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ACHIEVING INTEROPERABILITY

By H. MICHAEL NEWMAN,Manager of Utilities Dept. Computer Section,Cornell University,Ithaca, N.Y.

This article answers some of the most frequentlyasked questions about ANSI/ASHRAE Stan-dard135-1995,BACnetA Data Communi-

cation Protocol for Building Automation and ControlNetworks.

What is BACnet?As stated in the title of the above-referenced stan-

dard, BACnet is a data communication protocol for

building automation and control networks. A datacommunication protocol is a set of rules governingthe exchange of data over a computer network. Therules take the form of a written specification (in BAC-nets case, they are also on compact disk) that spellsout what is required to conform to the protocol.

What kinds of things are covered by BACnets rules?Everything from what kind of cable to use to how to

form a particular request or command in a standardway is covered. What makes BACnet special is thatthe rules relate specifically to the needs of buildingautomation and control equipmenti.e., they coverthings like how to ask for the value of a

temperature, define a fan operatingschedule, or send a pump status alarm.

But every manufacturers system isdifferent! How can BACnet possibly doall these things in a standard way?

The trick is that BACnet provides astandard way of representing the func-tions of any device as long as it has thesefunctions. Examples are analog and bi-nary inputs and outputs, schedules, con-

trol loops, and alarms. This standardized model of adevice represents these common functions as collec-tions of related information called objects, each ofwhich has a set of properties that further describe it.Each analog input, for instance, is represented by aBACnet analog input object that has a set of standardproperties like present value, sensor type, location,alarm limits, and so on. Some of these properties arerequired while others are optional. One of the objectsmost important properties is its identifier, a sort ofnumerical name that allows BACnet to access it un-ambiguously. Once devices have common appear-ances on the network in terms of their objects andproperties, its easy to envision messages that can ma-

nipulate this information in a standard way.

So what kinds of messages does BACnet define?Because BACnet is based on a client-server model

of the world, BACnet messages are called service re-quests. A client machine sends a service request to aserver machine that then performs the service and re-ports the result to the client. BACnet currently de-fines 35 message types that are divided into 5 groupsor classes. For example, one class contains messagesfor accessing and manipulating the properties of theobjects described above. A common one is the Read-Property service request. This message causes the

server machine to locate the requested

property of the requested object andsend its value back to the client. Otherclasses of services deal with alarms andevents, file uploading and downloading,managing the operation of remote de-

vices, and virtual terminal functions.

March 1997 HPAC Heating/Piping/AirConditioning 47

BACnet:Answers toFrequently Asked Questions

A primer on the revolutionary development

in the building automation and controls industry

interoperability among different vendors products

H. Michael Newman was the chairman ofASHRAE Standard Project Committee (SPC)135P, which developed BACnet. He iscurrently chairman of ASHRAE SSPC 135,which interprets and supports BACnet.

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BACnet Q & A

Is BACnet limited only to HVAC equipment? Can itbe used with fire/life safety, lighting control, andother building automation systems?

Absolutely. In fact, if you think about it, BACnetalready contains most of the capabilities required for

non-HVAC communications. These include the abil-ity to read and write binary, analog, and text data;schedule control actions; send event and alarm noti-fications; and so on. Nonetheless, the committee re-alized that these capabilities might not cover all sit-uations and developed the standard with an eyetoward accommodating future, unknown buildingautomation and control applications. As a result, oneof the real strengths of the BACnet model thatemerged from this consideration is that it can be eas-ily extended. If a vendor comes up with some newfunctionality for which communication is required,the vendor can add new properties to existing objecttypes or create new object types that are accessed in

exactly the same way as the 18 defined in the stan-dard. This is not only expected; it is encouraged.Moreover, a vendor could even dream up new ser-

vices that go beyond the standard ones. Of course,proprietary features may not be interoperable with-out vendor cooperation.

I keep hearing about interoperability, but I likethe vendor I deal with now. Does BACnet requiremulti-vendor installations?

Definitely not. BACnet is just a protocol. It makespossible the interconnection of different vendorsequipment that uses BACnet but in no way requiresit. Since many vendors will probably choose, sooner or

later, to use BACnet as their native protocol, youcould easily end up with a single-vendor BACnet sys-tem. Also, I agree with you. I would much prefer todeal with a single vendor or, at most, a couple of ven-dors. The issue is making sure their pencils staysharp at bid time!

What about connecting BACnet systems together?What networking options are there for BACnet?

Up until now, I have just been talking about theBACnet object-oriented model and the various ser-

vices or message types. You still need to pick an ap-propriate network technology to connect everythingtogether. The BACnet committee spent a lot of time

on this part of the standard. We ended up with fivedifferent options, each of which fills a particularniche in terms of the price/performance tradeoff. Thefirst is Ethernet, the fastest at 10 million bits per sec(Mbps), with 100 Mbps also recently available. Eth-ernet is also likely to be the most expensive in termsof cost per device. Next comes ARCNET at 2.5 Mbps.Both Ethernet and ARCNET can use a variety ofphysical mediacoaxial cable, twisted pairs, andeven fiber optic cable. For devices with lower re-quirements in terms of speed, BACnet defines the

master-slave/token-passing (MS/TP) network, de-signed to run at speeds of 1 Mbps or less over twistedpair wiring. Echelon Corp.s LonTalk network canalso be used on the various media listed above. All ofthese networks are examples of local area networks

or LANs. BACnet also defines a dial-up or point-to-point protocol called PTP for use over phone lines orhardwired EIA-232 connections. A key point is thatBACnet messages can, in principle, be transportedby any network technology, if and when it becomescost effective to do so.

You mentioned that BACnet can use LonTalk. Doesthat mean that any equipment that uses LonTalk canautomatically talk to BACnet systems?

Unfortunately not. LonTalk is Echelons specifica-tion for a recently developed LAN technology thatmany people thought would be a useful addition tothe BACnet standard. BACnet uses LonTalk to con-

vey BACnet messages in an identical manner to theway BACnet messages are transported by Ethernet,

ARCNET, and MS/TP. Confusion stems from the factthat Echelon has its own generic control languagethat is also transported by LonTalk. For LonTalk de-

vices to be interoperable, even using Echelons lan-guage, there has to be agreement among imple-menters as to what the generic messages mean in aparticular context. To obtain such agreements, Eche-lon has set up the LonMark Program, which hasworking groups, made up of people from each indus-try, that are trying to reach implementers agree-ments on how to use Echelons proprietary controllanguage in a common way for their applications. The

point is that the BACnet language and the Echelonlanguage are fundamentally different, and devicesusing one of the languages can never interoperate di-rectly with devices using the other, even though theymight possibly share a common LonTalk LAN.

I like the idea of BACnet, but what about all myexisting DDC systems? Can BACnet help tie themtogether, too?

Maybe, maybe not. For a BACnet device, say an op-erator workstation, to talk to non-BACnet devices likeyour existing DDC system from XYZ Controls, youneed an intervening gateway. A gateway is like aUnited Nations translator that can speak two lan-

guages. On one side, it speaks BACnet and on theother side, the XYZ protocol of your existing (legacy)system. Naturally, the most likely source for such agateway would be the XYZ company, and they may ormay not choose to develop one.

What if some of my DDC systems are on Ethernetand some are on, say, MS/TP. Is there any way toconnect them together?

Yes. Besides allowing the use of different LANs,the BACnet standard also specifies how to build

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routers. Routers are simply devices that connectmultiple networks together. The networks may be ofthe same or different types.

Now Im confused. Whats the difference between

a router and a gateway?I dont blame you for being confused because a lot ofpeople use the terms almost interchangeably. In BAC-net, a router is a device that passes a message from onenetwork to another without changing the form or con-tent of the message. If the networks are of differenttypes, the addresses and error checkingin short, thepackagingof the message may get changed, muchas you would repackage an ordinary U.S. Postal Ser-

vice letter if you were going to send it further usingFedEx or UPS. A gateway, on the other hand, opens theletter, translates it into a second language if possible,puts it back into some type of envelope, and then sendsit on. Obviously, it can take more time and energy to do

a translation than to simply forward a message, sogateways are more complicated machines than routers.

Do BACnet routers really exist?Sure. In fact, the National Institute of Standards

and Technology (NIST) BACnet InteroperabilityTesting Consortium sponsored a demonstration boothat the International Air-Conditioning, Heating, Re-frigerating Exposition in Atlanta in February 1996.Controllers were running on all the BACnet networktypes except PTP, and all were interoperating. AnEthernet-ARCNET router, an Ethernet-MS/TProuter, and an Ethernet-LonTalk router intercon-nected them. Each workstation was programmed to

display identical graphics and was able to carry outidentical control actions, even though three differentoperating systems and four different LANs were in-

volved. Fig. 1 is a schematic of this setup.

What is the NIST BACnet Interoperability TestingConsortium?

When BACnet was under public review, severalcommenters suggested that BACnet devices shouldactually be built and tested before releasing the

standard. The committee agreed but needed to find away to do the testing such that potential imple-menters could feel comfortable with the idea ofbringing their prototypes together with those oftheir competitors for a noncompetitive, nonjudgmen-tal evaluation. Also, we wanted to find a neutral

venue for the testingi.e., not one of the vendorsmanufacturing sites. Happily, NIST offered the useof its lab in Maryland. Moreover, NIST has a mecha-nism known as a Cooperative Research and Devel-opment Agreement, which allowed us to set up thetesting and still protect everyones proprietary inter-ests. There are currently 20 consortium members, 17of whom are manufacturers.

What about BACnet certification? Will NIST becertifying BACnet conformance?

BACnet certification is, for the moment, still anunresolved issue although everyone agrees that it isneeded. NIST will definitely not be doing any sort ofcertification nor wil l ASHRAE as a society. Twothings are needed: 1) a standard method of testingconformance and 2) a decision on who will be respon-sible for issuing the BACnet stamp of approval. De-

veloping such a test will be one of the top priorities ofthe ASHRAE Standing Standard Project Committee(SSPC 135) that was formed to interpret and extendBACnet.

Speaking of conformance, what can you tell meabout writing specifications for BACnet systems?

The first thing is that your specification must


Cornell PowerMacDelta PC

BACnet over Ethernet

Trane PC

BACnet over ARCNETCimetrics

router

Johnsoncontroller

Johnsonrouter

BACnet

overMS/TP

Honeywellcontroller

Siebecontroller

BACnet over LonTalk

NIST/PolarSoftdiagnostic software

Carriercontroller

Deltacontroller

Siebe PCSieberouter

Staefacontroller

Teletrolcontroller

Tranecontroller L&G

controller

1 Schematic of the demonstration network, featuring prototype BACnet devices, that operated at the International Air-Conditioning, Heating, Refrigerating Exposition in Atlanta, February 1996.

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BACnet Q & A

clearly state what kinds of network functions youneed. For example, if you want your field panel to beable to start and stop equipment based on date andtime, that does not necessarily require any networkfunctionality. But if you want to be able to manipulate

the controllers schedule from a remote workstation,both the workstation and the field panel have to sup-port the appropriate BACnet capabilities in terms ofobjects and services. Ideally, you should become fa-miliar with BACnets conformance classes and func-tional groups. These will help you translate your net-work needs into BACnet terminology. But even if youare not completely familiar with these things, you canstill be successful. Just carefully list the things youwant to be able to do across the networktime-basedthings, alarm and event requirements, points thatyou want to share between devicesand state thatthey must be accomplished using BACnet.

What are conformance classes?BACnet specifies a six-level hierarchy of device ca-

pabilities based on the implementation of certainBACnet objects and services. A Conformance Class 1device, for example, only needs to have a single ob-

ject and be able to understand and carry out a re-quest to read and send back the values of that ob-

jects properties (ReadProperty). A ConformanceClass 2 device, besides having Conformance Class 1capabilities, needs to be able to carry out a request tochange the value of an objects properties(WriteProperty). Higher numbered conformanceclasses tack on additional capability. This hierarchycould correspond to the capabilities that might be as-

sociated with a smart sensor at the Class 1 end and afull-featured, general-purpose direct digital con-troller at the Class 6 end. Please note that there is nocorrelation between conformance class and good orbad. A system made up of Class 2 or 3 devices maybe all that a particular application requires.

You also mentioned functional groups. What are they?Early on, it became apparent that something more

than conformance classes was needed to specifyBACnet capabilities. For example, suppose you havea simple Class 2 device like a smart actuator, butyou want it to be able to control the device connectedto it based on a time schedule. As mentioned earlier,

the ability to do time-related things over the net-work, such as setting a devices clock and alteringschedules, requires a certain collection of BACnetobjects and services. Each such collection, for a spe-cific function, is called a functional group. There arecurrently 13 functional groups in the standard, cov-ering such things as what objects and services wouldbe required to act as a BACnet handheld worksta-tion, a PC workstation, a device capable of initiatingor responding to alarms and events, a device capableof supporting virtual terminal logins, and so on.

