Difference Between PLC & DCS

Comparing PLC and DCS - Control.com

Please tell me about Alarm system & alarm list doc.

Anything else on you're homework list?

Damn, I walked right into that one didn't I? Where was my BS detector?

I feel so ashamed...

Here is how we trained people when I worked at Siemens.

PLCs make "things", DCS systems make "stuff".

The basic concept behind a PLC is a sequential logic execution system: do this, then do that, then do another thing etc. etc, then repeat and by the way, let me know if anything bad happens along the way. It is based on specialized hardware that is designed for this purpose. Initially PLCs were all about discrete digital I/O, but over the years that has expanded to include analog I/O, albeit in a limited capacity. Analog I/O requires an intense amount of math processing just to convert the signals, let along do anything with it. So initially, PLCs had a severe limitation as to the amount of analog I/O you could connect to them. As time marched on however, improvements in hardware have allowed that to increase significantly. But still, because of the inherent nature of how PLCs are designed as sequential machines, the more analog I/O you have, the longer it takes for the CPU to execute a program. This can then interfere with any deterministic aspects of a machine operating system that the PLC is supposed to support (deterministic means you tell it to do something in a specific amount of time and it does it).

Originally, a DCS was used SPECIFICALLY for handling a large amount of analog I/O, as one would find in a process environment like refineries, chemical manufacturing, pharmaceuticals and some complex food processing. The DCS was different in that the hardware was less important to the execution, a DCS was all about the software that was simultaneously executing thousands of analog I/O conversions as well as displaying an interface that made it all make sense to an operator, called an Operator Work Station, and the Engineers that need to tweak it in an Engineering Work Station. Later, they added the ability to store historical data and prepare reports. This was initially done with separate discrete sub processors all connected by a high speed network, but more recently has become extremely powerful computers running parallel sub process routines. So where an old PLC might have been able to handle a dozen analog I/O, a new one maybe a few thousand, a DCS has the ability to handle 20 or 30 thousand, depending on how the system is distributed.

However a DCS is NOT designed to handle sequential deterministic processes very well, such as machine controls, so that was/is usually off-loaded to a PLC and connected to the DCS network.

More recently, companies like Rockwell with their PlantPAx and Siemens with their PCS7 are blurring the lines even more however, because they are now both using the hardware from their PLC product lines as the field interface and sub process components of their DCS systems. The DCS processing still takes place in a parallel computing environment with all the bells and whistles a process system requires, but the field I/ now is connected to the same hardware used by the PLC systems; ControlLogix in the case of A-B Rockwell, S7-400 PLCs in the case of Siemens. This then makes the further integration of PLC subset control systems more fluid and simple, it also mean the field devices are designed with the same industrial environment considerations ad discrete machine controls, plus there are common replacement components as well.

...is a simple matter for a stroll through Wikipedia, looking at the articles on these topics and digesting their content.