# DCS vs PLC



## backstay (Feb 3, 2011)

I don't think it's cut in stone what they really are. DCS was distributive control system for us. Used in process control. Where a PLC was programmable logic controller. Used more as a single use machine control. That said, most systems are a hybrid with both mixed together.


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## JRaef (Mar 23, 2009)

The differences used to be greater, they are much more blurry now from a hardware perspective. The old definitions were that a DCS is used to make "stuff", and PLCs were used to make "things". The point of that was that originally a DCS (Distributed Control SYSTEM) was a system of controllers, all being watched by a bigger controller. Ages ago the individual controllers may have been pneumatic or even mechanical and the overall big controller was a room in which multiple operators,watched gauges and instruments, then turned switches and valves. That later became a computer and eventually things like the Automax came out that we're very specialized computers made for industrial applications, stripped of many of the overhead necessities that would be used to office tasks. That overall controller was being used to control the entire plant, or an entire section of a plane, as a process, a process of making stuff which consists of various ingredients, each of which has their own sub-processes. In that endeavor, one or more of those processes may have been controlled by a PLC. But that overall DCS was not a fast system, and was often too slow to be used on individual machines. 

A PLC on the other had was an optimized computer that executed repeated instructions over and over again, predictably and very fast, basically exactly what you want from a machine. Over time the computers used in them added more and more processing power and it became cheap to add what we're called "math co-processors" to do fast number crunching for things like analog to digital signal conversions, so the capabilities started to blend a little when PLCs became capable of performing PID loop controls, the domain of another of the sub-controllers found in a DCS. but originally, a PLC could do a few PID loops without affecting the sequential reliability of the machine, but it was limited, maybe a few to a dozen, depending on the size of the PLC. But in a chemical plant, you may have thousands and thousands of PID loops.

The blurring of the lines now are that the PACs, such as the Logix family, are no longer PLCs alone, they are a combination of PLC, small DCS (when combined with an HMI) and Motion Controller. Then if you want a full blown DCS you simply network a group of PACs together and add a server and overall control system software, running on what are called "Engineering Work Station" computers. For A-B, that is a Plant PAX system. Siemens does exactly the same thing with their PCS 7 system, Schneider is developing something similar using Modicon PLCs for the hardware.


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## ScooterMcGavin (Jan 24, 2011)

A lot of time nowawdays it comes down to tradition and semantics. We're currently doing the same thing your doing and upgrading to control logix systems. Depending on what it used to be we call it something different even though its the same system. 

If it replaces the old SCADA system from central control we call it the SCADA system. If it replaces the plant control system, we call it the DCS. If it just controls one machine, we call it a PLC. The operators always refer to it as the DCS because they only interface with the front end HMI and its all the same to them no matter where the information comes from. One thing we don't call them no matter how hard our Allen Bradley reps try is PAC's. I guess everyone is too old school for that.


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## KennyW (Aug 31, 2013)

A dcs was capable of running multiple loops within a process and multiple process in separate instances and time domains. 

A classic PLC can't really do this. You can kinda fake it with various timers and routines but purely software workaround and very difficult to manage on a large scale. 

A PAC like controllogix is not subject to these limitations. Multiple loops and equipment phases can run and be managed and altered completely independently of each other, execute as synchronized to I/O and/or in different timeslices or different exceptions with different priority level and this functionality is native to the hardware and development environment as opposed to needing workarounds that are done programmatically. 

So really the lines are blurred now. I would not hesitate to replace almost any DCS with a controllogix.


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## vileislepore (Jan 25, 2013)

Thanks for the insight guys.


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