in term of components both are same however the main difference is the implementation point of view. the DCS is more confined in a factory or industrial whereas the SCADA is not confined   

Well, I think DCS - Distributed Control Systems - is a control device that has higher capabilities compared to an ordinary PLC. On the other hand, SCADA - supervisory control and data acquisition - is mainly a software environment that can communicate with PLC's and probably DCSs also for that matter in order to monitor the functionality of those devices and perform data acquisition and data logging to a monitoring platform - say a computer in a control room etc. Such systems can also send in some control commands into the PLCs - like changing set points etc.

I agree with both Asif and Sanath however you can view a SCADA system based set-up at component level and becomes a DCS. Lets say we have a weighing system, a mixing system and a blending system. If each system has its own Controller and can operate autonomously then we can say its a DCS system because the control set-up is distributed, however if we use Controller Area Network (CAN) and link all the controllers to a single RTU then we have a centralized control system, If we then use a Software package to Monitor and carryout Data Acquisition with aid of a Historian and Alarm Management then we have a SCADA ..................

Thanks for all.

I think this two terms representing different control characteristics but overlapping in some senses.

For the control level, DCS (Distributed Control System) emphasizes the distribution control characteristics which include mainly control behaviors in field level (or local level), although DCS includes supervisory control level. However, SCADA (Supervisory Control and Data Acquisition) emphasizes the control function not only at the field level (mainly for monitoring the information by different sensors) at supervisory level.

Moreover, for the SCADA system, it was applied in the transportation field (railway and subway system). So, the function was emphasized on monitoring datum from distributed railway stations and lines. For the DCS system, it was applied in the field of process control including petroleum and chemical industry.

But I think you don’t need to pay more attention on the terms themselves, but on what kinds of techniques (PLC, DDC or others) you will use to solve the practical problems.

DCS is process oriented, while SCADA is data acquisition oriented, DCS is process state driven, while SCADA is event driven, DCS is commonly used to handle operations on a single locale, while SCADA is preferred for applications that are spread over a wide geographic location, DCS operator stations are always connected to its I/O, while SCADA is expected to operate despite failure of field communications.

 Programmable Logic Controllers (PLC) and Distributed Control Systems (DCS) are used to send, receive and process data from sensors, control valves and actuators as part of the control system in modern Manufacturing and Processing Plants. SCADA could be a system standing above DCS benefiting network technology for data acquisition, historical data management,  management level data access and management (rather than manufacture level) etc.

DCS is process state( plcs output) driven,  while SCADA is even driven. DCS does all its tasks in a sequential manner, and events are not recorded until it is scanned by the station. In contrast, SCADA is event driven. It does not call scans on a regular basis, but waits for an event or for a change in value in one component to trigger certain actions. SCADA is a bit more advantageous in this aspect, as it lightens the load of the host. Changes are also recorded much earlier, as an event is logged as soon as a value changes state.

In terms of applications, DCS is the system of choice for installations that are limited to a small locate, like a single factory or plant, while SCADA is preferred when the entire system is spread across a much larger geographic location,like a power station or  oil wells  which are spreaded  out in a large field.

http://www.ehow.com/facts_7588416_difference-between-scada-dcs-systems.html

In addition to the above Answers the Following three References will give you fair idea on the Topic.Particularly the third Reference will be Quite interesting.

1. A DCS solution does not require operator intervention for its normal operation, but with the line between SCADA and DCS merging systems claiming to offer DCS may actually permit operator interaction via a SCADA system.

  Difference between DCS and SCADA

      http://www.control.com/thread/1365080091

      Difference between DCS and SCADA

2. The first difference between the two is that DCS is usually a process oriented in contrast to SCADA, which is data-gathering oriented. DCS scans and concentrates on the controlled process, be it, a chemical plant or any other and presents the information to the operators. SCADA, on the other hand, concentrates mainly on the control centre as well as the operators. The remote equipment is mainly used to collect data even though it can be used to carry out other multifaceted process controls.

