing proposals, for local tool firms, who desired to evolve
into the new solid state manufacturing arena. These proposals used small computers, seemed to be repetitive in nature, and from one project to the next, there were many similarities. Eventually Mr. Morley got
bored with writing proposals, because of the repetitive nature, and began to wonder if he could create a
controller that could handle these every day jobs. Actually, during a hangover, Mr.
Morley created a draft for a proposed programmable
controller and took it to his team. They began designing the
programmable controller. After finding financial support, the company, Modicon was created. Unbeknownst to Modicon and during the design
phase of the programmable controller, a guy from GM had presented a paper, a request so to speak, for a solid state controller
that would make plants more reliable and durable, which would also replace the
hard wired relay systems that were pervasive in the
manufacturing industry. As the story goes, sometime later, GM hears about the work
being done at Modicon and eventually contracts with them
to purchase over $1,000,000 in PLC’s (at the time, the controllers were
called programmable controllers and the “Logic” part of the current name wasn’t added until the dawn of
personal computers or PC’s). Modicon was then baptized and
quickly became a business. The name has persevered through
a couple of acquisitions, the latest and current
being Schneider Electric. In the beginning, the PLC was used
primarily for discrete controls. After all, the large purchase by GM for
replacing hard wired relay systems. The programming of the PLC’s
was primarily in ladder logic, which is a format that is
very similar to a schematic. The PLC received device
information from the field, solved the logic and then energized the
outputs to produce the desired effect. Essentially, the PLC was invented to perform
repetitive tasks in a reliable and durable manner. As for the DCS, around 1975 a few companies
came out with a version of a DCS. Basically, the creation of a DCS system came about
because of the increasing use of microcomputers. There had been other computer
based systems in the industry since the late 1950’s but had limited scopes for scalability, robustness, and security. There were many benefits to a DCS
but one of the primary draws was that an entire plant could be
connected via proprietary communications and controlled by a distributed system. For instance, say you had a plant that
made an ice cream filled cookie sandwich. The plant would have a production
line for the ice cream and one of the autonomous controllers
would process the batch of ice cream. After the ice cream batch is complete, another autonomous controller may
process the freezing of that ice cream. Yet another controller may
process the cookie batch, while another may supervise
the baking process. With several autonomous controllers, if a controller failed, it would impact only
that process and not all of the others, which lead to a robust system that
virtually eliminated entire plant failure. The DCS was really good at autonomously
controlling single or multiple processes. Another major benefit of the DCS
was the integrated monitoring and control system similar
to today’s SCADA systems. The reason it’s a major benefit is
that the entire tag base is there, already created for the process control, available to use on the
monitoring and control screens. DCS’s also had function block programming. Function block programming,
if you are not familiar, is a section or several lines of
code behind a single interface. That interface may do something
like handling the manual and automatic operation of a valve. Function block programming saved a lot
of time and redundant programming. Essentially, the difference
40 years ago was considerable and if you owned a large plant
with continuous processes, you likely would have chosen a DCS. In today’s industries, the DCS
and PLC are quite similar, save for the integrated
monitoring and control. With open source
communications, fiber optics, Ethernet and the like, many PLC’s
can now communicate with each other and be autonomous PLC that communicate over
the network to other autonomous controllers. That wide communication would allow for
single or multiple processes being controlled by one PLC to communicate
with another PLC. Take our ice cream sandwich example. PLC-A could process
the ice cream batch. When the batch is complete, PLC-A would communicate with PLC-B
that the process was complete and PLC-B could then launch
the freezing process. You can see that with today’s technologies, a wide and robust PLC system could do virtually
the same thing that the DCS’s can do. An advantage of the DCS
is installation costs. This advantage occurs because of the
location of the autonomous controller to the process can be close in vicinity versus
pulling long runs of I/O wire across a plant. Another advantage is the onboard
monitoring and control system. One of the drawbacks to the DCS’s
is the scarcity of programmers that have some experience with a DCS’s. Most plant floor technicians are
familiar with ladder logic programming however, the DCS programmers and technicians typically need more specialized
experience in database functions as well as IT-related networking knowledge. Because of the specialized training, DCS programmers are a
bit harder to come by. In speaking of advantages, today’s PLC
systems can have nearly the same as the DCS, excluding the supervisory control
and data acquisition (SCADA). With a PLC system (multiple
PLC’s in a plant structure), you still need to create the
supervisory and control system. The entire DCS database would be available for
the creation of the monitoring and system, the PLC systems individual PLC databases would
need to be created in the SCADA system software. There are more programmers available
for hire in the PLC arena and with the new programming
languages such as function block, sequential function, etc., the advantage of function
block programming is no longer exclusive to the DCS. This saves in development
time when programming a PLC. As you can tell, there are likely advantages
and disadvantages in both systems. The take away is that with
today’s technologies, either system can control an entire plant. Which system is chosen will
likely take the advantages and disadvantages into account
as well as system costs. In summation, the DCS has autonomous controllers
dispersed throughout the entire plant. If a controller fails, the entire plant
doesn’t necessarily get impacted. It also has the onboard monitoring and control that saves development time. A single PLC is a single point of failure. You surely wouldn’t want to control
an entire plant with a single PLC however; a connected PLC system can have nearly
the same security and robustness as a DCS. Make sure that you head
over to realpars.com. To find even more training material
for all of your PLC Programing needs. We offer many videos to assist you
in learning PLC Programing and landing that job in a high-paying, highly thought after field of
automation and controls engineering. Go to realpars.com and subscribe to our
highly effective training series now!

70 COMMENTS

    Most electrical engineers working in an industrial environment , metals, materials handling, mining or processing are conversant in both PLC and DCS programming and as pointed out the differences in operation and implementations are very similar. That said, that it is why things like stuxnet became a reality and an on going problem that will not go away. While we know about stuxnet and the variants that are similar but different made up of different and often from unrelated and unexpected blends of vulnerability and hence they don't go away or are permanently fixed.

    hello Mr .
    I want to discusse with you do you have a gemail that allowed me to chat with you, I need your help.

    For anyone who's ever had a factory in MineCraft Tekkit and wondered if there was a real-life equivalent. ^^ (My proudest achievement was the fully automated kebab-stand, or the music trap, where if you fell into it, would be forced to listen to scary music)

    I want to know is there any compatibility issue if DCS and PLC's are used under same SCADA system? Meaning some equipment are controlled through DCS and Some are PLC's.

    Single-point failure in PLCs can be removed with redundant PLCs. Though its pain in the ass to program and test.

    Yeah all true but about 20 years out of date, that was the buzz word for about 10 years , oooooh "DCS", that the marketers sold to managers and purchasing agents who didn't have a clue about the difference.

    I worked a few place so far in my life, i encountered DCS only once. From my point of view, PLC are superiors in this time and age. A bit of background, my company have around 50 electricians, two of them are the "programmers" which deal with things like net/scada/etc and are only dedicated to that aspect. The main selling point for PLC is the ease of programming – any of the 48 others electricians can easily swap a card and reupload the programs in seconds on the fly with a laptop or ethernet point, downtime is minimum even on nightshift – add a modifications via logic block is almost child play for any edit except if it use i/o scanning. Logic blocks makes things so easier to edit or program, can create your own custom block for anything specific on your process. To be honest, PLC failure is rare/exceptional… most of my field calls are instead related to fiber optic/cat/mb+ communication problems or battery change on cpu.

    Nice video. One thing I might add is the DCS is usually designed with a narrower focus on process control. For example, implementing a complex cascaded multiple PID control scheme with constraints on a DCS can be done much more quickly and easily than a PLC. PLCs are more general purpose – you can implement the same control scheme but it will take longer, functions like tracking, reset limiting, signal selection, filtering and clamping sometimes need to be explicitly programmed so you really need to know the low level details (it could be argued PLCs are more powerful since you can make them do exactly what you want).

    DCS usually has a standard library of function blocks which cover pretty much everything you would ever need in process control. The PLC library is often more limited, extra functionality may be available at a cost however. Btw the function block diagram you've shown looks like modern ladder logic, quite a bit different to the typical DCS function block control drawing layout.

