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ACHIEVING INTEROPERABILITY BACnet: Answers to Frequently Asked Questions A primer on the revolutionary development in the building automation and controls industry— interoperability among different vendors’ products By H. MICHAEL NEWMAN, trol loops, and alarms. This standardized model of a Manager of Utilities Dept. Computer Section, device represents these common functions as collec- Cornell University, tions of related information called objects, each of Ithaca, N.Y. which has a set of properties that further describe it. Each analog input, for instance, is represented by a his article answers some of the most frequently BACnet analog input object that has a set of standard asked questions about ANSI/ASHRAE Stan- properties like present value, sensor type, location, T dard135-1995, BACnetE—A Data Communi- alarm limits, and so on. Some of these properties are cation Protocol for Building Automation and Control required while others are optional. One of the object’s Networks. most important properties is its identifier, a sort of numerical name that allows BACnet to access it un- What is BACnet? ambiguously. Once devices have common “appear- As stated in the title of the above-referenced stan- ances” on the network in terms of their objects and dard, BACnet is “a data communication protocol for properties, it’s easy to envision messages that can ma- building automation and control networks.” A data nipulate this information in a standard way. communication protocol is a set of rules governing the exchange of data over a computer network. The ■ So what kinds of messages does BACnet define? rules take the form of a written specification (in BAC- Because BACnet is based on a client-server model net’s case, they are also on compact disk) that spells of the world, BACnet messages are called service re- out what is required to conform to the protocol. quests. A client machine sends a service request to a server machine that then performs the service and re- ■ What kinds of things are covered by BACnet’s rules? ports the result to the client. BACnet currently de- Everything from what kind of cable to use to how to fines 35 message types that are divided into 5 groups form a particular request or command in a standard or classes. For example, one class contains messages way is covered. What makes BACnet special is that for accessing and manipulating the properties of the the rules relate specifically to the needs of building objects described above. A common one is the “Read- automation and control equipment—i.e., they cover Property” service request. This message causes the things like how to ask for the value of a server machine to locate the requested temperature, define a fan operating property of the requested object and schedule, or send a pump status alarm. send its value back to the client. Other classes of services deal with alarms and ■ But every manufacturer’s system is events, file uploading and downloading, different! How can BACnet possibly do managing the operation of remote de- all these things in a standard way? vices, and virtual terminal functions. The trick is that BACnet provides a standard way of representing the func- H. Michael Newman was the chairman of ASHRAE Standard Project Committee (SPC) tions of any device as long as it has these 135P, which developed BACnet. He is functions. Examples are analog and bi- currently chairman of ASHRAE SSPC 135, nary inputs and outputs, schedules, con- which interprets and supports BACnet. March 1997 HPAC Heating/Piping/AirConditioning 47 BACnet Q & A ■ Is BACnet limited only to HVAC equipment? Can it master-slave/token-passing (MS/TP) network, de- be used with fire/life safety, lighting control, and signed to run at speeds of 1 Mbps or less over twisted other building automation systems? pair wiring. Echelon Corp.’s LonTalkT network can Absolutely. In fact, if you think about it, BACnet also be used on the various media listed above. All of already contains most of the capabilities required for these networks are examples of local area networks non-HVAC communications. These include the abil- or LANs. BACnet also defines a dial-up or point-to- ity to read and write binary, analog, and text data; point protocol called PTP for use over phone lines or schedule control actions; send event and alarm noti- hardwired EIA-232 connections. A key point is that fications; and so on. Nonetheless, the committee re- BACnet messages can, in principle, be transported alized that these capabilities might not cover all sit- by any network technology, if and when it becomes uations and developed the standard with an eye cost effective to do so. toward accommodating future, unknown building automation and control applications. As a result, one ■ You mentioned that BACnet can use LonTalk. Does of the real