PLC vs. RTU: What Are the Differences When It Comes to Remote Monitoring?

· 16 min read
16 min read
Lucas Moreau
Lucas Moreau
OT/IT Network Engineer

In remote industrial monitoring projects, the question often comes up: Should you use a PLC or an RTU? These two types of equipment are frequently confused and sometimes used interchangeably, yet their design philosophies are radically different. Understanding this distinction is crucial for choosing the right architecture—and avoiding an over-engineered or underperforming deployment.

This comprehensive guide clarifies the definitions, compares the two technologies based on the criteria that matter for remote monitoring, and explains when and how to monitor each using SCADA, 4G LTE, or an IIoT cloud platform.

What is a PLC (Programmable Logic Controller)?​

Definition​

A PLC (Programmable Logic Controller), known as an API (Automate Programmable Industriel) in French, is a rugged industrial computer designed to execute a control program in real time for a local industrial process. Its primary role is to read physical inputs (sensors, buttons, flow meters), execute control logic, and act on physical outputs (actuators, valves, motors) in a deterministic and repeatable manner.

Unlike a general-purpose computer, a PLC is designed to operate 24 hours a day, 7 days a week, in demanding industrial environments—including vibrations, EMI, extreme temperatures, and humidity. Its reliability is ensured by a dedicated hardware architecture and a real-time operating system (RTOS).

The Scan Cycle​

A PLC operates based on a scan cycle that repeats indefinitely:

  1. Reading Inputs — The PLC samples all of its physical inputs and copies them into the input image table.
  2. Program Execution — The processor executes the user program (Ladder, FBD, Structured Text, SFC according to IEC 61131-3).
  3. Updating the outputs — the results are written to the physical outputs to control the equipment.
  4. System tasks — diagnostics, communications, timestamping.

The cycle time typically ranges from 1 ms to 100 ms, depending on the program's complexity and the PLC model. Safety applications (SIL 2/3) require cycle times of less than 10 ms with a hardware watchdog.

Examples of Typical PLCs​

  • Siemens: S7-1200 (compact, TIA Portal, native Modbus TCP), S7-1500 (high-performance, integrated OPC-UA), S7-300/400 (legacy systems still widely used)
  • Schneider Electric: Modicon M340 (mid-range), M580 (Ethernet-based, OPC-UA, redundancy), Modicon Quantum (high availability for energy/petrochemical)
  • Allen-Bradley (Rockwell): CompactLogix (versatile, native EtherNet/IP), ControlLogix (high performance, redundant chassis), MicroLogix (entry-level)
  • Mitsubishi Electric: MELSEC iQ-R, MELSEC-F (FX) — widely used in the automotive and pharmaceutical industries
  • Beckhoff: CX/BX series (PC-based automation, EtherCAT, OPC-UA, TwinCAT)

Typical Areas of Application​

Automotive production line, packaging machine, robotic cell, multi-axis conveyor, food processing with recipe management, complex wastewater treatment plant with multi-loop control, FDA 21 CFR Part 11-compliant pharmaceutical machine. The common characteristic: complex logic that must be executed locally, in real time, without relying on a remote command.

What is an RTU?​

Definition​

An RTU (Remote Terminal Unit), sometimes referred to as an automate de télégestion in French, is a device designed to monitor and control equipment at remote sites and transmit data to a centralized monitoring system (SCADA, MTU). Its local processing capacity is intentionally limited; its strengths lie in long-distance communication, resilience to network outages, and low power consumption.

An RTU is the quintessential field device in SCADA (Supervisory Control and Data Acquisition) architectures. While the PLC makes decisions locally, the RTU transmits information to a centralized control center.

How It Works​

The RTU monitors its inputs (4–20 mA, on/off, pulse counting), time-stamps the values locally, and transmits them—periodically or on an event-driven basis—to the central SCADA server via the WAN (radio, 4G, fiber). In the event of a communication outage, the RTU buffers the data with precise timestamps and retransmits it in bulk once communication is restored. This timestamped data storage feature is required in many water, energy, and gas specifications.