    The tight integration between the controllers, HMI (which you mentioned, but worth repeating) is a big deal. This can save many thousands of man hours on a large project if a DCS is chosen.

    Other notable differences:
    -Alarm configuration is typically more powerful in a DCS than a PLC/SCADA combination.
    -Fieldbus configuration is often tightly integrated with a DCS development environment, in fact some Fieldbus protocols common in continuous processing applications are not available or have only recently become available on PLCs.
    -System health assessment (via system alarms) in particular is tightly integrated within a DCS. You can see at a glance how the system is performing.
    -Operator keyboards providing key process control interaction funcitionality are standard with a DCS installation.

    Not to mention the seamless integration between DCS and SIS from the same vendor, but that's another story 🙂

    I am working as a Controls & Automation recruiter and trying to learn my market. Please keep up this amazing content as you're helping so much to learn about the things that Automation and Control engineers do everyday.

    If anyone can suggest anything else I should be trying to understand, i'd be most grateful 🙂

    I like that you touched on how using function blocks gives more capabilities and flexibility for uses of a PLC. However, I was a slight bit disappointed that you didn't touch on Script at all. This gives even greater flexibility to the programmer, and in some aspects even easier to program.

    @7:59 so true – used to work at a plant that had an old DCS that was slowly being made obsolete via ABB acquisition (to push there DCS platforms). One contractor with specific programming/hardware experience had 3rd party near monopoly for a good chunk of West coast US, made a career out of it.

    A PLC could have a Primary Processor fail and take out a whole system as compared to one system in a DCS that goes down. But I guess if you maybe build the PLC system right and add redundancy with a Primary & Secondary Processor network, if that system ever going down it should be far less an issue. It should switch and barely skip a beat.

    Great video. That fact that you missed was the DCS was used more in the past in large control systems because early PLC's were not capable of handling analog values. I saw a Honeywell DCS system completely replaced with Rockwell PLC's and a PC based HMI system. The 2 systems were almost identical to the process operator.

    You got a lot of things right on this video, but a lot of this information is extremely misleading. "You wouldn't want to control a single plant with one PLC," you said. I've never seen an entire plant small enough for a single PLC to control it, but PLCs are far more scalable than a DCS. Often a single PLC will be employed for a very limited system and will communicate with other such PLCs, creating a network of PLCs that is far more "distributed" than a DCS (Distributed Control System). And any DCS can have a controller fail, which would bring down a significant part of the process and other parts that relied on it in the flow of product. The platform you use is essentially irrelevant. You suggest that PLC programming is something of a commodity while DCS programming is highly specialized. That just isn't true; it's platform dependent. There are thousands of Rockwell programmers in the US, but far less Mitsubishi programmers, but both are PLCs. And for DCSs that have been around a while, you can find plenty of engineers with applicable experience. The thing you got most correct is that with the technology available today, almost any process can be successfully controlled with either a PLC or a DCS. The selection mostly depends on specific plant applications and company preferences.

    Many aspects are not mentioned. The DCS has been process oriented from day one where PLCs more discrete. PLC vendors are trying to get there and are better in process but still not equal. Redundant control processors for example whereby a control processor in a DCS can be removed during operation, fail over with no affect on process – have done it many times. Communications capabilities of the DCS vs most PLCs, network redundancy, as well as incremental downloads and online programming of the DCS. I have seen PLCs that will go offline with down load – and fault on divide by 0!. For plants that have maintenance shut downs measured in years this is unacceptable. Also, cost. A DCS was once much more expensive where now a PLC based system that comes close to capabilities of a DCS with redundancy on network and control levels is often more expensive in PLCs and much more difficult to implement. Depends on your needs.

    Thank you for the free educational material provided for us and the wonderful style of explanation could you please told me what the program you use to create these great videos to do the same for my channel

    I worked with Modicon PLC controllers 184, 384 ,484 and finally 1084 systems. They were really reliable and easy to program.Hats off to the rugged design . We had 25 systems and i should proudly say none of the I/O modules nor CPUs failed for 25 years ,except few power supplies and RIO interface.

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