Building automation protocols: BACnet and KNX

BACnet

BACnet (ISO 16484-5) is the dominant protocol for HVAC and building management systems.

• Devices expose standard objects: Analog Input/Output/Value, Binary Input/Output/Value, Multi-state, Schedule, Trend Log, Notification Class… each with properties (Present_Value, Status_Flags, Units, Priority_Array).
• Writes to outputs use a priority array (16 levels): a manual override at priority 8 wins over automatic control at priority 16; relinquishing the higher priority returns control.
• Transports: BACnet/IP on UDP 47808 (0xBAC0) with BBMDs to cross IP subnets, and MS/TP on RS-485 for field controllers.
• Services: ReadProperty, WriteProperty, COV subscriptions, alarms/events, Who-Is/I-Am discovery.

KNX

KNX (ISO/IEC 14543-3) is used for room-level electrical installations: lighting, blinds, room heating.

• Twisted-pair bus TP1 (9600 bit/s, bus-powered) plus IP and RF media.
• Devices communicate via group addresses written as main/middle/sub (e.g. 1/2/3), each carrying a datapoint type (switch, dimming, temperature).
• Projects are engineered with the ETS software.

Integration with industrial systems

• Gateways map BACnet or KNX points to Modbus, OPC UA or directly into a SCADA.
• Agree point lists early: object type and instance (BACnet) or group address and datapoint type (KNX), units, and which side has priority for commands.
• Keep building networks separated from control networks; BACnet broadcasts and Who-Is storms can load shared segments.