Areas of Application​

RTUs are deployed across geographically dispersed infrastructure:

  • Water Distribution and Sanitation: pumping stations, reservoirs, wastewater treatment plants. Measurements of flow, pressure, level, and pump status.
  • Electrical grids: HV/EHV substations, transformer stations, off-grid wind and solar farms.
  • Gas and hydrocarbon transportation: delivery stations, compressor stations, long-distance metering, pipeline monitoring.
  • Agriculture and irrigation: control of irrigation valves, weather measurements, remote field sensors.
  • Street lighting: control cabinets, dimming, consumption reporting.
  • Environment: air quality monitoring stations (PM2.5, NO2, O3), hydrological flow measurement.

The key feature: The RTU is a satellite device deployed at an unmanned site, whose purpose is to report the system's status to a centralized control center.

Comparison Chart: PLC vs. RTU​

CriterionPLC (API)RTU
Processing PowerHigh — 1–100 ms cycle time, complex sequencing logicLow to medium — simple processing, thresholds, alarms
I/O InterfacesModular, high-density digital and analog I/O4–20 mA analog, digital, pulse counting, measurement
Native ProtocolsModbus TCP/RTU, OPC-UA, PROFIBUS, EtherNet/IP, PROFINETModbus RTU, DNP3, IEC 60870-5-101/104, IEC 61968
Power consumptionMedium to high — stabilized 24 V DC power supplyLow — designed for solar/battery power
Environmental robustnessIndustrial grade, temperature-controlled cabinetUnattended sites, extreme cold, heat, and humidity
Typical UseLocal control and monitoring, machinery, production linesRemote management, water/energy SCADA, remote sites
Price range€500 to €20,000 depending on power and I/O€200 to €3,000 depending on I/O and protocols
Programming EnvironmentIEC 61131-3 (LD, FBD, ST, SFC) — automation engineerSimplified configuration or vendor-specific language — remote control technician
Remote communicationOPC-UA, Modbus TCP, via gateway or COM moduleDNP3, IEC 104, native Modbus RTU over 4G / radio / satellite
Redundancy / Offline BufferLimited — designed to remain constantly connectedNative — time-stamped data storage in the event of a network outage

When Should You Choose a PLC?​

A PLC is the right choice when the following criteria are met:

Complex local logic. If your system requires multi-step sequencing, multi-loop PID control, recipe management, multi-actuator coordination, or SIL safety logic, only a PLC has the processing power and the appropriate development environment.

Critical response time. A conveyor that must detect a jam within 10 ms and shut down the line cannot rely on a cloud-based command. The PLC makes the decision locally, in real time, without relying on the network.

Controlled environment. The equipment is installed in a workshop electrical cabinet with a stabilized power supply and controlled temperature. Constraints related to extreme cold, battery power, or unattended sites do not apply.

Real-world examples:

  • Welding robot on an automotive assembly line: The PLC manages axis synchronization, the welding sequence, and safety interlocks in less than 5 ms.
  • Pharmaceutical packaging machine: product flow management, weighing, labeling, and FDA traceability.
  • Wastewater treatment plant (WWTP) with a capacity of 100,000 PE: multi-loop control of the 6 tanks, fill/drain sequences, and management of air blowers.

When Should You Choose an RTU?​

Unmanned remote sites. A pumping station located 40 km from the monitoring center, powered by a generator with a backup battery, in a well or an un-air-conditioned room. The RTU is designed for this scenario.

A large network of measurement points. A water distribution network may include 200 pumping stations and 500 measurement points. Deploying PLCs at each location would be beyond budget and overkill. RTUs, which are more cost-effective and easier to configure for remote monitoring, are the natural choice.

Native SCADA protocols. If your central SCADA system uses DNP3 or IEC 60870-5-104 — protocols designed for remote management with timestamping, event sequences, and intermittent communication — an RTU will support these protocols natively, whereas a PLC requires an additional gateway.

Battery backup during power outages. The RTU stores measurements locally with precise timestamps and transmits them in batches once communication is restored. This feature is required in many water and energy specifications.

Specific examples:

  • Network of 80 air quality monitoring stations: PM2.5, NO2, and O3 readings transmitted every 15 minutes to the central server via 4G.
  • Gas delivery stations: IEC 101 RTUs connected via a PSTN line, followed by migration to IEC 104 over IP, with dispatching supervision.
  • Agricultural irrigation covering 500 ha: RTUs on each sector valve, controlled from the operations center.

