PLC vs DCS: Unraveling the Key Differences in Industrial Automation Systems
Are you a control System Engineer? worked with PLC or DCS earlier? One question must have been heard time and again, “ What is the difference between PLC and DCS ?”
How do I know whether to select a PLC or a DCS for a particular application?
We are trying to discuss this topic in a brief manner here for the larger good, because though the first PLC came in 1960, the above questions sound valid even today.
As we all know, the PLC Control system was introduced to replace Relay logic and was first used by the automobile industry for replacing Relay logic-based operations. The expectations were mine to replace a few 10s of relays.
Around 1975 the first commercial DCS was launched in both the USA and Japan. However, this was for controlling an entire plant, i.e. for Petrochemical, Chemical, Paper, etc.
When we closely examine, we can understand the sole purpose of introducing both the PLC and DCS were different and the objectives to be achieved were different. So the question is, should we really compare?
We can see rapid industrialization and utilization of large nos of PLCs within every automobile plant and every piece of equipment, every conveyor, and every machine was automated using PLCs.
As an evolution of the PLC System, the same was also brought to the process control market by the manufacturing companies to position it for new applications. Like conveyor automation, Cutting machines, quick and speedy processes, etc., and eventually, more and more functionalities of a DCS control system got adopted in PLCs. And it was all the more important for the PLC vendor to move up the game and start promoting against the DCS system. Thus the debate and comparison got more relevance.
Let’s look at the difference in the table below.
Sl.nos | PLC | DCS |
| 1 | Designed for replacing Relays. | Designed for complete plant control. |
| 2 | More Digital inputs and Outputs Suitable for faster operation | More of Analog Inputs and Outputs. Suitable for Analog Accuracy. |
| 3 | High-Speed Processor | Importance for Availability than Speed. |
| 4 | Capable of Large no of IO handling in one CPU. | Capable of relatively low no of Ios. |
| 5 | More Open Protocol at the SCADA Level to connect with Third party systems. | More Proprietary networks to avoid unwarranted access. |
| 6 | Engineering and Programming is a Three step process, ie.CPU programming, SCADA configuration, and Integration between the two. | Single Step configuration. |
| 7 | No concept of a single global database as a three-step process to configure. | Single Global Database in case Client Server Machine – which means entire program backup and data at a single location. |
| 8 | Single SCAN time for the entire operation. | Multiple scan times, even separate scan times possible to set in each Analog input. Thus the high performance of Analog Loops can be achieved. |
| 9 | More of Ladder Logic based on programming. | More of Function blocks-based Programming. |
| 10 | Application is more of equipment control or small process controls. | Mostly for complete plant control. |
We have now seen the differences in terms of Technicalities, this is largely for the academic purpose. However, the application-specific control system selection can be done keeping the above criteria.