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== Control operations ==
[[File:Functional levels of a Distributed Control System.svg|thumb|500px|Functional levels of a manufacturing control operation]]

The key attribute of a SCADA system is its ability to perform a supervisory operation over a variety of other proprietary devices.

* Level 0 contains the field devices such as flow and temperature sensors, and final control elements, such as [[control valve]]s.
* Level 1 contains the industrialised input/output (I/O) modules, and their associated distributed electronic processors.
* Level 2 contains the supervisory computers, which collate information from processor nodes on the system, and provide the operator control screens. 
* Level 3 is the production control level, which does not directly control the process, but is concerned with monitoring production and targets.
* Level 4 is the production scheduling level.

Level 1 contains the [[programmable logic controller]]s (PLCs) or [[remote terminal unit]]s (RTUs).

Level 2 contains the SCADA to readings and equipment status reports that are communicated to level 2 SCADA as required. Data is then compiled and formatted in such a way that a control room operator using the HMI ([[Human Machine Interface]]) can make supervisory decisions to adjust or override normal RTU (PLC) controls. Data may also be fed to a [[Operational historian|historian]], often built on a commodity [[database management system]], to allow trending and other analytical auditing.

SCADA systems typically use a ''tag database'', which contains data elements called ''tags'' or ''points'', which relate to specific instrumentation or actuators within the process system. Data is accumulated against these unique process control equipment tag references.