Remote Monitoring of a PLC​

Remote monitoring of a PLC involves retrieving process variables and transmitting them to a centralized monitoring system or a cloud platform. Several methods are used, depending on the PLC generation.

Recent PLCs (Siemens S7-1500, Schneider Modicon M580, Beckhoff TwinCAT, Allen-Bradley ControlLogix 5380) implement a built-in OPC-UA server. The IoT gateway or SCADA system connects as an OPC-UA client, subscribes to the relevant nodes, and receives updates in real time or when values change.

Advantages: built-in security (TLS, certificates, authentication), rich semantics (data types, units), cross-vendor interoperability.

Typical configuration: opc.tcp://[IP-PLC]:4840 endpoint, namespace index 2 (Siemens), username/password or client certificate authentication.

Modbus TCP: The Universal Method​

Modbus TCP is supported by virtually all modern PLCs and by many older models via communication modules (CP343 for Siemens S7-300, NOE 771 for Schneider Quantum). The IoT gateway polls the Modbus registers every N seconds and transmits the values to the cloud.

Advantages: fully universal, simple configuration, supported by all SCADA systems. Disadvantage: active polling (no event notifications), no native security (must be protected by a VPN).

IoT Gateway via RS-485 (Modbus RTU)​

For PLCs without Ethernet or with only a serial port (Siemens S7-200, older Modicon models), the Eziwan Gateway connects to the PLC’s RS-485 port via Modbus RTU and transmits the data to the cloud. This process is transparent to the PLC program; no code changes are required.

Remote Access via VPN for Maintenance​

For diagnostics and maintenance, the engineer accesses the PLC from their workstation via a VPN tunnel. They see the PLC as if it were on the local network: TIA Portal for an S7, Unity Pro for a Modicon, Studio 5000 for a ControlLogix. No ports are open on the industrial site side—zero-inbound-port architecture.

Remote Monitoring of an RTU​

Remote monitoring is the primary purpose of the RTU. The protocols used were designed from the outset for long-distance, intermittent WAN connections.

DNP3: The North American Utility Protocol​

DNP3 (Distributed Network Protocol 3) is the dominant remote management protocol in North America for electrical networks and water distribution systems. It natively supports intermittent communication, precise event timestamping, exception reporting (the RTU transmits only when a value changes), and data classes (classes 0, 1, 2, and 3 to prioritize critical alarms).

A DNP3 RTU connects to the master SCADA system via TCP/IP (DNP3 over IP) or via a serial connection over radio. Over 4G, the RTU establishes a DNP3 TCP session with the master server (standard port 20000).

IEC 60870-5-104: The European Standard​

IEC 60870-5-101 (serial version) and IEC 60870-5-104 (TCP/IP version) are the dominant remote management standards in Europe, particularly for power and water distribution networks in France. IEC 104 is the de facto standard for new deployments: the RTU establishes a TCP connection to the master SCADA system on port 2404 and transmits data either spontaneously or in response to general queries.

Modbus RTU over 4G​

For simpler deployments or existing equipment fleets, an RTU transmits its data via Modbus TCP encapsulated in a 4G VPN tunnel. The 4G gateway creates a VPN tunnel to the central SCADA system; the SCADA system queries the RTU as if it were on the local network.

Deployment example: A water district monitors 45 pumping stations via a central SCADA system. Each station has an RTU with 8 analog inputs (4–20 mA: flow, pressure, level), 16 digital inputs (pump status, alarms), 4 digital outputs (pump controls), and IEC 104 communication over 4G LTE with a secondary SIM for failover. The SCADA system polls each RTU every 5 minutes and receives alarms in real time in less than 10 seconds.

PLC/RTU Convergence in the IIoT​

The lines between PLCs and RTUs have been blurring over the past decade, driven by the IIoT:

Modern PLCs include RTU functions. The Modicon M262, Siemens S7-1500T, and CompactLogix 5380 feature native cloud connectors (MQTT, OPC-UA over MQTT), local storage capabilities, and remote management protocols. An S7-1500 with an IoT gateway can serve as a high-performance RTU.

Modern RTUs incorporate PLC logic. Devices such as the Schneider Electric SCADAPack 470i or the Wago 750-8207 offer processing power and an IEC 61131-3 programming environment comparable to compact PLCs, while retaining native remote management protocols (DNP3, IEC 104).

