What Is SCADA and What Is an RTU?

SCADA stands for Supervisory Control and Data Acquisition. It is the system that lets operators watch and control plant or infrastructure from one place: a water network, a substation, a production line, a pipeline. On a screen it looks like a live schematic of the process, with values updating, alarms flagging, and controls an operator can act on.

The key word is supervisory. SCADA does not usually run the fast control loops itself. That happens in controllers out in the field. SCADA sits above them, collecting their data, presenting it, storing the history, and passing operator commands back down. It is an architecture made of several layers, not a single box you buy.

A SCADA system is built from a stack of layers, each with a clear job:

• Sensors and actuators measure the process and act on it: a level transmitter, a flow meter, a valve.
• PLCs (Programmable Logic Controllers) run the real-time control logic close to the machine, fast and deterministic.
• RTUs (Remote Terminal Units) collect data from sensors and controllers at a remote site and telemeter it back to the center.
• The SCADA head-end gathers everything, drives the operator screens, raises alarms, and writes to a historian.

Data flows up from the field to the operator, and commands flow back down. The diagram below shows the path.

A Remote Terminal Unit is the field device that connects a remote site to the SCADA center. It reads sensors and controller registers, packages the values, and sends them back over whatever link the site has, historically radio or leased line, today usually cellular. It also receives setpoints and commands and applies them locally.

An RTU is built for the field: wide temperature range, wide voltage input, often low power for solar sites, and a telemetry protocol like DNP3 or IEC 60870-5 designed for slow or unreliable links. The line between an RTU and a PLC has blurred, but the emphasis is different. A PLC is built to control a process fast. An RTU is built to monitor and telemeter a remote site reliably.

Classic SCADA polls. The head-end asks each RTU for its values on a fixed cycle, over a protocol designed decades ago for serial links. It is proven and it is reliable, and in regulated infrastructure it is not going anywhere soon.

Modern IoT monitoring tends to be event-driven and to publish over MQTT to a broker or cloud, sending data when something changes rather than on every poll. The two are not rivals. A common pattern today is an edge gateway that speaks the field protocols and the SCADA protocol on one side, and MQTT to a cloud platform on the other, so the same field data feeds both the control room and the wider business.

An open Linux gateway can take the RTU role directly. It reads the sensors and PLCs, applies local logic, buffers through outages, and telemeters upward, either into an existing SCADA system over DNP3 or Modbus, or into a cloud platform over MQTT, or both at once.

The difference from a traditional RTU is ownership and openness. The protocols, the data model, and the software are yours, so adding a new sensor or a new destination is a configuration change rather than a vendor request. You get the field-hardened reliability of an RTU with the flexibility of an open edge platform.

Melqart Systems does not sell a SCADA system. We build the open edge layer that feeds one, or that feeds your cloud directly when a full SCADA is more than the job needs.

That layer is an open Linux gateway acting as a modern, ownable RTU: it reads your field equipment, telemeters reliably, and keeps the data model in your hands. See condition and process monitoring for the monitoring side, routers and gateways for the hardware, and the OTS datalogger where the job is acquisition rather than control.

What does SCADA stand for? Supervisory Control and Data Acquisition. It is the supervisory layer that collects field data, shows it to operators, raises alarms, stores history, and sends control commands back to the field.

What is an RTU? A Remote Terminal Unit: a field device that gathers sensor and controller data at a remote site and telemeters it to a SCADA center, while applying commands locally. It is built for harsh conditions and unreliable links.

What is the difference between an RTU and a PLC? Both are field controllers. A PLC is optimized for fast, deterministic process control. An RTU is optimized for monitoring and telemetering a remote site over a long, sometimes unreliable link. Modern devices often do both.

Is an RTU the same as a rooftop unit? No. In HVAC, RTU means rooftop unit, the packaged air handling equipment on a roof. In SCADA, RTU means Remote Terminal Unit. Same three letters, unrelated things.