Understanding BACnet vs Modbus: Choosing the Right Protocol for Your Building

If you are involved in building automation, you have almost certainly encountered the question: should we use BACnet or Modbus? Both protocols have been in use for decades and both remain widely deployed across Australian commercial buildings, but they serve different purposes and have distinct strengths and limitations. Understanding these differences is essential for making informed decisions about your building's control architecture. Modbus, originally developed by Modicon in 1979, is a simple, serial communication protocol that uses a master-slave architecture. It comes in two primary variants: Modbus RTU (serial RS-485) and Modbus TCP (Ethernet). Modbus is exceptionally reliable, easy to implement and universally supported by virtually every piece of building equipment from energy meters and variable speed drives to room sensors and valve actuators. However, Modbus is a register-based protocol. It transmits raw numerical values without context. There is no standardised object model, no built-in discovery mechanism and no inherent way to describe what a data point represents. This means that integration requires manual point mapping and detailed register documentation from the equipment manufacturer. BACnet (Building Automation and Control Networks), developed by ASHRAE and published as ISO 16484-5, was designed specifically for the building automation industry. Unlike Modbus, BACnet uses a rich object model with standardised data types for common building concepts, including analogue inputs, binary outputs, schedules, trends, alarms and more. BACnet devices can self-describe their capabilities, and the protocol includes built-in mechanisms for COV (Change of Value) subscriptions, alarm and event notifications, scheduling and trend log retrieval. For new building automation projects, BACnet IP is generally the preferred choice because of its richer functionality and interoperability. Modbus remains the go-to protocol for energy metering, VSD integration and interfacing with industrial equipment where its simplicity and universal support are advantages. In practice, most modern buildings use both, BACnet for the BMS backbone and Modbus for specialist subsystems, with the Niagara Framework or similar platforms bridging the two.