PLC vs. DCS

"Programmable Logic Controller" versus "Digital Control System". It's largely a matter of semantics and manufacturers' preferences, but "PLC" may be a more varied, because it contemplates analog input and /or output.

The definitions are more blurry than they originally were because of changes in technology. But as a general rule, a PLC is used to make "Things", a DCS is used to make "Stuff".

What that means is, a PLC is better at handling high speed sequential control of a volume of discrete inputs and outputs, with a somewhat limited amount of analog mixed in. This would be typical of what you would find in a typical machinery or manufacturing operation where you have lots of mechanical processes, measurements and on-off controls to perform reliably and deterministically. Determinism means you can calculate and predict the outcome of an action in real time. If you add a lot, by that I mean thousands, of analog PID loop controls to a PLC, it will have a lot of difficulty handling the mathematical processing of that data and still maintain the necessary deterministic processing of the discrete data.

A DCS on the other hand is typically used in process automation, where you have a continuous flow of materials into a process that involves analog information processing and PID loop control processing of temperature, pressure, flow etc.. A DCS can of course also do discrete I/O processing, but it is a very expensive way to go about that and sometimes people even use PLCs to do that function separately and just have the PLC communicate to the DCS separately. You will ofetn see in large processing facilities a DCS system running the entire plant, but individual PLCs running specific machines inside of that plant. In addition, a DCS is used where you have multiple high level GUIs (Graphical User Interfaces) that are allowing real-time monitoring and adjustment of these processes. GUI processing takes a lot of computing power and cannot be done by a PLC itself, you have to add external computers. In a DCS, the computer that is doing the control is also the one doing the GUI interfaces, and you will often have multiple GUIs on-line at the same time. That is difficult for a PLC system without affecting the operation of the PLC.

But like I said in the outset, technology advances are making these differences less and less distinct. In fact, Siemens uses their S7-400 high-end PLC as the I/O sub-processing platform of their PCS7 DCS system, which shows the melding of the technology right there.

Go to: http://www.innovativelogicalcontrols.com/PLCvsDDCinHVAC.html

for an example of the "melding" of the technology explained above. The differences between PLC, DCS and even DDC control are becoming very small. Only the communication protocols will differentiate the systems at this time.

All systems now incorporate stand alone control, networked control, distributed control, digital, analog and PID loop output.