strengths of the BACnet model that that mean that any equipment that uses LonTalk can emerged from this consideration is that it can be eas- automatically talk to BACnet systems? ily extended. If a vendor comes up with some new Unfortunately not. LonTalk is Echelon’s specifica- functionality for which communication is required, tion for a recently developed LAN technology that the vendor can add new properties to existing object many people thought would be a useful addition to types or create new object types that are accessed in the BACnet standard. BACnet uses LonTalk to con- exactly the same way as the 18 defined in the stan- vey BACnet messages in an identical manner to the dard. This is not only expected; it is encouraged. way BACnet messages are transported by Ethernet, Moreover, a vendor could even dream up new ser- ARCNET, and MS/TP. Confusion stems from the fact vices that go beyond the standard ones. Of course, that Echelon has its own generic control language proprietary features may not be interoperable with- that is also transported by LonTalk. For LonTalk de- out vendor cooperation. vices to be interoperable, even using Echelon’s lan- guage, there has to be agreement among imple- ■ I keep hearing about interoperability, but I like menters as to what the generic messages mean in a the vendor I deal with now. Does BACnet require particular context. To obtain such agreements, Eche- multi-vendor installations? lon has set up the LonMark Program, which has Definitely not. BACnet is just a protocol. It makes working groups, made up of people from each indus- possible the interconnection of different vendors’ try, that are trying to reach implementers’ agree- equipment that uses BACnet but in no way requires ments on how to use Echelon’s proprietary control it. Since many vendors will probably choose, sooner or language in a common way for their applications. The later, to use BACnet as their “native” protocol, you point is that the BACnet language and the Echelon could easily end up with a single-vendor BACnet sys- language are fundamentally different, and devices tem. Also, I agree with you. I would much prefer to using one of the languages can never interoperate di- deal with a single vendor or, at most, a couple of ven- rectly with devices using the other, even though they dors. The issue is making sure their pencils stay might possibly share a common LonTalk LAN. sharp at bid time! ■ I like the idea of BACnet, but what about all my ■ What about connecting BACnet systems together? existing DDC systems? Can BACnet help tie them What networking options are there for BACnet? together, too? Up until now, I have just been talking about the Maybe, maybe not. For a BACnet device, say an op- BACnet object-oriented model and the various ser- erator workstation, to talk to non-BACnet devices like vices or message types. You still need to pick an ap- your existing DDC system from XYZ Controls, you propriate network technology to connect everything need an intervening gateway. A gateway is like a together. The BACnet committee spent a lot of time United Nations translator that can speak two lan- on this part of the standard. We ended up with five guages. On one side, it speaks BACnet and on the different options, each of which fills a particular other side, the XYZ protocol of your existing (legacy) niche in terms of the price/performance tradeoff. The system. Naturally, the most likely source for such a first is Ethernet, the fastest at 10 million bits per sec gateway would be the XYZ company, and they may or (Mbps), with 100 Mbps also recently available. Eth- may not choose to develop one. ernet is also likely to be the most expensive in terms of cost per device. Next comes ARCNET at 2.5 Mbps. ■ What if some of my DDC systems are on Ethernet Both Ethernet and ARCNET can use a variety of and some are on, say, MS/TP. Is there any way to physical media—coaxial cable, twisted pairs, and connect them together? even fiber optic cable. For devices with lower re- Yes. Besides allowing the use of different LANs, quirements in terms of speed, BACnet defines the the BACnet standard also specifies how to build 48 HPAC Heating/Piping/AirConditioning March 1997 routers. Routers are simply devices that connect ■ What is the NIST BACnet Interoperability Testing multiple networks together. The networks may be of Consortium? the same or different types. When BACnet was under public review, several commenters suggested that BACnet devices should ■ Now I’m confused. What’s the difference between actually be built and tested before releasing the a router and a gateway? standard. The committee agreed but needed to find a I don’t blame you for being confused because a lot of way to do the testing such that potential imple- people use the terms almost interchangeably.

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