Differences & Similarities of SCADA Vs DCS

Simply put, a DCS (Distributed Control System) is PROCESS ORIENTED,while a SCADA (Supervisory Control And Data Acquisition) is DATA-GATHERING ORIENTED:

DCS performs more like a controller who processes the data to provide control to the systems;

SCADA collects and presents the data to the operators who give control to the systems.

From MMI point of view, a DCS operator station is normally connected with its I/O (or RTU). When DCS operators need to check up the site information, they usually send requests directly to the field I/O to get the response. Field events can directly interrupt the system and advise the operator. A SCADA operator station present the operator all the data through SCADA systems.

There are some other differences, technically and conceptually, betweent the two systems but that would take some more words to describe.

Thanks!

Nicely put.

Aren't there cases / situations where the two are integrated? The "Supervisory Control" part of the SCADA being implemented by a DCS while gathering the I/O data for logging and trending? And how does the "distributed network architecture" protocol fit into the discussion?

Thanks

PLC vs DCS:

Programmable Logic Controllers (PLC) are mostly set of Input and Output Cards installed in strong (industrial) frame (mostly metallic) constructions. In their processor + memory module they have a logical (discrete) program to set on/off "commands" to output like motors to run, coils to energize /change position etc, according to specific combination of inputs. E.g. run the pump to fill the tank when the liquid level is too low. Programs have also timers and counters, sequencers and some other function required by a process. Some PLC have so called analog modules that fulfill functions usually assigned to DCS. In PLC it is something like "remote analog type control".

DCS is time to time Destinate Control System. Also it could be just a set of individual controllers trying to keep the process variable as close as possible to required (set point) value, e.g. temperature of 80 +/- 2 degrees Celsius, pressure 120 +/-3 psi, level 3 +/- 0.1 m. etc

SCADA begins with Data Acquisition System that collect process data to one central panel in Control Room but being extended to elements of control with final control elements like valves, contactors etc.

Modern Control Rooms have Displays /computer monitors that have process diagrams with input signals, value indicators, On/Off position lights etc.

Individual machines, process cells could have their individual designed HMI /MMI /GUI panels (Human -Man Machine Interface, Graphic Users' Interface) which help operators to start - run - stop motors / machines.

Modern designers have many options to select needed hardware as well as software and do program them to run simple machinery to very complicated processes with required security means.

There are two major differences between scada and DCST These are amplified in large systems such as in oil field automation. The first is that there is a limit to what a SCADA system can poll in 24 hrs. Regardless how small the scan time (which is a function of data aqusition and telemetry), you can only scan so many stations in a 24hr period. DCS systems report on an exception bases and can store thier data till polled.

Second is "Distributed Risk": If the supervisory goes down on SCADA the entire network is blind because it relies on the base station for instructions. In DCS if you loose an RTU the remainder are still functioning

While all of the technical discussions concerning the difference / similarities between SCADA and DCS are valuable and correct, I would like to add that many actual field installations exploit the best features of both - Supervisory Control and Data Acquistion applied using Distributed Network Archetecture - the nodes act as (or are) in many cases discrete DCS systems which report data (and exceptions) through communications to one or more "centralized" stations from which an operator can control set points etc. The advantage comes when a comm failure cuts communications off to the centralized server since the discrete systems continue to operate (using last received set point commands) until the supervisory computer is re-engaged. Also, since the DCS can be configured to store data, the server can update it's database on a scan / call basis which overcomes the polling limitation previously mentioned - data transmission for record is set at a lower priority than data transmission for control / alarm

I came up with this definition for a DCS introduction training course:

A PLC is like a brother who tends to make "things"; trucks, mud balls, rock piles etc.

A DCS is like a sister who tends to make "stuff" that requires a process; pies, cakes, salad dressing etc.

A SCADA system is like a baby sitter who watches them both, keeps them out of trouble, supplies them with materials and tells the parent what they are doing.