Another DCS vs PLC thread.

[kallen88]

Hello People,

Sorry for another DCS vs PLC topic but I have a few questions that aren't really addressed in most of the thread questions I have found.

I have been looking to SCADA and DCS systems and there is a lot of information out there defining them but no real 'practical' information. It seems the definitions of PLC/SCADA and DSC systems, by what I can gather from several forums and posts up to 10 years old, are nowadays almost the same thing.

No one really talks about specific hardware or software that helps split the definition. From what I understand they achieve the same purpose; gather field data, control what needs to be controlled, display to the operator, log the data and use it to generate alarms. So this requires some IO, communications, processors and display interface, which is the same for both systems. The only major difference is the 'brains' are spread across multiple separate processing units in a DCS rather than one processing unit for the entire plant.

Now, I understand a PLC can be used in a DCS as a standalone process controller but when purchasing hardware and software are there clear proclamations that say this is a DCS component and this is a SCADA/PLC component? Meaning SCADA software like Citect or Wonderware are only used in a 'PLC/SCADA' systems or can they be used to configure a DCS interface/database? Are there any specific DCS suppliers of hardware and software? For instance, I am familiar with Redlions modular controller series. This uses a master controller that handles the data logging, alarming and communications with add on expansion modules that handle the individual processes. Would this be referred to as a DCS?

So in each system are there any differences to where the alarms configured and set? Or where the data would be collected and stored?

 

[djs]

To the best of my knowledge, DCS systems are "closed" in that all hardware and software must be purchased from the same vendor and for the same system. PLC systems on the other hand tend to be much more open. In fact some hardware platforms support hardware from multiple vendors! And of course the SCADA side of the software is wide open.

In certain ways the DCS platform has better support. Because all hardware and software comes from the same supplier, there is no finger pointing.

 

[danw2]

>No one really talks about specific hardware or software that helps split the definition.

Details on DCS's are difficult to come by, because information beyond the glossy 'executive overview' brochures is generally limited to those who own the systems ("registration required").

My view is that a DCS is server-centric, with a central database served from a high end server, with distributed control hardware. PLC's are controller-centric, no server needed.

>Meaning SCADA software like Citect or Wonderware are only used in a 'PLC/SCADA' systems or can they be used to configure a DCS interface/database?

A DCS is configured only by its own branded software. Emerson's Delta V software is used to configure and run Delta V; Honeywell's software is used to program and run Experion PKS.

I call Wonderware HMI software, it sits over almost any PLC with the appropriate driver.

>Are there any specific DCS suppliers of hardware and software?

Typical DCS vendors are Emerson's Ovation or DeltaV, Honeywell's TDC/Experion, Invensys/Foxboro's I/A, Yokogawa Centum, ABB Symphony 800xA. GE has a DCS for the power industry whose name I don't even know and latecomers Siemens' and Rockwell are touting their PLCs on steroids as a DCS.

>Redlions modular controller series. This uses a master controller that handles the data logging, alarming and communications with add on expansion modules that handle the individual processes. Would this be referred to as a DCS?

The Red Lion controller is a PAC, a Process Automation Controller, a controller-centric hybrid, whose primary function is process control, with discrete logic control a secondary

 

[ScottyUK]

"GE has a DCS for the power industry whose name I don't even know"

I think you're taking about their Mark VI turbine controller. It's far from being my favourite turbine control system: both Emerson's Ovation and Siemens PCS-7-based system are superior in capability and lower in cost, but with a GE turbine you don't get the luxury of choosing for your controller. After all, you're only the customer.