A BACnet MS/TP server for fan coil units, on an STM32 with an RTOS

The challenge

A fan coil sits in every room, so there are a lot of them, and each one is a small microcontroller board with an RS-485 port, not a computer with an Ethernet jack. In most buildings those units are wired on a BACnet MS/TP trunk back to the building management system.

The developer needed a BACnet MS/TP server that fits the memory of an STM32, runs under the board’s RTOS, and takes its turn on a shared token-passing trunk with every other unit on the floor.

Why the CAS BACnet Stack

• The stack’s embedded build target starts with every object type, service and data link switched off, and turns on only what the device uses, so an MS/TP fan coil carries only MS/TP and the objects it presents.
• The stack is portable C++ behind a flat C interface. The product supplies the serial port and a clock through callbacks, and the stack handles MS/TP framing and token passing on top of them.
• Source is included and royalty-free, which matters on a product that ships one per room.

The solution

The stack runs as a BACnet MS/TP server in an RTOS task on the STM32. The developer’s code passes bytes between the RS-485 UART and the stack’s receive and send callbacks, and calls the stack’s tick on a regular schedule.

Each fan coil appears on the trunk as its own BACnet device, and the room’s values become BACnet objects the building management system reads and writes.

Results

• A central building management system monitors and controls every room’s fan coil over BACnet MS/TP.
• The BACnet server runs on the existing STM32 microcontroller, with no separate network processor.