The term SCADA describes centralized systems which monitor and control sites, or complexes of systems distributed over considerable areas in other words anything between an industrial plant and a country. A majority of the control actions are executed automatically by Remote Terminal Units also known as RTUs” or by Programmable Logic Controllers also known as PLCs. Host control functions are normally restricted to basic overriding or supervisory level intervention. For instance, a PLC may control the flow of cooling water in a section of an industrial process, but the SCADA system may enable operators to alter the set points for the flow, and permit alarm conditions to be exhibited and recorded. The feedback control loop goes through the RTU or PLC, while the SCADA system monitors the complete operations of the loop.
A SCADA System is typically comprised of subsystems which include
- A Human-Machine Interface or HMI which is the apparatus that presents process data to a human operator. In doing this, the human operator monitors and controls the process.
- A supervisory or computer system, which acquires data on the process and sends commands to the process.
- Remote Terminal Units or RTUs which connects to sensors in the process, converting sensor signals to digital data then sending digital data to the supervisory system.
- Programmable Logic Controller or PLCs which is used as field devices because they are more affordable, multi-purpose, flexible, and modifiable in comparison to special-purpose RTUs.
- Communication infrastructure which connects the supervisory system to the Remote Terminal Units.
There is some amount of ambiguity related to the differences between SCADA systems and Distributed control systems or DCS. In general, a SCADA system normally refers to a system that coordinates, but does not actually control a set of processes in real time. The idea of real-time control is clouded in some way by newer telecommunications technology, permitting reliable, low latency, high speed communications over extensive areas. The disparities between SCADA and DCS are culturally determined and can generally be overlooked. As communication infrastructures with greater capacity are made available, the differences between SCADA and DCS will fail to be.
SCADA systems usually incorporate a distributed database which is normally referred to as a tag database. This database is composed of data elements called tags or points. A point is representative of a single input or output value monitored or controlled by the system. Points can either be soft or hard. A soft point is the result of logic and mathematical operations applied to other points while hard point is representative of an actual input or output within the system. Points are usually stored as value-timestamp pairs: a value and the timestamp when it was recorded or calculated. A series of value-timestamp pairs will give the history of that point. It is also common to store extra metadata with tags, inclusive of the path to a field device or PLC register, design time comments, and alarm information.