OPC-UA, MQTT, Modbus: Choosing an Industrial Gateway in 2025

· 8 min read
8 min read
Équipe Eziwan
Équipe Eziwan
Infrastructure IoT

OPC-UA, MQTT, Modbus RTU, Modbus TCP—every industrial integrator faces the same question: Which gateway should I choose to connect my field devices? The answer depends on your devices, your infrastructure, and your goals. Here is the decision matrix.

Why There Is No "Best Protocol"​

The debate over OPC-UA vs. MQTT vs. Modbus is often framed incorrectly. These protocols are not competitors—they are complementary and operate at different levels:

An effective industrial gateway can translate between these levels—for example, reading Modbus RTU data from the field and publishing it to the cloud via MQTT.

Modbus RTU/TCP: The Universal Field Protocol​

Why Modbus Will Remain Essential in 2025​

Modbus has been around since 1979. It is the most widely used protocol in French industry:

  • Variable-frequency drives (Schneider, Danfoss, ABB, Siemens)
  • Industrial sensors (temperature, pressure, flow, level)
  • Analyzers (water quality, energy, gas)
  • Compact PLCs (Schneider TM3, Siemens S7-200, Wago)
  • UPS systems, inverters, industrial chargers

Modbus RTU operates over RS-485 (2-wire bus), supporting up to 32 slaves and a range of 1,200 meters. Modbus TCP is the Ethernet version, with the same registers and the same logic.

Strengths and Limitations​

Modbus RTUModbus TCP
InfrastructureRS-485 (2-wire)Ethernet
Data Rate9600 – 115200 bps100 Mbps
Number of slaves32 per busUnlimited (network)
SecurityNone (native)None (native; TLS possible)
StandardizationYes (Modicon)Yes
OverheadLowModerate

When to Choose Modbus: Always, when your field devices support it. It is the foundation of field data collection.

OPC-UA: The Standard for SCADA Interoperability​

What OPC-UA Offers​

OPC-UA (Unified Architecture) addresses the shortcomings of Modbus:

  • Native security: TLS encryption, certificate-based authentication, access control
  • Semantics: Data is typed, named, and organized into an object hierarchy
  • Bidirectional: read AND write, subscriptions, remote methods
  • Guaranteed interoperability: certified by the OPC Foundation
Modbus : Registre 0x0054 = 184 (valeur brute)

OPC-UA :
  ns=2;i=1234
  DisplayName: "Main Motor Current"
  Value: 18.4 A
  DataType: Float
  Quality: Good
  Timestamp: 2025-08-12T10:23:45.123Z
  EngineeringUnit: "A" (ampere)
  Description: "Effective current measured on phase R"

OPC-UA gives meaning to data—that's what allows a SCADA system to consume data from a PLC without manual configuration.

OPC-UA in Practice​

When OPC-UA is appropriate:

  • SCADA connection (Ignition, Wonderware, WinCC) → modern PLCs
  • Mixed environment: multiple PLC vendors (Siemens, Rockwell, B&R)
  • High security requirements (audit, NIS2 compliance)
  • Data exchange between different SCADA systems

When OPC-UA is overkill:

  • Simple sites with 1 to 5 PLCs from the same vendor
  • Limited budget (OPC-UA requires more configuration resources)
  • Older equipment that does not support it (Modbus-only)

MQTT: The IoT Cloud Protocol​

The publish/subscribe model​

MQTT is designed for unreliable networks and limited resources:

MQTT Architecture:

vs. Modbus (request/response):

MQTT "pushes" data to the cloud. The field doesn't need to be accessible from the outside—which works perfectly with NAT on LTE networks.

MQTT Sparkplug B: The Industry Standard​

Sparkplug B is an industry-defined MQTT payload standard:

{
  "timestamp": 1691843400000,
  "metrics": [
    {
      "name": "Inputs/Pressure",
      "alias": 1,
      "timestamp": 1691843400000,
      "datatype": "Float",
      "value": 3.45
    },
    {
      "name": "Outputs/PumpEnable",
      "alias": 2,
      "datatype": "Boolean",
      "value": true
    }
  ],
  "seq": 12,
  "uuid": "pompe-station-001"
}

Sparkplug B is supported by Ignition, HiveMQ, EMQX, and most modern SCADA platforms.

