ROS2 Modbus

This package is an ultimate C++ implementation of Modbus for ROS2.

Most server use cases require Modbus RTU or Modbus TCP instead. However sometimes the servers use this library to simulate the devices or to expose a non-Modbus device using a Modbus interface.

ROS2 clients use this library to communicate with other ROS2 nodes in a way that is abstracted away from the particular method (RTU or TCP).

Architecture

The below diagram explains how the modbus library fits into the overall ROS2 architecture both on the client and server sides. See the following sections for more details.

flowchart TB
    cli_serial["# Modbus debugging\n$ ros2 topic echo /modbus/example_bus"] .-> topic_modbus[/ROS2 interfaces:\n/modbus/example_bus/.../]
    subgraph modbus["Library: remote_modbus"]
      topic_modbus
    end
    users["Client's\nconsumers"]
    subgraph your_exe["Client process"]
      config["Register layout\nin YAML"]
      subgraph modbus_client_svcs["Services: .../modbus/..."]
        generated_services["Register-specific\nservices generated"]
        generated_services --> topic_modbus
      end
      code_dds["Client code\nthat consumes\ntype definitions"]
      code_api["Client code\nthat consumes\nnative API"]
      code_dds --> topic_modbus
      subgraph modbus_client_lib["Library: remote_modbus"]
        generate["Generate\nregister\nmappings"]
        modbus_topics["ROS2 interface declarations"]
        modbus_client["Modbus client API"]
        modbus_topics -.- modbus_client
      end
      config ---> generate
      modbus_topics ---> code_dds
      code_api ---> modbus_client
    end
    users --> generated_services
    generate --> generated_services
    modbus_client --> topic_modbus
    subgraph modbus_exe["Server process"]
      modbus_server["Modbus server API"]
      subgraph modbus
        topic_modbus --> modbus_server
      end
      modbus_server <--> server["Server code"]
    end
    server .-> device{{"External devices"}}
    style device stroke-dasharray: 3 3

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Modbus clients

The client may consume ROS type definitions and send the messages directly to the Modbus service providers using ROS communications.

Alternatively, the client may consume a native API which can send simple messages.

Modbus servers

The servers use the modbus library to instantiate ROS2 topics for communication with clients.

This interface is used by Modbus RTU and Modbus TCP libraries. But it can be used to implement alternative access to hardware interfaces that the clients are expecting to access using Modbus.

ROS2 interface

Topics:

• /modbus/<bus-name>/status/
  • leafs_seen
  • last_leaf
  • last_seen
• /modbus/<bus-name>/<leaf-id>/
  • last_seen
  • last_function_code
  • last_error_code

Services:

• /modbus/<bus-name>/<leaf-id>/
  • holding_register_read
  • holding_register_write (not yet)
  • holding_register_write_multiple (not yet)

Native interface

The native interface duplicates the ROS2 service interfaces. The same request and response data types are reused.

Known Limitations

• As of now, the only supported object type is 'Holding register'