What is SCADA and how it is built
Supervisory control and data acquisition: the components of a SCADA system — I/O servers, tag database, alarm server, historian, clients, engineering station — and how data flows from the PLC to the operator.
Purpose
SCADA systems supervise a plant or a geographically distributed process (water network, pipelines, power distribution): they acquire data from controllers and RTUs, present it to operators, record history and alarms, and send supervisory commands (setpoints, start/stop). Closed-loop control stays in the PLCs/RTUs.
Components
| Component | Function |
|---|---|
| I/O server / data server | Communicates with PLCs/RTUs using drivers (S7, CIP, Modbus, OPC UA, IEC 104, DNP3) |
| Tag database | Definition of every point: source address, type, scaling, alarm limits, history |
| Alarm server | Evaluates alarm conditions, keeps alarm state, acknowledgement, logging |
| Historian | Stores time series and events long-term — see historian basics |
| Runtime server | Runs the project: graphics data, scripts, calculations |
| Clients | Operator stations, web/thin clients, mobile views |
| Engineering station | Develops and deploys the project; not used for operation |
Small systems put all functions on one PC; large ones distribute them over redundant servers — see architecture and redundancy.
Data flow
PLC → driver/OPC UA → tag database → (alarms, history, scripts) → clients. Each value carries quality (good/bad/uncertain) and a timestamp; a communication failure must turn values "bad" on the screens instead of freezing the last number.
Typical functions beyond graphics
- Reports (shift, daily, batch), calculated tags, scripting.
- User management and audit trail — see alarms, events and audit.
- Interfaces upward: MES/ERP, databases, OPC UA/MQTT.
Security note
SCADA connects many controllers and often the business network: it is a prime target. Segment networks, harden servers, control remote access and patching — NIST SP 800-82 gives an overview of OT security practice.
Connections
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Sources
Updated 2026-10-02