SCADA Software Functions

In general, SCADA’s many functions and capabilities fall into three categories:

Data acquisition is the scanning function SCADA performs to get real-time data about the monitored system. RTUs wired to specific devices are scanned at a rate specified in the SCADA database. The RTUs send the data they receive (referred to as “raw data”) to the CFEs, which perform some conversion and checking on the data before sending it to the SCADA Front End, and then to the host.

Supervisory control is the SCADA functionality that lets operators and applications issue commands to RTUs from the control site. A control command causes a change in the operating state of a device. For example, an operator can issue a command that causes a closed valve to open.

System monitoring refers to the capabilities that allow operators and applications to change the way that SCADA processes and displays data. For example, an operator can enter a value for a device whose measurement is not retrieved from an RTU.

Data acquisition, supervisory control, and system control are explained in more detail in the following sections.

Data Acquisition

“Data acquisition” is the process of SCADA acquiring data about the monitored system.

To acquire data in the monitored system, the SCADA Front End performs the following functions:

Scans the monitored system at prescribed times and receives data from RTUs via the CFEs.

Performs conversion and checking on raw data to make it usable for display to operators and to other applications.

Determines the quality of data and marks data that may not be reliable.

Allows SCADA operators to request scanning of individual scan groups.

To acquire data about devices in the monitored system, performs the following functions:

Performs calculations on data for other applications.

Determines the quality of data and marks data that may not be reliable.

Marks data to indicate its source.

SCADA can acquire, process, and display data from four types of devices in the monitored system. These device types are:

Variable-state devices, which have numerical measurements, referred to in SCADA as “analog points”

Discrete-state devices, such as pumps, which have status measurements, referred to as “status points” (extendable up to a 16-state device)

Pulse accumulator devices, whose numerical measurements are referred to as “count points”

Text, such as batch identifiers

For more details about data acquisition functions, see section 1.5 .

Supervisory Control

SCADA’s supervisory control function allows operators and applications to control devices in the monitored system. From the control center, the SCADA operator can issue control commands to change the state of such devices as pumps and valves.

Specific instructions for performing supervisory control functions are included in later chapters and sections of this guide.

System Monitoring

The system monitoring function allows operators and applications to change the way in which SCADA processes and displays data. One example of a system monitoring operation performed at the SCADA Front End is controlling the configuration of the communications system by removing from service RTUs, scan groups, and communication lines.

Listed below are examples of system monitoring operations that are performed on the host SCADA system:

Removing a point from service – this operation disables the processing of scanned data for a selected device in the monitored system.

Restoring a point to service – this operation enables SCADA to process scanned data for a selected device.

Entering a value manually for devices whose measurements are not telemetered (not retrieved by RTUs and not retrieved from remote sites).

Overriding a telemetered value with a manually entered one.

Manually entering limit values for variable-state devices, and instructing SCADA to ignore limits for selected devices.

Inhibiting and enabling activities associated with alarmed and abnormal events.

Operator acknowledgment of alarms.

Specific instructions for performing system monitoring functions are included in later chapters and sections of this guide.