Building an Industrial HMI and Edge Controller on STM32 with FreeRTOS and Modbus TCP

The article discusses the development of an industrial HMI and edge controller on an STM32 microcontroller using FreeRTOS and Modbus TCP. The project, called SLX FlowControl, aims to demonstrate the behavior of a buffer tank system by simulating a real-world process. The system incorporates a FreeRTOS control task, an LVGL touch HMI, a Modbus TCP/IP server, and a Factory I/O Fill Tank digital twin.

The main objective is to allow behavior testing before connecting a physical machine, ensuring that commands, field demand, automatic control, conflicts, alarms, and recovery are treated as explicit machine states rather than special cases. The architecture consists of a periodic FreeRTOS control task running every 100 ms, with Factory I/O acting as a Modbus TCP client and the STM32 acting as the server.

The Modbus map separates measurements, field commands, valve outputs, and diagnostics. The LVGL dashboard provides a comprehensive view of the system's state, including alarms, process views, controls, session recipes, diagnostics, setup, and more. The demonstrator focuses on testing a complete operational sequence, such as the controller starting in STOPPED mode, handling field requests for fill or discharge, and returning to the AUTO mode when both commands are released.

The HMI is designed to explain the controller's behavior rather than just displaying alarms, with alarm pages providing severity, context, cause, required action, and a guided reset path. The article also highlights that SLX FlowControl is not a substitute for risk analysis, hardwired safety functions, physical-process validation, authenticated access, OT firewalls, or persistent production logging.

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