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!

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