Even though you might not be in the Building Automation industry, it’s important to have a working knowledge of all the major technologies that shape the buildings and factories where we work. BACnet is now the technology of the Building Automation industry. Now as we continue to integrate the enterprise, the facility, and the factory, it’s important to have a good handle on all the relevant technologies.
Prior to BACnet, Modbus RTU and Modbus TCP were the primary technologies for many years. These technologies are fading away as newer, object-oriented protocols like BACnet that are more easily able to share information take their place.
You probably already know that BACnet is used in heating and cooling, but BACnet is also used in lighting, access control, video surveillance, water chilling, and security and safety systems. BACnet uses an object-oriented data organization and services that are very similar to EtherNet/IP and OPC UA. There are standard services to access data in those objects. BACnet standardizes access to all the different disparate devices that are needed to give the user an integrated view of the system (usually an entire building).
Here are the top 10 things you need to know about BACnet:
- BACnet (Building Automation and Control Network) is a completely non-proprietary, open specification that defines both how data is organized and represented on the network and the services users can use to move data from one BACnet device to some other BACnet device. Services include messages to identify data and network nodes such as Who-Is, I-Am, Who-Has, and I-Have.
- Like the Control and Information Protocol (CIP) used in many industrial applications, BACnet defines a set of standard objects to represent data on a network. And just like the EtherNet/IP implementation of CIP, these objects have both required and optional data. There are many objects defined by the specification, but the primary ones are analog input, analog output, binary input, and binary output.
- BACnet devices represent data and information in terms of one or more of its object collections. An object can embody information regarding some physical input or output, or it can stand for a logical point that performs some function, such as a setpoint.
- A BACnet system identifies objects by a mnemonic (such as AI-1 for analog input). Objects are more than a data point. Objects consist of a number of properties (called attributes in CIP), the most important of which is the present value (the data point) of the object. Objects are monitored and controlled through their properties.
- There are 123 object properties in the BACnet specification. The majority of these are meta-data type properties such as high and low limits, status flags, data types, and so on. There are three required properties, object-identifier, object-name, and object-type, that are required to be present in every object. The type of device and type of an object determines which properties are present. Like factory floor technologies, some of these properties are required and some are optional. Some properties are limited to be read-only and some as read-write.
- Services are mechanisms that a building automation system uses to access a property or request an action from a BACnet object. Services are how one BACnet device gets information from another device, commands a device to perform certain actions (through its objects and properties, of course), or communicates events to other objects. There is one required service: the read-property service must be supported by all devices. The specification names 32 standard services including services for file access, alarming and eventing, object access, and network management.
- Users and system developers don’t need to understand how services are executed by a BACnet device; those operations are transparent and automatic. But to design a system or configure a Building Automation System (BAS), you will need to know what objects and services are supported by which devices. This information is found in the device’s protocol implementation conformance statement (PICS).
- Not all devices need to have the same level of functionality. BACnet defines conformance classes that categorize the capabilities and functionality of devices. A device conformance class contains a minimum set of required features (in the form of objects and services). Other features are optional. This information is required to be public in the PICS, the list of features that the device supports. The PICS lists what objects are present in the device and whether the device initiates a service request (asks or commands) or executes the request (responds or acts). The PICS also provides you with the conformance class of the device. By comparing a device’s PICS with project requirements or with another vendor’s PICS, you can determine how well a BACnet product “fits” a given application.
- BACnet is an unconnected, peer to peer network. A device can send service requests to any other device at any time. Unlike connected protocols where devices have an ongoing communication connection, communication in BACnet is unscheduled without any time-critical operations.
- BACnet is a certifiable standard, but certification is not required by the BACnet trade organization that supervises the BACnet specification. There is a BACnet Testing Laboratory (BTL) that provides services to validate the operation of devices against the specification. Prices vary with the device complexity, but it costs several thousand dollars to certify a BACnet device.