A DCS operator station is usually linked to its I/O via the local wiring, networks and FiedBus. The operator makes a request directly to the field I/O whenever he wants to get information and gets a response for the same. This means that field events can directly disrupt the system and counsel the operator. On the other hand, whenever field communication fails, SCADA has to operate reasonably. Its main concern of a SCADA system operation is the quality of the data that is shown to the operator. In short, a DCS system is principally concerned with the process trends while a SCADA system is driven by the process events.

Another outstanding difference between SCADA and DCS is that the former maintains a database of previous good values and secure data and control whenever there are slow and undependable communications, which makes it fast for urgent operator display. It handles redundancy in a distributed manner. DCS, on the other hand, does not need to keep a database as it is always connected to its data source hence it doesn’t need to be connected to its data source. In the event of a redundancy in DCS, parallel equipment handles that in contrast to diffusion of info around a distributed database.

It is also crucial to note that in a DCS system, the operator has a closed-loop control at the PCS/RTU levels. However, closed-loop control is unavailable in a SCADA system. In its place, you can access the control via the HMI, using human as supervisory control. In this regard, most DCS systems today have integrated HMI and supervisory facilities, DB connectivity as well as all basic facilities that come with SCADA system. Due to the time limitations and urgency, control is done without engaging Human in control flow at the PLC/RTU.

Despite the aforementioned differences, most DCS and SCADA systems today come with same standard facilities like event, archiving, HMI, reporting, DB management and logging as well as control centre remote control. However, the solutions that are contained in either SCADA or DCS system depend on the need of the customers. They can be customized to contain the expected solutions.

    http://www.synergistscada.com/dcs-vs-scada-systems/

    Key Differences between DCS and SCADA Systems

3. Article dealing with differences between the two most common types of industrial control system

Posted by Austin Scott in ICS, SCADA, SCADA 101 on February 27, 2013

   http://www.synergistscada.com/key-differences-between-scada-dcs-and-hmi-systems/

 Key Differences Between SCADA, DCS and HMI (Human–machine    interface) Systems

P.S.

Conventionally SCADA was more for data-gathering (though it can carry out control actions) where as DCS was for controlled processes. SCADA generally supports storing of database where as DCS generally does not support database. In recent times due to technology advancement features of the two are getting blended and both on foreground appears similar.

In addition to the Morteza Razei's answer I may add, after that you may go to other sites in Google Search. For Further Information you may go to Google Scholar Search.

P.S.

1)

DCS is the acronym for Distributed control systems;

SCADA is the acronym for Supervisory Control And Data Acquisition

2)

DCS is process oriented;

SCADA is data-gathering oriented.

3)

DCS is process state driven; 

SCADA system is event driven.

4)

A DCS is a process control system that uses a network to interconnect sensors, controllers, operator terminals and actuators. A DCS typically contains one or more computers for control and mostly use both proprietary interconnections and protocols for communications.

SCADA may be called Human-Machine Interface (HMI). SCADA systems are used to monitor or to control chemical, physical or transport processes.

5)

Distributed Control System (DCS) consists of one or more controllers used to implement advanced process control techniques.

Supervisory Control and Data Acquisition (SCADA) systems cannot carry out advanced process control techniques.

6)

DCS is more integrated and can do more higher-end stuff, but

SCADA systems are more flexible.

scada is more bigger than dcs

You can image the DCS system= PLC + Scada (of course only imagine)

I mean that , DCS systems are using sharing database between Controller and Scada.

In addition , DCS is controller plus Scada . 

You can use SCADA in control but in my fiels , I never seen that. Scada is using only data input or visualiztaio of datas. And ofcourse some trend functions, alarm messages , events.

This link may help

http://controlreal.com/en/dcs-or-plc-or-pac-or-rtu/

Thanks you very much..

PLC = Programmable Logic Controller, it can be a small standalone microprocessor based controller or a complex networked architecture of controller. Used for process control or automation in a small to medium setup.

DCS = Distributed Control System, A bigger system to do what a PLC can do, for more complex and mission critical operations. With an architecture that is 'distributed' in its resources and processing capabilities. This term was initially used to refer to the systems that were designed to continue to function controlling other areas of the process, when one part of the system fails. However, the demarcation of PLC and DCS is becoming less distinct with modern PLC generations in use.

