What is the adventage of DCS (Distributed Control System) over PLC (Programmable logic controller) ?

PLCs may need to interact with people for the purpose of configuration, while a DCS solution does not require operator intervention for its normal operation.

Derived from wikipedia.

Wow, that's quite the abstract and meaningless answer.

Truth is that a PLC is a component of a DCS. The PLC's (programmable logic controllers or controller) are distrubuted about a plant and in many cases control discrete signals (on/off). They are located in the field and are the 'work horses'. The controllers are networked to the DCS (Distributed Control Station). PLC's only offer control.

The DCS offers central control over the entire plant from a single location (usually but not always in the plant) in an environmentally controlled room.

Where the controllers (PLC) handle discrete information only, analog signals are grouped together and connected to a DCS controller that is located in an evironmentally conditioned room.

A DCS has a Human to Machine Interface (HMI) and provides much more than central control. It provides graphical monitoring, trending and maintenance scheduling among many more functions (equipment warehousing, instrument specs, ect).

PLC's only offer control. They can have an HMI but it is another package. They would use an HMI for control only and the HMI would serve to replace hard-wired control. Some like operator stations with a television screen; some don't.


I have seen many PLC's without a DCS but I have yet to see a DCS without a PLC.

A new system is being introduced for small installations. It is called a PAC (process automation controller). A PAC is essentially a PLC with many fuctions previously only available with a DCS.

DCS having a central contoller and can control over all plants
# It has huge I/O s(more input and output - AI,AO,DI,DO),
#It has Reduntancy(If one controller fails with in a second another controller comes on line).
#on line trend and historical trend are very useful in dcs

I suspect the underlying criterion of your question will have to do with the complexity of the overall system to be controlled (&/or monitored), including the system control.
I want to be careful not to treat DCS & PLCs as mutually exclusive terms.
I refer you to Ask Tom!
Ask Tom! Archive:  Distributed Control Systems Primer, by Dan Capano (http://www.waterandwastewater.com/www_services/ask_tom_archive/distributed_control_systems_primer.htm)
"distributed control systems offer the advantage of centralized control, while retaining the capability of distributed, or local control."
"A DCS may be as simple as one PLC (Programmable Logic Controller) remotely connected to a computer located in a field office. Larger systems may be PLC based, but will most likely consist of specially designed cabinets containing all of the equipment necessary to provide I/O and communication. One point to consider during the design and specification of a control system is the level of autonomy each node will have in the event of a network or system failure. A true distributed system will allow most remote nodes to operate independently of the central control facility should the facility go off line or lose communication capability. Each remote node should be able to store the minimum process data required to operate in the event of such a failure. In this manner, costly and potentially disastrous process upsets can be avoided."