Three Quick Questions

The debate: DCS or PLC?

1. What is the difference between a DCS and a PLC?

In the past, the strengths and weaknesses of both automation technologies were well understood. It was easy for an engineer to select one that suited their application. These days, the criteria which were thought to be exclusive to one technology can now be found in the other. The line has been blurred.

PLC stands for Programmable Logic Controller, which is mainly used for interlocking various instruments and equipment together to make a system. PLCs are commonly used for on/off or digital control and may be expanded with analog I/O modules. PLCs have traditionally been used for manufacturing, small batch processes, and individual units within a plant.

DCS stands for Distributed Control System and does exactly what a PLC can do, but is generally used in much larger and complex applications such as power generation plants, water and waste-water treatment facilities, and petrochemical plants. More and more, we are seeing the DCS applied in smaller applications for its all-in-one capabilities. Total plant automation becomes more of a reality with the DCS. DCS systems include multi-disciplined controllers for logic, sequential and process control, HMI, historian, alarm management, and business applications on one platform.

A PLC can be fully integrated into a DCS... PLCs generally perform a single task, whereas a DCS is a system of PLCs/RTUs that communicate to accomplish a particular task. For example, in a power plant, there might be a PLC that is used to operate a duct burner. In that same power plant a DCS may be communicating with 14 duct burner PLCs and starting the burner management system when required. In these instances where PLCs are a subsystem, many applications still require an interface back to the DCS for interlock and control reporting.

2. Where is a DCS practical over a PLC?

Initially, a PLC was a very cost-effective solution for low I/O count circumstances. But with technological advances, the DCS has become a competitive option in the market. I/O count is no longer an issue. The PLC is cost-effective from zero to a few thousand I/O points. Whereas, the DCS was thought to be more applicable in much larger situations.

With the release of the new Symphony Plus SD Control and I/O, a DCS can now be used in applications with as little as 150 I/O. The SD Series platform can withstand up to 70 °C ambient temperatures and harsh environmental conditions, have very low power consumption, require smaller power supply, produce a small footprint, and be very cost competitive. These were once limiting constraints that prohibited the DCS from being practical in many applications where PLCs were used. Now, it is difficult to imagine a situation where the DCS would not be a practical choice.

3. What is the system lifecycle of a PLC and a DCS?

Evolution and obsolescence are major differences between a DCS and a PLC. PLCs do not typically have an evolution path forward once the hardware reaches maturity. Historically, they need to undergo costly and risky “rip and replace”. Conversely the DCS system evolves over time to offer an evolution path that does not require the owner to replace the equipment, only to evolve it while maintaining original integrity of engineering logic and design. This is where the Symphony Plus DCS has a clear edge in the market.

Symphony Plus SD Series provides complete backwards compatibility and interface hardware that allows Infi-90 customers to install the SD hardware on autonomous applications while providing a communication path back to their original system. Users can easily utilize their existing rack solution controllers to interface to remote mounted SD I/O.

In short, the release of the SD Series provides solution functionality that was not available in the past for installed Infi-90 users. The question is not if you should use a PLC, but why would you?


David Thompson
David is a Senior Analyst and Product Development Manager for ABB’s PSPG - Power Generation. An expert in control systems with a knack for reading and writing logic, David has continuously enjoyed all work related to developing control logic and graphical HMI interfaces for various units. David began his work with Programmable Logic Controllers in 1978 as an Instrumentation Technician with a Petro-Chemical Company on the Houston Ship Channel where he ultimately installed the company’s first Distributed Control System in 1988. David also held the position of System Manager for 3 processing units in the company’s engineering and O&M projects department.

David joined ABB’s Field Service division in 2001 and further continued to broaden his knowledge and experience on control systems. In 2012, David’s career transitioned to the field of sales as an account manager, a progression that David himself says “felt natural” to him. As a Field Service Engineer, David interacted regularly with Power customers and was able to form lasting relationships. Once officially making the switch to account management, David was able to act directly in the best interest of these customers. David continues to provide support to PLC Replacements with SD Controllers and I/O account managers in his current role at ABB.

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