Communication Overview#

The WMX ROS2 application uses three communication layers:

  • ROS2 DDS (CycloneDDS) – All inter-node communication via topics, services, and actions over the ROS2 middleware.

  • WMX API – C++ shared libraries at /opt/lmx/ (coremotionapi, advancedmotionapi, ioapi, ecapi, wmx3api) that bridge ROS2 nodes to the motion engine via shared memory.

  • EtherCAT – Real-time fieldbus managed by the WMX engine for deterministic, low-latency servo drive communication.

Note

The system does not use TCP/IP to communicate with the robot controller. The WMX motion engine runs on the same machine as the ROS2 nodes and communicates with servo drives over EtherCAT.

The diagram below shows how the three layers stack and which libraries and buses connect them:

        %%{init: {"theme": "base", "themeVariables": {"primaryColor": "#1a73e8", "primaryTextColor": "#fff", "primaryBorderColor": "#1558b0", "lineColor": "#555"}}}%%
flowchart TB
    subgraph L1["Layer 1 — ROS2 DDS  (CycloneDDS middleware)"]
        direction LR
        N1["manipulator_state"]
        N2["follow_joint_trajectory_server"]
        N3["wmx_engine_node"]
        N4["wmx_core_motion_node"]
        N5["wmx_io_node"]
        N6["wmx_ethercat_node"]
    end

    subgraph L2["Layer 2 — WMX API  (C++ shared libraries at /opt/lmx/)"]
        direction LR
        A1["libcoremotionapi<br/>Axis position & velocity"]
        A2["libadvancedmotionapi<br/>Cubic spline execution"]
        A3["libioapi<br/>Digital I/O"]
        A4["libecapi<br/>EtherCAT diagnostics"]
        A5["libwmx3api<br/>Device lifecycle"]
    end

    subgraph L3["Layer 3 — EtherCAT  (real-time fieldbus, dedicated NIC)"]
        direction LR
        E1["WMX3 Motion Engine"]
        E2["EtherCAT Master"]
        E3["Servo Drives  J1 → J2 → ... → J6  (daisy-chained)"]
        E4["I/O Module  (gripper digital output)"]
    end

    L1 -->|"C++ library calls  (same machine, shared memory)"| L2
    L2 -->|"WMX3 real-time process"| L3
    E1 --> E2 --> E3 & E4

WMX ROS2 — three-layer communication architecture#

For the full list of ROS2 interfaces, see: