A fieldbus is the information highway that connects the components in a BMS system to allow monitoring and controlling of air-conditioning systems, heating systems, lighting systems, ventilation, wastewater systems etc.

A fieldbus facilitates data communication and logging of data from equipment in a BMS system such as air handling units, fans, pumps, lighting components, sensors, weather stations etc.
One language
Devices from different manufacturers are not automatically able to communicate with each other, as they do not speak the same “language”.
The technology used to connect the equipment in communication networks is in broad terms called fieldbus.
A fieldbus specifies the rules and data formats that all the equipment must follow. In simple words, it specifies that all the equipment must “speak the same language” in order to communicate.
Different fieldbusses
Various fieldbusses exist, as each fieldbus has its own strengths in certain areas, and each application/building has its own requirements and priorities.
For building automation, two of the most common fieldbusses today are LONworks and BACnet, both used in great scale world-wide.
All equipment in such a system should be able to communicate data, such as running hours, energy consumption, status information (alarms/warnings), temperature, pressure or flow measurements etc.
A Building Management System makes central monitoring and control of all electrical equipment in a building possible. A BMS increases the overall comfort level in a building, reduces energy consumption and also saves man hours, because the need of visual inspection of the equipment is reduced.
Building automation is normally seen in hospitals, airports, universities, hotels, office buildings, stadiums and sports arenas etc.