What is a PICS?PICS stands for protocol implementation confor-

mance statement. It is basically a BACnet spec sheetlisting a devices BACnet capabilities. Every BACnetdevice is required to have one. It contains a generalproduct description, the conformance class of the de-

vice, the functional groups supported, a list of all theservices supported as a client and/or server, whichLAN options are available, and a few other items re-

lating to character sets and special functionality. APICS is the place to look to see what a devices capabil-ities are. Additionally, a specifier could draft a PICS asa way of conveying what BACnet capabilities are de-sired for a particular job.

What is happening next within ASHRAE?The SSPC will be working on a number of refine-

ments or extensions to the standard. As I mentionedearlier, the top priority is to finalize a conformancetesting addendum. We will also be looking at ways toenhance the use of BACnet with the Internet proto-cols (IP). This refinement, BACnet/IP, will hopefullybe available for public review later this year. We are

also planning to revisit the whole subject of confor-mance classes and functional groups to see if it canbe streamlined. Another working group is looking atfine-tuning the operation of the MS/TP LAN. Fi-nally, a working group is looking at specifying thedefinitions of several new macro objects whoseproperties would be the parameters required for themonitoring and control of a large-scale device like aheat pump, chiller, or packaged rooftop air condi-tioner.

What else is going on?Besides the SSPC activities, there are some BAC-

net demonstration projects underway. One that has

been in the news quite a bit is the General ServicesAdministrations rehab of the Phillip Burton FederalBuilding at 450 Golden Gate Ave. in San Francisco.*The neat thing about this project is its scale andcomplexity. It will involve more than 1000 con-trollers, four (possibly five) different vendors, sev-eral applications including lighting control, and aninterface to the local electric utility. It is scheduledto be completed by the end of this year.

50 HPAC Heating/Piping/AirConditioning March 1997

The BACnet demonstration exhibit at the 1996 Interna-tional Air-Conditioning, Heating, Refrigerating Expositionin Atlanta.

*See related story in Youll Want To Know on page 8 of this issue.

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There is also some activity on the internationalstandards front. BACnet has been selected by theEuropean Community (EC) standards committeeCEN TC247 as a European pre-standard, meaningthat it will undergo an evaluation process over the

next couple of years to see whether it should becomea full-fledged EC standard. The International Orga-nization for Standardization (ISO) committee TC205is also considering whether BACnet should be desig-nated an ISO standard. All of this is quite remark-able, considering that BACnet was published only 14months ago (January 1996).

Are there any BACnet products actually installedand in use today?

Definitely. Of course, it is hard to get a precisetally, but if NIST consortium members are to be be-lieved, there are more than 1000 BACnet sites up andrunning at this moment. Of these, roughly one-third

are multi-vendor installations. Most of the sites arein the United States, but members also claim to haveinstallations in Canada, Mexico, Brazil, Germany,Switzerland, the U.K., Hong Kong, Singapore, Aus-tralia, and Taiwan. It wouldnt surprise me if therewere more installations that I havent heard about.

When will more products be available?This, of course, is a key question. There is a bit of

chicken and egg here: the vendors will probably ac-celerate their product development if they see agrowing user demand, and users will probably in-

crease their demand if they see more BACnet prod-ucts in the marketplace. If you are a user, the bestthing to do would be to express your interest in BAC-net products directly to your friendly local vendor.

How can I learn more about BACnet?Continue to readHPAC and other publications;

get a copy of the standard, either in hard-copy formor on CD, or both; and attend seminars or shortcourses offered by ASHRAE and other organiza-tions. Dont overlook vendor literature. A number of

vendors now have tutorial information on BACnet ingeneral and their BACnet products in particular.There is also a book calledDirect Digital Control of

Building Systems, immodestly enough by me, thathas a lot of information on both data communicationfundamentals and their application to BACnet. HPAC

This article is an Interactive Feature. Visit the HPAC InteractiveWebsite at www. penton.com/hpac/ for supplementary informa-tion and to explore related Websites.


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