I will mention two differences in simple terms:

1. If you want to monitor and receive data on continuous basis and perform automated logic on the the values of those data then DCS is to be used. In SCADA automated logic is not possible. Theoretically we call this feature of receiving and processing automated logic on data is called closed loop control system and SCADA is mainly open loop control system. Most of the time operator driven command is issued from SCADA, in case automated command is required from SCADA then additional controllers will be required.

2. DCS is mainly used in process/industrial automation where as SCADA is used in power/Electrical automation. Typical response time for power system protection is less than 1 seconds. All protection and protection related control function for electrical equipment are configured in the relays and BCPUs which are located in control relay panel near equipment itself. This level is self-supporting, that means all control and protection equipment on this level are capable of supporting the primary function of this part of the network, without depending on any communication with the SCADA. For process equipment protection, the first layer of protection is process controller of DCS, if process controller fails then 2nd layer is alarm actuation and human intervention and if human intervention also fails then third layer is pressure safety valves, Emergency Shut Down System (ESD) etc. So response time for process equipment protection is more.

A distributed control system (DCS) is a central control system having distributed autonomous controllers for one process or full plant operation.

My opinion;

DCS is process oriented and needs to PLC/Controllers and also uses for a high-speed communications .

SCADA system is data-gathering oriented and needs to RTUs and also covers larger geographical areas.

I wrote a detailed answer on Quora which I am posting it here again:

PLC stands for Programmable Logic Controllers. In the olden days, interlocks and plant sequential operation were implemented using relay logic. The problem with relay logic is that it is difficult to change the logic. If you wanted to change the logic, you had to change the wiring. PLCs came into existence to address this problem of relay logic. PLCs were microprocessor based devices which could be reprogrammed any number of times. If the operation logic changed, only the PLC ladder program had to be changed. The ladder language was very intuitive to the plant technicians and they didn't need to have any experience in computer/ microprocessor programming. Initially PLCs were meant to handle digital relay logic where as Distributed Control System handled most of the analog control.

Distributed Control System architecture had analog and digital controllers (microprocessor based devices) spread across the plant. These controllers sent data to the control room through proprietary communication networks. The control rooms had VDU (video display unit) where plant data was displayed. DCS systems were proprietary in nature meaning that the complete solution was given by a single vendor. Major DCS system suppliers were Honeywell, Yokogawa, ABB, Foxboro etc. The distributed nature of the technology made it inherently reliable. The feature that made DCS so popular was the fact that the DCS vendor took complete responsibility of the implementation. Evolution of the DCS technology made it extremely reliable over time. DCS technology had fault tolerance and reliability inbuilt and the DCS vendors utilised the expertise of the best designers. DCS systems are still used in the process industry because of the reliability.

SCADA stands for Supervisory Control and Data Acquisition. SCADA technology was used to connect remote sites spread geographically across thousands of kilometres. The remote sites has RTU (Remote Telemetry Units) which transmits data to central control rooms using radio communication. Since there is a lag in data transmission to control room, real time control is done by RTU. Control room does supervisory control by providing set points to the RTUs. To begin with, SCADA vendors supplied complete SCADA solution with RTUs and SCADA software. Now there are many vendors who supply only SCADA software packages (based on Windows platform) with which the user can build his own HMI or control application by using PLC as RTUs.

Now PLCs have advanced to include analog control and many other advanced control functions. So a combination of PLC with SCADA software packages works out to be functionally similar to DCS technology. Such implementation will also be much cheaper than DCS technology. This type of solution will also not have proprietary control by one vendor. But the most important advantage of DCS technology is the reliability.

It depends on which industry you are in (and how long you've been around for). Petrochem have used the term DCS, water and waste water use the term SCADA; others just say control system. DSC used to mean "digital control system" but then became "distributed control system". Most large control systems are distributed in that there are IO modules in different parts of a plant. PLCs have grown from small "bricks" with only on/off capability to huge systems with >10000 IO and full functionality of PID control, alarms, advanced logic, etc. Once you add a user interface you can call a PLC whatever you want.