SCADA: Data Acquisition

PLC/RTU Level Data

The PLC/RTU performs the data acquisition like metre readings and equipment status which they communicate to the SCADA as per requirement. SCADA compiles and formats this data in a way that makes possible for a control room operator to make appropriate supervisory decision using the Human Machine Interface (HMI) to adjust or even change PLC/RTU controls. Often for studying trends or other analytical works, a commodity specific database management is developed from which data are collected.

Data Points

A tag database is a distributed database typically implemented by SCADA systems. The data elements contained in such database are called tags or points. The value of a single input/output represented by a point is monitored and controlled by the SCADA system. There are two types of points, viz. hard and soft. An actual input/output connected to the system is represented by a hard point. Applications of the result of logical and mathematical operations to other hard and soft points are represented by a soft point. However in most actual implementations, this distinction between the two points are removed and each property is made conceptually a soft point that in the simplest case can be made equal to a single hard point. The storage of point values are in the form of value-timestamp combinations representing the value and the time when the value was recorded or calculated. History of a point is represented by a series of value-timestamp combination of that point. Additional tag data like PLC Register, path to field device, comments on design time, alarm information, etc. are also commonly stored.

Obtaining SCADA

Purchase of DCS (Distributed Control System) or SCADA can be made from a single vendor. However assembling of SCADA system from hardware/software component is more common. There are many suppliers in the market for these components. The SCADA system requires HMI packages also which have to be resourced from different suppliers.