What Is BACnet?

BACnet (Building Automation and Control Networks) is a communication protocol developed by ASHRAE and published as ASHRAE Standard 135. Released in 1995 and updated continuously since, it has become the dominant open protocol for building automation — used in HVAC controls, lighting systems, access control, fire alarm integration, energy management, and virtually every subsystem in an intelligent building.

Before BACnet, building automation systems used proprietary protocols: a Honeywell DDC system couldn't communicate with a Siemens chiller controller or a Johnson Controls AHU. BACnet created a common language that allows equipment from different manufacturers to share data and work together in a unified BMS.

The BACnet Object Model

BACnet represents every controllable element — a temperature sensor, a VAV box damper, a chiller setpoint — as a BACnet Object. Objects have standard types: Analog Input (AI), Analog Output (AO), Analog Value (AV), Binary Input (BI), Binary Output (BO), Schedule, Calendar, Trend Log, and many more.

Each object has Properties — data elements that describe it. The Present_Value property holds the current measured or commanded value. The Object_Identifier uniquely identifies the object (e.g., Analog Input 5 on Device 1001). The Object_Name is a human-readable label ("Supply Air Temperature").

This object model is what makes BACnet integration powerful: a BMS operator can browse any compliant device and see a consistent data structure regardless of manufacturer.

BACnet Network Types

BACnet/IP — BACnet encapsulated in UDP/IP packets. Runs over standard Ethernet infrastructure. This is the predominant BACnet physical layer for modern installations — it leverages existing IP networks and allows communication across routers and VLANs. A BACnet Broadcast Management Device (BBMD) handles inter-subnet broadcasting.

BACnet MS/TP (Master-Slave/Token-Passing) — a serial bus protocol running over RS-485 at 9,600–76,800 baud. MS/TP is used for connecting low-cost field devices (VAV controllers, zone sensors, small AHU controllers) on a trunk line. Devices pass a "token" that grants permission to communicate; only the device holding the token can transmit. MS/TP devices (field buses) connect to IP networks through BACnet router devices.

BACnet/Ethernet and BACnet/ARCNET are older physical layers still found in legacy systems.

BACnet Services

BACnet defines standardized services for device-to-device communication:

  • ReadProperty / ReadPropertyMultiple — read one or more properties from a remote device
  • WriteProperty — write a value to a remote device (e.g., send a new setpoint)
  • SubscribeCOV (Change of Value) — ask a device to proactively send updates whenever a value changes by more than a specified increment; more efficient than constant polling
  • Who-Is / I-Am — device discovery; a controller broadcasts Who-Is and all devices on the network respond with I-Am (their device instance number)
  • Alarm and Event Notification — proactive alarm reporting from field devices to a supervisor

BACnet Profiles (BIBBs)

BACnet Implementation Building Blocks (BIBBs) define what services a device must support for a given interoperability role. A device that supports the DS-RP-B BIBB can respond to ReadProperty requests; one that supports DM-DDB-B can be discovered by Who-Is. When specifying BACnet equipment, list the required BIBBs to ensure true interoperability rather than assuming "BACnet-compatible" labels are sufficient.

OPC-UA: BACnet's Complement

OPC-UA (Open Platform Communications Unified Architecture) is increasingly used alongside BACnet for connecting building systems to higher-level enterprise platforms, cloud analytics, and digital twin systems. Many modern building automation controllers now support both BACnet for peer-to-peer control communication and OPC-UA for northbound integration with IT systems and cloud platforms.