I thought I had understood the difference between a DCS and a PLC. I
read somewhere , possibly here , that if the vendor showed you a
picture of an oil refinary then it was a DCS , if the picture showed a
complicated machine then it was a PLC.
However I read "using PLCs leaves you lacking many desirable DCS
functions" in the Foxboro A2 system overview at
What are the desirable DCS functions which you dont get with a PLC?
If anybody has better thoughts on the PLC , DCS definiton then I would
also like to hear them.
I hope I am not the only one with a feeling of Deja-Vu. I seem to
have read a massively long thread on this not so long ago. The
concencus then, ISTR, was that a PLC, Embedded PC or Embedded
Controller might be components of a DCS. DCS, like SCADA, was a term
applied by the vendors to describe the style of the overall system.
If you haven't got DCS or SCADA they were most probably stand-alone
DCS will involve quite a bit of networking (mixed topology) and
remote control of various nodes. They may use a mix of PLC. Embedded
PC Embedded Controller and Intelligent Instruments to obtain the data
they need to effect the appropriate controls over the process. DCS can
be in control of complex machinery or massive processes.
SCADA systems are simpler in concept. They collect data from a wide
range of sensors (usually, but not always, via star-form communication
networks) then determine the required state for the controls from that.
They can seem like huge state-machines (although the better ones are
For more on the discussion try and find the thread (with the same
title) in Google Groups.
I won't address hardware issues, but from a software standpoint, in my
experience, DCSs usually use function block programming (and, so I've heard,
boolean logic), and PLCs ladder logic.
DCSs were designed for apps that required a lot of analog monitoring and
control, and PLCs for machine or sequential control apps. These days there
is vast overlap.
In my limited experience, function block programming is well suited to
analog control applications; doing the same thing with PLC software is often
quite a bit more tedious. On the other hand, implementing sequential control
is generally easier to do with ladder logic than the boolean logic I've seen
used in DCS software.
Some PLC and DCS providers allow both function block and ladder logic
programming. Modicon, the venerable PLC co., does, and Moore Process
Automation DCS does, or did. Moore was bought out by Siemens in 2000.
Moore's software allowed programmers to integrate use of function block,
ladder logic, "instruction list" and sequential function charts (SFCs). I
hope I remember this right from 4+ years ago, anyway. I'm sure there are
others doing the same thing these days.
As to the hardware, costs and reliability issues, I leave that to others.
One unfortunate thing that too many users of PLCs and DCSs do is get too
many of brands in their plant, or plants. Too often an accountant's
perspective of the purchase price is what comes into play. Overlooked or
underplayed is all the time and money, that it takes for people to learn how
to use different stuff.
Many providers, like Rockwell Software/ Allen-Bradley, the PLC supplier,
offer a suite of proprietary hardware and software meant to work with each
other, and, of course, give you more reasons to be married to them as a
With the new 1131 standard for programming and the ability for PLC's to
process analogs very well, the distinction between a DCS and a PLC is
blurring quite a bit. Many DCS systems use the same HMI software that you
might see with a stand alone PLC system (in particular Wonderware and
As many have said previously in this thread, many times PLC's are components
of a DCS or SCADA. The term DCS is being used more like SCADA, describing
the overall configuration of a system which is comprised of many subparts
(or truly distributed control!!!). If intelligent Profibus or Foundation
Fieldbus is added on top of this where programming can be done internal to
the field instrument, then we will really take the final giant leap towards
true distributed control.
DCS is more "high level", good if you need HMI or integration with many
different systems, but more complex requiring more programming skill.
PLC is low level and better for fast, dependable, dedicated operation with
minimal human input.
Let the PLC handle the nitty gritty, the DCS to interface with the PLC's.
Below is my experience on DCS and PLC applications in large chemical
plants involving batch, continuous control and automatic machinary.
In the past, PLCs were based on bit processing and had very rudimentary
analogue capabilities. Whereas the DCS was essentially an analogue
control system. Speed of response was poor on DCS and PLCs were quick
on digital applications. This made it very easy to pick the right tool
for the job.
As things developed, PLCs adopted more analog functionality and DCS
gained digital and advanced analog control functionality. The advent of
good digital display screens started to change things.
DCS gave the impetus to changing control room design to screen-only.
PLCs tried to compete by buying in SCADA packages, but these were much
inferior to the main DCS vendor HMI screens. DCS configuration was
carried out by the vendor, whilst PLC vendors made their profits from
the hardware and solutions were provided by Systems Integrators.
The position now is very confusing. The PLC vendors have improved their
HMI and now provide higher levels of analog control. Whereas, the DCS
vendors seem to be trying to compete with the PLCs and producing cheap i/o.
On the configuration front, the modern DCS is the easiest to configure
and modify. In recent projects I have been involved with, problems at
FAT can be solved on the run, due to the on-line modification facility.
Also, plant uptime can be maintained by the quick modification of the DCS.
PLCs need more engineering hours to develop a solution, and invariably
that solution is different with different configuration engineers.
Modifications are a nightmare.
Possibly PLCs can be developed with sufficient macro libraries to match
DCS, but then aren't they the same as DCS?
PS. The desirable functions in a DCS are the deadband on alarm
functions, input filtering, non-linear gain controllers,
gap-controllers, alarm inhibit, alarm priority change dependent on
process conditions, custom control modules, flexible batch, etc. etc.
Mike D2 wrote:
You are partly right and partly left. If you have a DCS you only need one
It's when you have a PLC that you start your brand collection. You will
need cabinets, power supplies, HMI software, HMI hardware. Things really
start to get entertaining if you want advanced functions like historization
to repeat, a PLC is a component, a DCS is a system. Buying a DCS is like
buying a bus. Buying a PLC is like buying an engine.