What is the Basic Difference between PLC , DCS and SCADA?

* PLC and DCS are functionally the same. Of course, for huge plants with I/O's ranging from 10,000 to 30,000 I/O points, and beyond, it is silly to even think about PLC. A DCS is not the same with PLC in the realm of huge systems such as oil and gas plants, as the following points show:

1. A PLC is cost-effective up to a certain I/O count, and so is the DCS. But the difference is in their starting points: the PLC is cost-effective from 0 to a few thousand I/O points; the DCS becomes cost-effective starting from a few thousand points and beyond.

2. A PLC becomes a subsystem of the DCS in rare occasions when the situation calls for it, i.e., purchase of huge package systems with engineering schedules incompatible with the DCS schedule (I/O lists cannot be submitted on time before the DCS hardware freeze date). Note that this package system is a process system using continuous control, not discrete. Based on this, a PLC can never be larger than a DCS in terms of I/O count.

3. In large plants the DCS is king because most owners want a single source of hardware support and service, and this mentality naturally denies the PLC a foothold. Package vendors are no longer required to provide PLC for their system. Everything is connected to the DCS.

* SCADA is the Software tool in which you can able to view/Monitor/Control your Process Variable data and Graphical representation of your plant etc...It's an operator front end display Example of few SCADA S/W is Wonderware, IFix, RS View, WinCC etcSCADA is Supervisory Control and Data Aquisition which is used to control and Monitor small number of Equipments in a field.SCADA systems are typically used to perform data collection and control at the supervisory level. Some systems are called SCADA despite only performing data acquisition and not control.The supervisory control system is a system that is placed on top of a real-time control system to control a process that is external to the SCADA system (i.e. a computer, by itself, is not a SCADA system even though it controls its own power consumption and cooling). This implies that the system is not critical to control the process in real time, as there is a separate or integrated real-time automated control system that can respond quickly enough to compensate for process changes within the time constants of the process

SCADA is the Software tool in which you can able to view/Monitor/Control your Process Variable data and Graphical representation of your plant etc...It's an operator front end display....

for self study please visit:

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

* http://www.controlengeurope.com/article/40827/DCS-and-PLC-SCADA-a-comparison-in-use.aspx

* http://blog.cimation.com/blog/key-differences-between-scada-dcs-and-hmi-systems

Difference between SCADA, DCS and PLC Systems.

A SCADA (or supervisory control and data acquisition) system. It consists of many remote terminals units for collection of data (field),that is being connected with master station through any communication system, having main task of collection of accurate data and controlling of process for smooth operation.

It stands for distributed Control System, controlling is performed by embedded system (Microcontroller based or Microprocessor based controlling unit for device or instruments from which data is to be collect. It provides very intelligent analog control capability. It is very sensitive for HMI (Human machine Interface) for easy and smooth control of process.

It stands for Programmable Logic controller, having get this name from the fact that it replace the relay logic at the initial stage then it get the capability for analog channels also for display then it get the ability for close loop control and after some time it has the ability for redundant operation, and also its HMI having the ability for Indicatiion,controlling,data logging ,Alarming and backup data facility. It is also defined as below: “A digitally operating electronic apparatus which uses a programmable memory for the internal storage of instructions for implementing specific functions, such as logic, sequencing, timing, counting and arithmetic, to control through digital or analog input/output, various types of machines or process.” National Electrical Manufacturers Association (NEMA), 1978

Source: http://what-instrumentation.blogspot.com/p/difference-between-scada-dcs-and-plc.html

A detail answer has been alreday added by Nasir Khan