Legacy Modbus Devices Connected to Azure IoT


In the world of industrial automation and IoT, Modbus has been a reliable protocol for connecting devices in various industries. As we continue on the path of digital transformation, integrating legacy systems like Modbus with modern cloud services becomes crucial for unlocking new possibilities in data analysis, insights, and operational efficiency. This article will explore the significance of Modbus, its compatibility with Azure IoT services, and how this integration bridges the gap between the past and the future of industrial connectivity.

What is Modbus?

Modbus is a widely used communication protocol that facilitates data exchange between industrial devices, sensors, and control systems. It has been the backbone of automation systems for decades, known for its simplicity and versatility. Modbus can operate over serial or Ethernet connections and supports various modes, including Modbus RTU (binary) and Modbus ASCII (text-based). It finds applications in manufacturing, energy, utilities, and transportation industries.

Integration with Azure IoT Services

By bringing Modbus into the realm of Azure IoT services, we open doors to enhanced data processing, analytics, and management. Here’s how Modbus and Azure IoT services can work together:

  • IoT Edge Gateway: With Azure IoT Edge, an edge computing solution, Modbus devices can communicate with the cloud without major changes to the existing infrastructure. This allows for seamless integration.
  • Azure IoT Hub: Azure IoT Hub acts as the central hub for bi-directional communication between IoT applications and devices. By developing custom device connectors or using pre-built solutions, Modbus devices can connect to Azure IoT Hub for data ingestion and remote device management.
  • Azure IoT Central: Azure IoT Central, a SaaS solution, simplifies the creation of IoT applications. It can be configured to connect with Modbus devices, enabling quick deployment of custom IoT solutions without extensive coding.
  • Azure Functions: Leveraging Azure Functions, developers can build serverless applications that respond to Modbus device data events. This enables real-time data processing, transformation, and triggering of specific actions based on Modbus data.

Modbus Architecture

The Modbus architecture consists of several components that enable communication between master and slave devices. These components work together to facilitate data exchange, control, and synchronization in industrial automation and control systems. Here are the key components:

1. Master Device

The master device initiates communication with one or more slave devices. It controls the flow of communication by sending queries or commands to the slaves. Examples of master devices include programmable logic controllers (PLCs) and industrial computers.

2. Slave Device

The slave devices respond to the master’s queries or commands. They provide data or execute commands as requested by the master. Slave devices can include sensors, actuators, drives, and other control devices.

3. Data Registers

Data registers store the actual data exchanged between master and slave devices. There are different types of data registers in Modbus:

  • Coils: Coils represent single bits and are used for control purposes, such as device activation or deactivation.
  • Discrete Inputs: Discrete inputs represent the status of devices or sensors and are read-only.
  • Input Registers: Input registers store analog data values, such as temperature readings or sensor outputs. They are read-only.
  • Holding Registers: Holding registers store analog data values that can be read from or written to by the master. They are used for configuration, control settings, and data exchange.

4. Modbus Messages

Modbus communication is based on messages containing information about the communication request or response. There are different types of Modbus messages:

  • Request Messages: Sent by the master to request data or initiate an action from the slave. These messages include function codes that specify the action to be taken (e.g., read coils, write holding registers).
  • Response Messages: Sent by the slave in response to a request from the master. They contain the requested data or an acknowledgment of the action taken.

5. Function Codes

Function codes are numerical values indicating the type of action to be performed in a Modbus message. Each function code corresponds to a specific Modbus command, such as reading or writing data. Examples include reading coils (Function Code 01), writing single coil (Function Code 05), reading input registers (Function Code 04), and writing multiple holding registers (Function Code 16).

6. Physical Layer

The Modbus architecture operates over different physical layers, depending on the communication mode:

  • Serial Communication (Modbus RTU and Modbus ASCII): This layer includes physical interfaces like RS-232, RS-422, or RS-485, along with the corresponding electrical specifications for transmitting binary or ASCII data.
  • Ethernet Communication (Modbus TCP/IP): Modbus TCP/IP operates over standard Ethernet networks, using TCP/IP packets to encapsulate Modbus messages for communication over local area networks (LANs) and the internet.

7. Network

The network encompasses the physical connections, cables, and devices that facilitate communication between master and slave devices. It can be a serial network or an Ethernet network, depending on the communication mode.

8. Addressing

Each Modbus device has a unique address for identification within the network. The addressing scheme differs for coils, discrete inputs, input registers, and holding registers.

Modbus Pin Configuration

Modbus pin configure

The pin configuration for Modbus communication can vary depending on the communication mode (Modbus RTU or Modbus ASCII) and the type of physical interface (RS-232, RS-422, or RS-485). Here, we’ll provide a general overview of the pin configurations for Modbus RTU over RS-485, commonly used in industrial settings.

Modbus RTU over RS-485 Pin Configuration

RS-485 is a differential signaling standard commonly used in industrial applications for long-distance communication. Modbus RTU is a binary protocol that utilizes RS-485 for communication. Here’s a typical pin configuration for connecting Modbus RTU devices over RS-485:

  • A (Positive or Non-Inverted Data Line): Carries the positive (non-inverted) signal of the differential pair. Also known as “non-inverting” or “+”.
  • B (Negative or Inverted Data Line): Carries the negative (inverted) signal of the differential pair. Also known as “inverting” or “-“.
  • Ground (GND): Provides a common reference voltage for the communication lines, connected to the system’s ground reference.
  • Shield (Optional): In noisy environments, a shielded cable can reduce electromagnetic interference. The shield is connected to the metal housing or chassis of the devices.

Connecting Modbus RTU Devices

To connect Modbus RTU devices using RS-485, follow these steps:

1. Connect the “A” pin of one device to the “A” pin of the other device.

2. Connect the “B” pin of one device to the “B” pin of the other device.

3. Connect the Ground (“GND”) pin of both devices to the common ground of the system.

4. Optionally, connect the shield wires of the cables to the metal housing or chassis of the devices to reduce interference.


For reliable communication, especially over longer distances, it’s common to add termination resistors to the communication lines. Termination resistors (120 ohms) are typically connected across the “A” and “B” lines at both ends of the communication bus. This helps reduce signal reflections and ensures proper signal integrity.

Benefits of Modbus and Azure IoT Integration

  • Data-Driven Insights: Integration of Modbus with Azure IoT services allows businesses to leverage data analytics and gain actionable insights from real-time device data. This enables predictive maintenance, performance optimization, and data-driven decision-making.
  • Scalability: Azure’s cloud infrastructure ensures seamless scaling as the number of Modbus devices grows, accommodating the increasing data load.
  • Real-Time Monitoring: Integration enables real-time monitoring of Modbus devices, triggering alerts for anomalies and allowing immediate actions to prevent downtime and improve operational efficiency.
  • Global Connectivity: Azure IoT services enable global device connectivity, making it possible to monitor and manage Modbus devices across different geographical locations.


The integration of Modbus with Azure IoT services represents a crucial milestone in industrial connectivity. By combining the reliable Modbus protocol with the scalability, analytics, and agility of Azure IoT services, organizations can drive digital transformation, streamline operations, and uncover valuable insights from existing infrastructure. This integration bridges the gap between traditional industrial systems and the transformative potential offered by the cloud, enabling businesses to transition seamlessly into an era of data-driven decision-making and IoT innovation. As the world continues to evolve, the compatibility of Modbus with Azure IoT services exemplifies the harmonious coexistence of legacy and modern technology, shaping a future where efficiency and innovation thrive.

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