BACnet vs Modbus — which protocol should I choose for building automation?

BACnet (ASHRAE Standard 135) is purpose-built for building automation systems and natively models HVAC objects like schedules, trends, alarms, and analog/binary/multistate values, making it the preferred choice for system-level integration between controllers, supervisory devices, and operator workstations. Modbus RTU/TCP is a simpler, lower-cost protocol that works well for field-level device integration (sensors, VFDs, power meters) where you only need to read/write registers, but it lacks the object model, alarm management, and scheduling capabilities of BACnet. ASHRAE 135-2020 added BACnet/SC (Secure Connect) which provides TLS-encrypted, certificate-based security that Modbus cannot match without IT-managed VPNs. Most modern BMS designs use BACnet at the network and management layers with Modbus at the field device integration layer through protocol gateways.