What does a SCADA system actually do?
A SCADA system (Supervisory Control and Data Acquisition) performs four core functions in an industrial installation:
- Data acquisition: it collects in real time the value of the process variables (temperatures, pressures, flow rates, equipment status) from PLCs, RTUs and other field devices.
- Supervision: it presents the data on process screens that the operator can understand: animated synoptics, gauges, trends, alarms.
- Control: it lets the operator act on the process: start or stop equipment, change setpoints, force states — always within the configured limits.
- Historical logging: it stores the values of every variable over time, enabling retrospective process analysis, report generation and compliance with traceability requirements.
Architecture of a modern SCADA system
Field level
The devices that generate the data: PLCs, RTUs, variable frequency drives, analysers, smart meters, protocol gateways. They communicate with the supervision level using industrial protocols: OPC-UA (the modern standard), Modbus TCP, Profinet, DNP3, IEC 60870-5-104 or proprietary protocols.
SCADA server
The core of the system. It receives data from the field level, processes it (calculations, unit conversions, alarm logic), stores it in the historical database and serves it to clients. In high-availability systems, the server is deployed in a redundant configuration (primary server + standby server) with automatic failover.
Historical database (Historian)
Conventional relational databases are not suitable for storing process data (millions of data points per day). Specialised historians such as OSIsoft PI, Ignition Historian, InfluxDB or TimescaleDB are optimised for time-series data: high write rate, efficient compression and fast queries over time windows.
SCADA clients
The user interfaces: control-room screens, operator stations on the plant, remote web access, mobile apps. Modern SCADAs allow browser-based access with no software installed on the client (Ignition Perspective, WinCC Unified Web Client).
Alarm management: the SCADA's Achilles heel
A poorly configured SCADA generates hundreds or thousands of alarms a day, many of which are false, redundant or non-actionable. This trains the operator to ignore alarms — with the obvious risk of missing a real critical alarm. Effective alarm management is one of the most important parts of SCADA design and one of the most frequently overlooked.
Alarm management good practice under the ISA-18.2 standard includes: defining the priority of each alarm according to its real consequence, removing redundant or low-value alarms, configuring delays to avoid alarms on transient states, implementing alarm grouping for cascade situations, and periodically reviewing the most frequent alarms to identify those that can be removed or improved.
Redundancy and high availability
On critical installations (utilities, infrastructure, continuous processes), a SCADA outage can have severe operational and financial consequences. Modern SCADA systems offer multiple layers of redundancy:
- Redundant server: primary server + standby server with real-time synchronisation. If the primary fails, the secondary takes over automatically and transparently for the operator.
- Redundant communications: dual communication path to the PLCs — for example, Profinet primary + Ethernet secondary — with automatic switchover on failure.
- Redundant storage: RAID for historical data, with automatic backups to a secondary location.
- Backup clients: operator screens with direct access to the PLC in case of a full SCADA-server outage.
Cloud SCADA vs on-premise SCADA: when does each make sense?
On-premise SCADA: the server is on the customer's plant. Pros: lower latency, more control over security, operation independent of internet connectivity, easier compliance with regulatory requirements (pharma, critical infrastructure). Cons: infrastructure cost, server management, updates.
Cloud SCADA: the server is at a cloud provider (Azure, AWS, Google Cloud). Pros: no infrastructure investment, automatic scalability, access from anywhere, managed updates. Cons: dependency on internet connectivity, higher latency, recurring cost, security and data-sovereignty considerations.
Hybrid SCADA: the most common architecture in practice. Real-time processing and critical data on premise; long-term history, reporting and management access in the cloud.
At Bluemation we design and implement SCADA systems tailored to each project's needs: platform, architecture, redundancy, alarms and reporting. Tell us about your project.