Industrial Gateway Selection Chart​

Selection Criteria​

SituationRecommended ProtocolGateway
Existing Modbus RTU devices → cloudModbus RTU → MQTTEziwan Gateway
Modbus TCP devices → Ignition SCADAModbus TCP → OPC-UAEziwan + Edge App
Mixed network (Modbus + Profibus + BACnet)Multi-protocol → MQTTDedicated gateway
LTE network (no static IP)All → MQTT (outbound)Eziwan Gateway
Remote sites + low bandwidthModbus + MQTT QoSEziwan Gateway
NIS2 security requirementsSecure OPC-UA or OpenVPNEziwan + VPN

2025 Industrial Gateway Checklist​

Before you buy, check the following:

Hardware:

  • DIN rail (standard rack mount)
  • Temperature range: -40°C to +75°C
  • Wide-range power supply (9–48V DC)
  • Hardware watchdog (automatic reboot in case of a freeze)
  • Dual SIM with automatic failover

Field Protocols:

  • Native Modbus RTU (RS-485)
  • Native Modbus TCP
  • 4-20 mA / 0-10 V analog inputs
  • Digital I/O (digital inputs/outputs)

Cloud Protocols:

  • MQTT with TLS
  • MQTT Sparkplug B (if SCADA is already in place)
  • REST API (for custom integrations)
  • OpenVPN/IPsec VPN

Administration:

  • Zero-Touch Provisioning
  • OTA firmware updates
  • Fleet monitoring dashboard
  • Remotely accessible logs and diagnostics

Use Case: Modbus → MQTT → Grafana Integration​

A simple and widely used architecture:

This architecture can be deployed in less than a day and scales to handle any volume of data.

See the Eziwan protocols documentation → · Get started integrating your devices →

FAQ​

Can OPC-UA and MQTT be used simultaneously on the same gateway? Yes. These protocols operate at different levels and are complementary. A gateway can act as an OPC-UA client for an S7-1500 PLC and simultaneously publish MQTT data to the cloud. This is a common architecture: OPC-UA for local data collection (data quality, precise timestamps), and MQTT for cloud transmission (lightweight, firewall traversal, TLS).

Is MQTT Sparkplug B required for industrial use? No — Sparkplug B is an MQTT extension that standardizes topics, payloads, and device management. It is required only if you are connecting to a SCADA system or a platform that requires it (Ignition Cirrus Link, Canary Labs, certain AWS IoT platforms). For custom integrations (InfluxDB, custom APIs), standard MQTT is sufficient.

What is the difference between OPC-UA and OPC-DA (classic)? OPC-DA (Data Access) is the Windows-only version of OPC, which requires DCOM and runs only on Windows. OPC-UA (Unified Architecture) is cross-platform, secure (TLS, authentication), and suited for the cloud and modern architectures. All recent SCADA systems support OPC-UA; OPC-DA is considered a legacy technology.

Can the Eziwan Gateway act as an OPC-UA client for an S7-1500? Yes. The Eziwan Gateway includes a native OPC-UA client. It connects to the S7-1500’s OPC-UA server, authenticates using a certificate (SignAndEncrypt mode recommended), and publishes the selected variables via MQTT. No additional software is required on the PLC side.

Why choose MQTT over a REST API for IoT data collection? MQTT is optimized for IoT constraints: low overhead (2-byte header), asynchronous publishing (no HTTP round-trip), guaranteed QoS (at-least-once, exactly-once), and automatic reconnection. REST is easier to debug and test, but less efficient for hundreds of data points published every second. Use REST for one-off integrations and configuration APIs, and MQTT for continuous field data streams.

Further Reading​

Conclusion​

By 2025, the ideal industrial gateway must support Modbus RTU/TCP for the field, MQTT for the cloud, and optionally offer OPC-UA for demanding SCADA environments. The priority remains reliable connectivity (Dual SIM, watchdog) and ease of large-scale deployment (ZTP, fleet management).

Don't look for the gateway that supports the most protocols—look for the one that perfectly handles the three that cover 95% of your needs.

Compare Eziwan models → · Try it free for 30 days →

Additional Resources​