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Since starting work after leaving university, I have found several terms thrown around when talking about the control system. In every factory I have worked in, they have had a DCS, though some parts of it were controlled by PLCs. In some documents or training courses, I have seen references to SCADAs. I didn't really understand what these terms meant, other than they seemed to be talking about the control systems.

PLC stands for Programmable Logic Controller. These are small computers that receive inputs and adjust outputs depending on these inputs. They tend to be associated with a single piece of equipment or plant and are capable of operating mostly independently, though it may have some communication capability with other control systems.

SCADA stands for Supervisory Control And Data Acquisition. A SCADA system is a bit like a glue that joins other individual pieces of equipment such as PLCs or individual instruments and valves together. It usually provides a method of looking at the whole system from a central location, recording what is happening and making changes to the way the plant is getting controlled.

DCS stands for Distributed Control System. These are large systems with multiple control loops. There is no central controller, but multiple controllers throughout the plant. Each controller looks after a local set of equipment, though they can communicate with other controllers. If the communication between controllers or the operators is broken, the individual controllers should be able to continue to keep their sections of plant in control and it is only the higher level control functions that get lost.

There is little practical difference between a set of PLCs joined together with a SCADA system and a DCS, though a DCS tends to be made by a single vendor so the controllers, network and consoles are all better integrated. This has become less of a problem more recently as systems are getting better at talking together.

For the most part, they are just different words for a control system.

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