What Is an HMI?

An HMI is the local operator interface at the machine or process level — showing the status of one machine and allowing local control. SCADA (Supervisory Control and Data Acquisition) is a higher-level system that aggregates data from multiple HMIs, PLCs, and field devices across an entire plant or facility. In the ISA-95 hierarchy, embedded HMIs sit at Level 2 (process control); SCADA spans Levels 2–3 (manufacturing operations). A production plant typically has dozens of embedded HMI panels, each feeding data upward to a single SCADA system in the control room.

A panel HMI is a standalone touchscreen or keypad-operated display terminal mounted on or near a machine for local operator control. Unlike an embedded HMI (which is integrated into the machine itself), a panel HMI is a separate, purpose-built unit connected to the machine controller via industrial protocols such as Modbus, EtherNet/IP, or PROFIBUS. Panel HMIs are manufactured by companies like Siemens, Schneider Electric, Weintek, and Delta, and are sold as general-purpose industrial HMI hardware.

Not always. Simple embedded HMIs with a single display task and no concurrent real-time requirements can run on bare-metal hardware — the HMI loop is the main application loop. An RTOS is beneficial when the HMI must coexist with time-critical background tasks: communications (Modbus polling, CAN bus), sensor sampling, motor control, or safety monitoring. In RTOS configurations, the HMI typically runs as a lower-priority task that yields CPU time to real-time tasks. Sparklet runs on bare-metal, FreeRTOS, Azure RTOS, Zephyr, QNX, and embedded Linux.

Industrial HMIs are designed for 7 to 15+ years of continuous operation, and many are still running after 20 years. This long service life creates specific requirements for the HMI software and framework: no licensing model that creates a retirement obligation at a fixed date, stable APIs that do not break with updates, and robust operation without memory leaks or undefined behaviour. Royalty-free licensing models are particularly important for long-life industrial HMIs — a per-unit royalty or subscription model can force unplanned replacement when licences expire.

C is the dominant language for embedded HMI programming, used directly against a C-language embedded GUI framework. C++ is used by some frameworks (Qt for MCUs, TouchGFX) and is common on higher-resource embedded platforms. For safety-critical HMIs, C is strongly preferred due to MISRA C compliance requirements — MISRA C cannot be achieved with C++ code. Sparklet is written in pure C, MISRA C compliant throughout, and produces C code from its Flint UI Designer — no C++ in the safety stack.