Supervisory Control And Data Acquisition (SCADA)

SCADA is the acronym for Supervisory Control And Data Acquisition. SCADA may be called Human-Machine Interface (HMI) in Europe. The term refers to a large-scale, distributed measurement (and control) system. SCADA systems are used to monitor or to control chemical, physical or transport processes.

The term SCADA usually refers to a central system that monitors and controls a complete site. The bulk of the site control is actually performed automatically by a Remote Terminal Unit (RTU) or by a Programmable Logic Controller (PLC). Host control functions are almost always restricted to basic site over-ride or supervisory level capability.

Data acquisition begins at the RTU or PLC level and includes meter readings and equipment statuses that are communicated to the SCADA as required. Data is then compiled and formatted in such a way that a control room operator using the HMI can make appropriate supervisory decisions that may be required to over-ride normal RTU (PLC) controls. (A SCADA system includes all the pieces, HMI, controllers, I/O devices, networks, software, etc.)

SCADA systems typically implement a distributed database which contains data elements called points. A point represents a single input or output value monitored or controlled by the system. Points can be either "hard" or "soft". A hard point is representative of an actual input or output connected to the system, while a soft point represents the result of logic and math operations applied to other hard and soft points. The point values are normally stored as value-timestamp combinations; the value and the timestamp when the value was recorded or calculated. A series of value-timestamp combinations is the history of that point.

http://en.wikipedia.org/wiki/SCADA#Communication_infrastructure_and_methods

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