Briefly About BACnet
BACnet is not a thing you can buy. BACnet is literally a book that defines methods which a manufacturer can use, if it chooses to, to make systems which can be interoperable with other BACnet systems in various ways. Owners and specifiers can also use BACnet as a tool for specification of interoperable systems.
BACnet does not replace the need for specifying what one wants or needs. It simply provides some standardized tools to help enable the creation of systems that can interoperate.
BACnet is not specific to HVAC, but is intended to apply to all types of building systems: fire, security, lighting, HVAC, elevators and so forth. The existence of BACnet does not mean that all of these options are automatically available today. But it does provide a context which could enable the creation of those systems if the marketplace indicates a demand for interoperable products.
BACnet achieves these goals by defining a generalized model of how automation systems work, and how to describe the information that they contain, and how to describe standard methods that one device may use to ask another device to perform some desired action.
BACnet works by dividing the problem of interoperability into three distinct areas, and by defining methods and standards for implementing each.
All information within an interoperable BACnet device is modeled in terms of one or more information objects. Each object represents some important component of the device, or some collection of information that may be of interest to other BACnet devices.
BACnet devices ask each other to perform services. For example, a device to which a temperature sensor is attached, may perform the service of reading the temperature and providing this information to another device that needs it. The model of objects and services is realized by encoding messages into a stream of numeric codes that represent the desired functions or services to be performed. The "language" of this encoding is common to all BACnet devices. BACnet devices actually exchange information and do things by sending and receiving electronic messages containing this coded language.
BACnet provides flexibility by allowing multiple types of transport systems to be used to convey these coded messages between devices. The transport system uses different types of electronic messaging standards and methods to convey coded messages. Even though different transport methods are used, the coded message content remains the same. This philosophy allows the designer or specifier to choose the most cost-effective transport method for a given application.