An “open protocol” allows multiple products to communicate information with each other using a common language. The possible benefits also include a common operator’s workstation, although programming must still be accomplished through each manufacturer’s programming tool.

BACnet is defined as a data communications protocol for "Building Automation and Controls networks." BACnet is simply an agreed-upon set of rules that apply to a computer's hardware and software. A key design criterion was that the protocol had to be applicable to all building automation needs such as HVAC control, fire detection, lighting control, and security. To accomplish this, BACnet specifies most all of the most common functions that clearly apply to almost any kind of monitoring or control application: analog and binary input, output, and values; control loops; schedules, etc.

BACnet was first published in 1995 after 8 ½ years of development under ASHRAE’s guidance and has been updated many times since. It was approved as ISO Standard 16484-5 in 2003. One of the positive aspects of the arduous standards development process is that BACnet has a large measure of industry acceptance due to its thorough industry review. Available products include workstations, controllers, gateways, routers and diagnostic tools.

The BACnet Protocol Rules apply to:

• Electrical Signaling
• Addressing
• Network Access (Master/Slave, Peer-to-Peer)
• Error Checking
• Flow Control
• Message Sequencing, Segmentation, Checkpointing
• Presentation (compression, encryption)
• Message Format

This is accomplished by introducing "objects." An object is simply a collection of information related to a particular function that can be uniquely identified and accessed over a network in a standardized way. All information in a BACnet system is represented by such data structures. The object concept allows us to talk about and organize information relating to physical inputs and outputs, as well as non-physical concepts like software, or calculations. Objects meet the design requirement of providing each device with a common "network view," i.e., all objects, regardless of the machine in which they reside, look alike!

Each object is characterized by a set of standard properties that describe its behavior or govern its operation. Some properties are “read only”; you can look at the property value, but not change it. Some properties can be changed (written).

As an example, a temperature sensor would be represented as a BACnet Analog Input object. Some properties would include a name property (RM112 SPACE TEMP), an object type (ANALOG INPUT) and present value (72.3 degrees). Other properties show us other information about the sensor object, such as whether it appears to be functioning normally, or High and Low Limits for alarming purposes.

Although there are thousands of potentially useful object types which might be found in building automation, BACnet defines 23 standard object types in some detail. This set of standard objects represents much of the functionality found in typical building automation and controls systems today. BACnet devices are only required to implement the objects as appropriate to the device’s functions.

The second part of the development challenge was to agree on what kinds of messages building automation and control devices might want to send to each other. Because BACnet is based on a "Client-Server" communication model, these messages are called "services" which are carried out by the server on behalf of the client. The classes of Application Services are as follows:

• Alarm and Event Services
• File Access Services
• Object Access Services
• Remote Device Management Services
• Virtual Terminal Services

The third part of the BACnet standard involved selecting the appropriate networking technologies from the dozens available. The set of LAN options specified in BACnet was chosen because it seemed to span the real-world requirements of building control systems in terms of speed, throughput, cost, familiarity, etc.:

• Ethernet
• ARCNET
• Master-Slave/Token-Passing (MS-TP)
• Point-to-Point (PTP)
• Echelon’s LonTalk
• BACnet/IP and “Virtual LAN’s” (allows for TCP/IP, ATM, etc.)

Ethernet, ARCNET, and LonTalk are off-the-shelf LANs, requiring minimal work on the part of the committee to specify how BACnet messages are to be transported on them. For low-cost EIA-485 and dial-up communications, BACnet created its own protocols in the form of the BACnet "Master-Slave/Token-Passing" protocol and the "Point-to-Point" protocol. The development of BACnet/IP provided not only the specification for transporting BACnet messages between IP devices but also the framework for embracing other new networking technologies with a minimal impact on existing BACnet technology. BACnet/IP is the best network technology selection when exchanging larger amounts of information (chillers, air handlers, etc.)

To summarize, BACnet consists of a specification for the BACnet object and services model, a network layer protocol, and a selection of various network transport technologies appropriate for various size and capability systems.

The drivers behind BACnet’s success to date include: User Demand, a very flexible architecture, easy extension of object model, lack of dependence on current technology, large vendor commitment, broad participation in development of the standard and global interest.

The major benefits to BACnet as an “Open Protocol” standard for the industry are:

• Designed specifically for building control
• No charge for its use
• Maintained by an ASHRAE committee representing all sectors of the industry
• Can be implemented in devices of any size
• Can be readily enhanced and improved
• Not tied to present technologies

The primary source for this document is the official ASHRAE BACnet website at www.bacnet.org and the presentation: “BACnet – a Tutorial Overview” by Mike Newman.