A Supervisory Control and Data Acquisition (SCADA) system is software that allows operators to centrally monitor and control industrial processes. It collects real-time data from PLCs, sensors and field devices, displays them on clear control screens and records alarms, trends and process variables. Whereas a PLC directly controls a machine or process, a SCADA system provides the overview of an entire plant.
In practice, SCADA and PLC are often confused. Yet they are two different layers that complement each other. Below you can read about what exactly a SCADA system does, how it relates to a PLC and an HMI, and where it is used in industrial environments.
Table of Contents
What does a SCADA system do?
A SCADA system forms the supervisory layer above the control system. It does not take over control tasks itself, but makes a process visible and controllable. The core functions are:
- Data acquisition: continuous reading of measurement and control signals from PLCs, Remote Terminal Units (RTUs) and sensors.
- Process visualization: Showing the current state of the process on control screens, with graphs, mimics and real-time values.
- Monitoring and alerting: Detect anomalies early via an alarm system so operators can intervene before a failure escalates.
- Data logging and trends: Capture historical process variables in a database for analysis, reporting and optimization.
- User Management: Control access and operating privileges by role.
These features give operators and technicians a centralized view of the process, while historical data is used to analyze and improve plant performance.
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SCADA versus PLC: two layers in the automation pyramid
The difference between a PLC and a SCADA system becomes most obvious through the automation pyramid. This is the layer model by which industrial automation is usually described:
- Field level: sensors, actuators, motors and valves.
- Control level: The PLC or RTU, which directly controls the process.
- Supervision level: The SCADA system and HMI, for monitoring and visualization.
- Production level: MES systems for production management.
- Business level: ERP software for planning and administration.
A PLC (Programmable Logic Controller) sits at the control level. It executes logic and controls (think inputs, outputs and PID controllers) and controls the machine in hard real time. A SCADA system sits one layer higher, at the supervision level. It aggregates data from one or more PLCs and presents it centrally.
Aspect | PLC | SCADA System |
|---|---|---|
Function | Direct, real-time control of a machine or process | Monitoring, visualization and data acquisition over an installation |
Level in the pyramid | Control level | Supervision level |
Operation | Performs control logic and schemes | Collects and displays data from multiple sources |
Time behavior | Hard real-time (milliseconds) | Near-real-time visualization and logging |
Operation | Local, machine-oriented | Central, plant or factory-wide |
Dependency | Can drive independently | Needs PLCs or RTUs as a data source |
In short, a PLC controls, a SCADA system monitors. A PLC can function without SCADA, but a SCADA system always needs a control layer that provides the data.
SCADA, HMI and DCS: what’s the difference?
Three terms are often mentioned in the same breath. The distinction:
- HMI (Human Machine Interface): A control panel or touch screen at one machine or plant component. An HMI is local and machine-centric. SCADA operates at a higher, plant-wide level and can include multiple HMIs and PLCs.
- DCS (Distributed Control System): A distributed control system that highly integrates control and visualization, traditionally applied in continuous process industries. SCADA and DCS are growing toward each other functionally, but SCADA is more often used for geographically dispersed or machine-oriented applications.
To be clear, a SCADA system typically includes HMI visualization, but a separate HMI is not yet SCADA.
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How does a SCADA system work?
A SCADA system basically consists of servers, clients and communication links to the controllers. The architecture is as follows:
- Data sources: PLCs, RTUs and sensors provide process data and measurement and control signals.
- Data communication: data is sent to the SCADA server via industrial protocols. Kwadrant IA applies protocols such as Profinet, Modbus TCP and OPC UA, over Ethernet and, in older installations, serial connections such as RS232 and RS485. An OPC UA connection between PLC and SCADA ensures an unambiguous, future-proof data structure.
- SCADA server: Collects, processes and stores data, often in SQL databases for trends and reporting.
- Clients and operator screens: Operators view and control the process through real-time visualizations, possibly web-based.
- Functions: alarms, data logging, trend modules and user management are standard components.
A well-thought-out data structure and structured engineering (documented in EPLAN at Kwadrant IA) make the system maintenance-friendly and expandable.
Where is SCADA applied?
SCADA is deployed where processes must be continuously monitored or where facilities are geographically dispersed. Examples from the industries Kwadrant IA works in:
- Water management: Central monitoring of pumping stations and treatment plants.
- Food industry: Process monitoring and traceability in production lines.
- Infra: Remote control and monitoring of scattered objects.
- Petrochemical and marine & offshore: Safe process monitoring in demanding environments.
- Machine building and OEMs: Making machine data uniformly available for central monitoring.
SCADA as part of a reliable operating system
A SCADA system delivers value only when it works seamlessly with the underlying hardware and controls. Industrial automation requires more than just a correctly constructed panel. At Kwadrant IA, the approach starts with the architecture of the system, with conscious choices in component selection, standardization and scalability.
The SCADA and PLC software is developed according to international standards such as IEC 61131-3 and S88, with a modular and maintenance-friendly structure. The control cabinets are built according to NEN-EN 61439 and, for the North American market, UL 508A. Because hardware engineering, software engineering, panel construction and commissioning are under one roof, hardware, software and field components function as one system.
Thus, Kwadrant IA realizes not a standard solution, but a technically sound operating system that performs reliably and has a manageable Total Cost of Ownership.
Trends: SCADA in a digitizing industry
SCADA systems are evolving along with Industry 4.0. Real-time data from the process forms the basis for predictive maintenance, predicting failures before they occur. Links to MES and ERP software make process data available company-wide, and a digital twin uses that same data for simulation and optimization. With this increasing connectivity, OT cybersecurity becomes a regular concern: the separation and security of IT and OT networks should be included from the design stage.
Need a SCADA system designed or integrated?
Want to know how a SCADA system connects to your machines and equipment, or how best to set up the link between PLC and SCADA for your process? Schedule a technical consultation with Kwadrant IA engineers.
