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SCADA Systems: Automatic Change-of-State (COS) Control

Supervisory Control and Data Acquisition (SCADA) systems are used for monitoring and controlling industrial processes at the supervisory level. Usually, these systems make it possible to manage networks by collecting and analyzing real-time data from remote locations. Sites where human presence is not practical.

Examples of companies that deploy SCADA systems include water and waste control, oil and gas refining, transportation, energy and telecom.

SCADA Components and Functions

SCADA systems involve many components. Some of the SCADA monitoring equipment and elements are:

  • Sensors

  • Control relays

  • Remote terminal units (RTUs)

  • Master station or HMI software

  • Communication channels

These components perform the following functions:

  • collect data

  • network data communication

  • data presentation

  • process control

Control Relays That Automatically Respond to Change-of-State Events

Advanced SCADA systems support control functions. SCADA control functions are what enable a system to automatically respond to certain situations with a programmed response. Sensors cannot generate or interpret communications protocols.

Remote Telemetry Units (RTUs) along the network interpret the information from these sensors. And translate it into a language the master can understand.

SCADA Control is a core component of a SCADA Monitoring System

The master can utilize the information it receives from various inputs to enact control relays at the RTU level. This means that whenever a user-specified combination of alarms occur, the RTU will automatically respond with a SCADA control relay that has been programmed into the system. It will secure the network by responding to the Change-of-State (COS) event indicated by the alarm.

A Change of State (COS) Alarm list displays all new events that happen in your network, including alarm points that go into an alarm state and alarm points that are cleared. If your alarm master supports both kinds of view, you have the quickest and most accurate picture of your network's current status.

In other words, a control relay output allows you to remotely activate virtually any piece of equipment at your remote sites.

Derived Controls provide for instant reactions of the monitoring system in the event that user-specified alarm combinations occur. For example, if two individual alarms occurred at a single site, indicating that there was a power failure and a battery failure, a Derived Control would be activated, powering on a back-up generator.

So, with derived controls, you can apply rule sets to incoming alarms to control complex automatic responses to emergencies. There are two types of derived controls: echo and formula.

An echo derived control creates a one-to-one relationship in which a particular alarm input is echoed by a relay. For example, if your alarm monitoring device senses that a tower light has gone out, you can have a backup light automatically turned on in seconds.

A formula derived control monitors multiple alarm inputs. As well as evaluates them by a user-defined formula to determine if a relay should be activated. Derived control formulas use Boolean operators to specify under what conditions relays should be activated. Our generator failure example might be written like this:

If ( (generator=down) AND (battery=bad) )
then ( (page=technician) AND (backupGen=start) )

This is just a basic example, but it gives you an idea of the power of derived controls. And this power is easily expanded, because derived control formulas can themselves be used as terms in larger derived control formulas. Using derived controls, extremely complex and intelligent responses to emergencies can be completely automated.

Derived controls can trigger a control relay latch on a NetGuardian RTU - or an SNMP GET command to any SNMP device.

SCADA controls prevent the interruption of mission-critical operations. Even when a network operator is unavailable to manually respond to an alarm notification.

Lesser monitoring systems do not provide SCADA control functions. They simply allow for monitoring of a network. Operators using these types of systems must manually respond to individual alarms. This can be time consuming and very inefficient when dealing with complex processes at multiple sites.

NetGuardian 832A Supports up to 8 Controls for Added Network Security

This is why it is critical for operators to look for RTUs that can support both discrete and analog alarms, as well as control relays, when deploying a SCADA system. RTU devices, such as the NetGuardian 832A, are capable of all of these functions. Which eliminates inefficiencies by responding to alarms with control relays.

The NetGuardian 832A can support up to 32 discrete alarms, as well as 8 analog alarms and 8 controls. With a large capacity such as this, operators can create controls to respond to numerous alarm combinations along their network. While easily accommodating additional devices as their network expands.

Resources Related to SCADA Tutorials

SCADA Tutorial White Paper


To see additional information related to a SCADA Manual, please visit the SCADA Monitoring page.