SCADA vs DCS

[gorichan 0]

Hello, Just want to know, what is the difference between SCADA and DCS? Im quite confused with the fact that SCADA and DCS are the same thing. Some also say that they are not.. Pleaze

[IO_Rack 181]

I have become very impressed with the Wikipedia site. SCADA at Wikipedia DCS at Wikipedia In a nutshell, DCS is the actual control system that runs plant, factory or other complete system. The SCADA will monitor the control system, do analysis, and allow access at specified levels. Their differences can be crossed but that is really up to the programmer. There discussions here about "Where to draw the line?". I will post the link if I find it. Edited 21 Apr 2007 by IO_Rack

[IO_Rack 181]

I can't find the post I was looking for but here (at MrPLC) is a discussion as to why the lines should not be crossed.

[paulengr 44]

It's even more confusing than that. At one time, the analog side of the world was poorly supported in PLC's. Any time you needed to implement control loops (PID, etc.), a DCS was the way to go. If you want a totalizer, there is a specific block to do just that. They have what are usually called "continuous time variables" or something like that. Most DCS systems have a very solid and intimate understanding of time so that you can more or less directly implement blocks referring to flows and totals without introducing any extra baggage to deal with the time aspect of this kind of thing. The typical DCS structure is to have a bunch of PLC's everywhere doing essentially low level stuff. The DCS runs the control loops and serves essentially as the control interface with inter-PLC stuff. The SCADA/HMI is implemented on top of that. Most of the time, the DCS systems I've seen also tended to be the HMI. In spite of the fact that this is not a necessity, DCS/PLC programmers tend to move all the PID loops, totalizers, etc., into the DCS, leaving the PLC fairly "barren". In reality, there is one feature that I liked about DCS systems. They seemed to all have a really nice graphical block language that made implementation of complex control systems for chemical plants really simple and visual. Even with the newer languages available in PLC's, it just doesn't seem to be quite as nice as what I remember. The closest thing to it is the Agilent/HP "G" language, but that one is very confusing for the unitiated (learning curve is fairly steep). The downside of a DCS system is that in general, they were slow, they implemented proprietary communication systems that would make even the PLC manufacturer's blanch, and they implemented pricing that made HMI/SCADA vendors green with envy. Every plant I'm aware of that had one eventually threw in the towel and disbanded them for these reasons.

[Ken Moore 23]

I work in a Chemical plant that has both PLC's with SCADA's and a DCS system. The DCS is used for control of critical systems, and plc's for non-critical. The DCS is more expensive, but is much more reliable and robost. The DCS handles redundant processors, I/O etc... much, much, better than the PLC's. No special logic, just set a bit. The DCS only has one database, so HMI, historians etc... are easier. But as mentioned before, the DCS is much slower than a PLC. The fastest scan rate is 100 msec, with 500msec being more common. However, in process control, you don't need really fast scans. For machinery control PLC's shine due to the fast response.