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BACnet Unraveled



Steven T. Bushby


Publication of ASHRAE and ANSI's BACnet communication protocol in 1995 ushered in a new era for building automation and control systems, making it possible to integrate building control products designed by different manufacturers. This change has proved to be as profound for consulting engineers as it has been for building owners and manufacturers of building automation system (BAS) products. Despite this breakthrough, and the fact that there are hundreds of thousands of installed BACnet system control products, consulting engineers have had difficulty understanding how to write quality BACnet specifications. It has been difficult because, in many instances, specifiers have no background in computer communications and the tools provided in the 1995 standard to bridge this gap have not worked well in practice.ASHRAE has recognized this and at its annual meeting in late June, the board of directors approved final publication of three new addenda to ANSI/ASRAE Standard 135-1995. These addenda added many new capabilities to the standard. Including features specifically designed to make it easier to integrate fire and life-safety systems with other building automation systems. It is also a mechanism for making nonstandard extensions to BACnet network visible. For consulting engineers, Addendum d stands out because it was designed specifically to make the job of specifying BACnet systems easier.Addendum d replaces Clause 22, Conformance and Specification, and introduces a new concept called BACnet Interoperability Building Blocks(BIBBs). As the name suggests, each BIBB defines a small portion of BACnet functionality needed to perform a particular task. BIBBs are combined to build the BACnet functional requirements for a device in a specification.In developing BIBBs, ASHRAE SSPC 135, the committee charged with maintaining the standard, turned to a work in progress for inspiration. At the time, a separate ASRAE committee had been developing a tutorial on BAS , now published as ASHRAE Guideline 13-2000. Specifying Direct Digital Control Systems.
Consulting Specifying Engineer
No. 2


ANSI/ASHRAE Standard 135, BACnet, building automation and control, communication protocol, direct digital control, energy management systems


Bushby, S. (2001), BACnet Unraveled, Consulting Specifying Engineer (Accessed May 24, 2024)


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Created August 1, 2001, Updated February 19, 2017