difference betn plc&dcs

Dear All,
wats the basic difference between plc and a DCS?
regards
Prasad

Prasadtoke,

Basically a PLC is used for your discrete applications ie ladder logic. A DCS is aimed at the process industry using FBD as the programming language.

Peterboy

More typical question is difference between DCS and SCADA - that really depends on who you ask and their particular industry biases. Modern systems really do both (Distributed Control System, and Supervisory Control and Data Aquisition, respectively). Wikipedia has good info. I just got a post on the SCADA threads where an Oil and Gas guy went into detail from his perspective. I can post that if anyone's interested.

surferb, I would like to see that!

i have worked mainly on PLCs but the question that is haunting me is what the DCS can do that PLC can't. coz nowadays with tchnolgy we have much more powerful features in a PLC .so my question really is why ppl go for a DCS and not a PLC in say oil and gas application

I would say it's probably more habit than anything. As you say nowadays we have the ability to incorporate the functionality of a DCS into a PLC. The best example i know of is Control Logix

DCS v PLC is a topic that can run forever, with no real agreement. My take is that DCS vendors supply better lunches.

I say that DCS can handle much bigger systems than PLCs.

If a project is a factor 1000 bigger than your average PLC project, then a number of factors become really important.
If you only have a factor 20 more personnel to configure and maintain such a project, then the system must be accordingly more easy to configure and troubleshoot. Therefore the configuration tools are on a higher level than with PLCs. You configure preprogrammed "objects" rather than fiddle with bits at a lower level as you do with PLCs.
And if you have a project that is a factor 1000 bigger, then downtime is also a factor 1000 more important = downtime must in principle be zero. So there must be redundancy on all levels, and there must be full configuration in run, also for hardware exchanges.
I believe that some DCS have triple redundancy, and can achieve SIL4. The highest I have seen for PLCs is double redundancy and SIL3.

Of course PLC and DCS share many common technologies, but there is a real difference in scale.

As I said this can go on forever. My impression is that the largest systems are probably PLC/SCADA! I have seen such systems with 10's of thousands of IO.

I would say that PLC's were the original intended to replace relay logic. They started out using ladder logic and only one controller was used per system.

Distributed Control Systems could have two or more controllers in the same system running different programs that would share the same I/O and registers. DCS would use ladder logic, block logic and/or a simple form of basic.

Over time, the companies that started with one or the other have become more kown for the product than the actual functionalty. For example AB is known for its PLC's despite Controllogix having all the functionality of a DCS. I believe that is why they bought out the Reliance Automax, in order to obtain the DCS know-how.

The reason oil and gas may be using dcs is a legacy issue. Once you go one route and have invested a great deal of money and training in one system, you will find it very difficult to move away from it. You will encounter that, especially techs, will resist anything different.

i was reading through some stuff on the DeltaV site and got some features in a DCS which i guess are not part of PLC today :
1.Sequence of Events Recorder.
2.Standard function blocks for emergency shutdown i.e. they are pre-programmed blocks
3.and the software contains virtually unlimited SCADA tags but the process tags(Emerson calls them DST)are to be selected for Licensing

helloooooooooo

i think plc and dcs you can get better difference if you consider a single manufacturer.
for example if you ask about the difference between plc and dcs according to siemens system then i said there is no difference between plc and dcs if you take plc and wincc in account you can use same susytem for plc and dcs.

and when you are aking about hony well system the use dedicated cpus for dcs and dedeicated for plc

in the bigning its not possible to archive analoge data in plcs and we canot manuplate analoge value in plcs so they use sapacial computers for that purpose and now a days cpu can handle analoge data

With Siemens PCS7 (the DCS from Siemens) is not just the same as PLC+WinCC. PCS7 uses S7 and WinCC as subcomponents, but that doesn't tell the complete story.
The licensing is completely different for example. WinCC uses "powertags" whereas PCS7 uses "proces objects".

but i think

ok i agree pcs7 include s7 and wincc as a sub components. if you use wincc alone it use powertags and when with pcs7 it use process object. my opinon is that siemens use its plc cpus for analoge data also and for storing data it use wincc but some vendors use their dedicated cpus for that purpose and for discrete logic they use other cpus.

i am saying siemens pcs7 or plc+wincc as a DCS because you can use same system for plc purpose and for dcs purpose

if i am wrong pls correct me thx

garciaortiz said:

Over time, the companies that started with one or the other have become more kown for the product than the actual functionalty. For example AB is known for its PLC's despite Controllogix having all the functionality of a DCS.

The reason oil and gas may be using dcs is a legacy issue. Once you go one route and have invested a great deal of money and training in one system, you will find it very difficult to move away from it. You will encounter that, especially techs, will resist anything different.

Click to expand...

I have to take issue here.

ControlLogix doesnt have anywhere near the functionality of a DCS.
Wheres the HMI?
Wheres the Data Historian?
Wheres the Enterprise Level functionality?
These all come pretty much built in if you buy one of the mainstream DCS systems.

As for the 'legacy' comment, oil refineries in particular right now are spending millions getting rid of their old DCS systems. What are they putting in? New DCS systems of course, not a PLC in sight. They use DCS systems because it is the best solution for their needs, not because they 'feel like it'.

I've seen this topic discussed several times on here and its been answered over and over again.