[BLOG] How do PLCs and DCSs differ, and what’s the best option for your business?


PLCs and DCSs are the most widely used control systems for robotic applications.

Over the past two decades, the world has been rapidly adopting smart manufacturing and automation. A lot of global companies and even small businesses have automated their manufacturing to increase production, safety, and efficiency.

Despite the fact that they were invented for different purposes, PLCs and DCSs are now being used to control factory automation in similar situations.

Some functions that were unique to DCSs are now found in PLCs, and vice versa.

Although these two controllers are similar, each controller is specialized and brings a specific set of benefits to each application.

In order to choose which one is best suited for your business, it is critical to understand the differences of these control systems and how your industry and business size come into play.

What is PLC(Programmable Logic Controller)?



The first prototype of PLC was invented in early 1970’s to mechanize repetitive manufacturing in the automotive industry.

PLCs are able to conduct simple logic on a palm-sized circuit board without any monitor.

As with other technologies, the memory capacity of PLCs has grown a great deal since the 1970’s.

Even though PLCs are able to control a series of different automated process and machines, it is not able to manage other PLCs who are managing their own processes.

The managing PLC is not able to modify the processes running on these other PLCs as needed and has no way of providing information about the processes beneath into the operator.

To solve this technical limitation, programmers can integrate an HMI or HMI/SCADA package which is easily compatible with PLCs in many different kinds of network environments.

By providing the software which can correct, formalize, and combine the data from different devices, general technicians are able to use the HMI hardware to set up the program easily by themselves.


– Economical: usually range from a few hundred to a few thousand dollars

– Correcting errors extremely fast: Able to shut down the applications and prevent hazardous situations

– Easy to program: programmers can easily control the connections between inputs and outputs

– PLC programming language is easy to learn

– One PLC can control several machines

– Small footprint and short implementation time


– Difficult to extend/expand hardware such as the memory

– Not easily integrable with another manufacturer’s products


What is DCS(Distributed Controlled System)?

Distributed controlled systems(DCSs) were developed to improve upon the limitations of PLCs and DDCs(Direct Digital Control).

Even though PLCs and DDSs have been used for most automated robotics applications in the world, they present some risks.

For example, the whole plant would be out commission if once computer goes down: the entire system is controlled by one operating system.

To prevent this problem, programmers distributed the main computer to several small DDCs, and then connected to the whole system to manage the complicated and large set of data.


– Lots of space for H/W

– Able to generate complicated arithmetic and logical circuits

– Able to correct Data and write reports

– Easy to access other plants by adding individual systems


– More expensive than PLCs

– Requires another controller to deal with low-level errors and faults

– Need a skilled operator since all information and data is not accessible via the HMI

– Failure of one controller often affects more than one control loop






Future of the DCS/PLC


Based on the current rate of technological advancement, most experts agree it will be pointless to differentiate between DCSs, DDCs, and PLCs in the future.

However, consumers willing to adopt automated robotics applications in their manufacturing processes still need to understand their differences.

Having the understanding allows the user to choose, which control system should be integrated with their automated machines based on their specifications, budget, and other aspects of their project.

PLC Expertise and DCS possibility

Promation has deployed over 5o automated robotics applications over 23 years both in Canada and Worldwide in Automotive, Nuclear, and Industrial industries by integrating PLCs with HMIs or and HMI/SCADA system.

In addition, Promation has hired some professional DCS software developers to expand the company’s capabilities to bring DCS architecture to other industries that may benefit.

So, time to answer. What’s the best option for your business?