BACnet, which stands for Building Automation and Control Networks, is a data communication protocol designed specifically for building automation and control systems. Developed under the auspices of the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), BACnet has become an international standard (ISO 16484-5) since its introduction in 1995. It provides a comprehensive set of standardized object types and services that allow for seamless integration of various building systems, including HVAC, lighting, access control, and fire detection.

Compared to the simplicity of the Modbus protocol, BACnet is much more complex, providing features at a high level of functionality, such as the auto-discovery of devices, and its auto-discovery tool can significantly speed up the time needed to configure and set up large systems.

Physical layer

One of BACnet's key strengths is its flexibility in network architecture. It supports multiple network types, including BACnet/IP (for Ethernet and Wi-Fi networks), BACnet MS/TP (Master-Slave/Token-Passing for RS-485 networks), BACnet ARCNET, and BACnet LonTalk. This versatility allows BACnet to be implemented across a wide range of building sizes and types, from small commercial buildings to large industrial complexes. The protocol also includes provisions for secure communication, addressing cybersecurity concerns in modern building management systems. MS/TP and BACnet/IP have traditionally been the most popular, with BACnet/IP slowly gaining share as LAN infrastructure becomes more commonplace.

Discovery

Because BACnet provides auto-discovery, configuring the software for a system is usually an easy task. The primary headache caused by BACnet systems is typically ensuring compatibility between devices. Beyond ensuring that the same physical layer is being used (eg. RS-485, RS-232, BACnet/IP), the user must also confirm compatibility of the BACnet object types. Some of the most common object types are described below:

Object types

BACnet uses an object-oriented approach to represent the functions of building automation devices. These objects include both standardized types (such as Analog Input, Binary Output, and Schedule) and vendor-specific types, allowing for both interoperability and innovation. Each object has a set of properties that define its characteristics and behavior. BACnet devices communicate using a client-server model, where devices can request or provide information as needed. This model supports both polling (where a client regularly requests data) and Change of Value (COV) reporting (where a server sends updates only when significant changes occur), allowing for efficient network utilization.

Analog Input

Sensor values can be read using the analog input object. BACnet analog input objects contain the engineering unit used, as well as allowing floating point values, so there is usually no configuration needed. Analog input objects are very widely supported, and can typically be used even if no other higher-level objects are supported by a device.

Trend Log

The trend log object was added to the BACnet standard later on, so it is not as widely supported. It provides a mechanism to retrieve "logged" data from a sensor, as opposed to only being able to read the most recent sample using the analog input object.

Analog output

Control values can be set using the analog output object. Similar to the analog input object, it provides a lower-level interface for direct control of device outputs.

Hardware and software

Paragon's N-series controllers and BACnetDiscover software allow interfacing with any existing BACnet infrastructure that may exist at a site. Two of the most common applications which require BACnet hardware integration:

• Connecting to legacy BACnet hardware devices - An N-series controller can be installed to communicate with legacy BACnet hardware, and provide gateway translation to any number of other output types. Wireless BACnet can also be easily provided with this setup.
• Converting non-BACnet sensor and controls to a BAS - Large commercial buildings may have an existing BACnet based building automation system (BAS). It may be desirable to install non-BACnet sensors and controls in the building, and a N-series controller can be installed to communicate between the systems.