A single PLC controls one local process. SCADA sits above dozens or hundreds of them at once — pulling status, raising alarms, and giving one operator a unified view of a plant or network they could never physically walk in real time.
SCADA (Supervisory Control and Data Acquisition) sits above local control systems — PLCs and remote terminal units (RTUs) at individual sites — aggregating their data into a centralized human-machine interface (HMI), so one operator or small team can monitor and, when needed, remotely control equipment spread across a wide area: a water utility's pump stations, an oil pipeline's valve sites, or a power grid's substations.
Each local PLC handles its own fast, real-time control loop autonomously — a pump station's PLC keeps running its logic even if communication to the central SCADA system is temporarily lost. SCADA's role is supervisory: gathering status and measurements from many such sites, presenting them in one consolidated view, logging historical data for trending and analysis, and providing a path for an operator to issue remote commands or adjust setpoints when necessary.
With dozens or hundreds of monitored points across a system, an operator cannot watch every value continuously — SCADA alarm systems are designed to surface abnormal conditions (a tank running low, a communications failure, a value outside its normal operating band) immediately and distinctly, so operator attention goes where it's actually needed rather than requiring constant manual scanning of every data point.
A site that stops communicating with the central SCADA system isn't necessarily broken — but the operator has genuinely lost visibility into whatever is actually happening there, which is itself a serious operational concern. This is exactly why 'communications lost' is treated as an alarm condition in its own right in the example above, not merely a missing data point to ignore.
A PLC executes fast, local, real-time control logic at a specific site or process. SCADA is a supervisory layer above many PLCs/RTUs, aggregating their data for centralized monitoring, historical logging, alarming, and (where permitted) remote operator control — SCADA doesn't replace local control, it supervises it.
Typically no — day-to-day, fast control logic runs locally on the PLC/RTU at each site, which continues operating even during a temporary loss of SCADA connectivity. SCADA primarily monitors and, when an operator initiates it, sends supervisory commands (like changing a setpoint) rather than executing the moment-to-moment control loop itself.
Because SCADA systems often supervise critical infrastructure (water, power, oil and gas) and were historically designed assuming a physically isolated network — as SCADA systems have become more networked and, in some cases, internet-connected, they've become a serious target, which is why frameworks like NIST CSF and standards like ISA/IEC 62443 specifically address industrial control system (OT) security.
Try our STEM Learning Studio
More calculators, simulators, and guides for this discipline.