Edge computing is blurring the lines. The trend is toward versatile "edge controllers" capable of advanced local processing (machine learning on sensor data, anomaly detection), multiprotocol communication, and simultaneous cloud monitoring—combining the best of both worlds.

Practical Recommendation. For new installations, the PLC/RTU dichotomy is becoming less and less of a defining factor. The real question is: Do you need local real-time logic (→ PLC), remote management using utility protocols (→ RTU), or both (→ edge controller)? In any case, remote monitoring today relies on an IoT gateway that abstracts the field protocol and exposes a standard cloud API.

Checklist for Choosing Between a PLC and an RTU​

For each new project, answer these 5 questions in order:

1. Is there a complex local control logic?
   (sequencing, PID control, SIL safety)
   OUI → PLC obligatoire
   NON → continuer

2. Is the site remote, unguarded, and has an unreliable power supply?
   OUI → RTU ou edge controller
   NO → PLC if local logic is required; otherwise, a simple IoT gateway

3. Are there any utility protocols (DNP3 or IEC 60870) that must be followed?
   OUI → RTU avec protocol natif
   NO → Modbus + MQTT IoT gateway is sufficient

4. Le budget par site est-il < 1 000 € ?
   YES → Simple RTU or IoT gateway
   NO → PLC with an IoT gateway for cloud-based monitoring

5. Do you need both local logic AND SCADA remote management?
   YES → Edge Controller (hybrid) with IEC 61131-3 and DNP3/IEC 104 protocols
   NO → single-function solution based on answers 1–4

Hybrid PLC + RTU Architecture in a Complex System​

In many real-world industrial facilities, the two pieces of equipment coexist and play complementary roles:

In this configuration:

  • The S7-1500 PLC handles real-time control of the aeration tanks and the fill/drain sequences—logic that an RTU could not execute.
  • The SCADAPack RTU provides remote management to the regional SCADA system (IEC 104) with precise event timestamping and a 30-day buffer—optimized native functions.
  • The Eziwan Gateway aggregates the two data sources, publishes them to the cloud via MQTT, and provides VPN access for maintenance from the office.

Three devices, three distinct roles, a single unified monitoring platform.

FAQ​

What is the main difference between a PLC and an RTU?​

A PLC is an industrial controller designed to execute complex control logic locally, with cycle times ranging from 1 to 100 ms. An RTU is a remote monitoring device designed to collect data from a remote site and transmit it to a monitoring center. The PLC makes decisions, and the RTU reports the information. Both can coexist in the same facility: the PLC controls the machine locally, while the RTU reports the data to the central SCADA system.

Can a PLC and an RTU be monitored remotely on the same platform?​

Yes. An industrial IoT gateway like the Eziwan Gateway natively supports Modbus RTU/TCP (for PLCs), DNP3, and IEC 60870-5-104 (for RTUs), as well as OPC-UA for modern controllers. It centralizes all these sources on a single cloud platform, regardless of the field equipment. This is the recommended approach for mixed PLC/RTU fleets.

Which protocol should be used for remote access to a PLC?​

  • OPC-UA: Recommended for modern PLCs (S7-1500, M580, Beckhoff). Built-in security, rich semantics, interoperability.
  • Modbus TCP: universal, compatible with all modern and legacy PLCs. Easy to configure and secure via VPN.
  • Modbus RTU via RS-485: for PLCs without Ethernet or with a serial port only. Via a local gateway.
  • Direct VPN Access: For maintenance and diagnostics, the engineer accesses the PLC via its native software (TIA Portal, Unity Pro) through a VPN tunnel.

RTU or PLC for Water and Wastewater Management?​

For remote water management systems (remote pumping stations, reservoirs, measurement points), the RTU remains the optimal choice: native DNP3/IEC 104 protocols, low power consumption for solar/battery power supplies, time-stamped data logging, and rugged design for unattended sites. For complex treatment plants (WWTPs with multi-loop control), a PLC manages the local process while an RTU or an IoT gateway handles data transmission to the central SCADA system.

Do you oversee a fleet of PLCs or RTUs and want to centralize remote monitoring? Discover Eziwan's solutions for remote access to PLCs.

Additional Resources​