Selecting a protocol is only the first step to ensuring that you get what you want. With building automation protocols, the devil is in the details.
For example, each of the three protocols offers options for communication and networking technologies. Those choices are of great practical importance. Andres Szmulewicz, a consultant with Smart Buildings LLC, describes one installation where a LonTalk network, running at 78Kbps fed to a BACnet gateway. The installation used daisy chained serial bus to connect up to 10 floors. The result was a very slow system.
There is a place for serial buses, says Szmulewicz. They offer an inexpensive option for using lower end controllers. “But you want to make sure you’re not overdoing it,” he says. Ideally, he says, serial buses should be limited to a single floor or part of a floor, depending on floor area and device density.
Another problem with the project involving the 10-floor daisy chain is that it actually used a LonTalk-to-BACnet gateway. Gateways may be the only option in some cases, but they are bad news when supposedly “open” new systems are being installed. Facility managers should be aware that manufacturers sometimes put a gateway on top of a proprietary system and call the result “BACnet compatible” or “Modbus compatible.”
“Compatible” is a word to watch out for. Instead of relying on gateways, the system should run the protocol “natively.” That is, the system should utilize that protocol for its own internal communication and be able to communicate with other systems running the same protocol (assuming the protocol has been properly implemented) without the need for a gateway.
Each of the three protocols has an organization devoted to ensuring that devices and systems using the protocol have implemented it properly. BACnet Testing Labs lists products that have been tested to verify that they correctly implemented the BACnet features their manufacturers claimed they implemented. LonMark International http://www.lonmark.org/ certifies products that have completed the LonMark conformance tests and have been designed to interoperate across a LonWorks network. Modbus offers the Modbus TCP Conformance Testing Program, which offers a self-test program for companies that belong to The Modbus Organization, as well as testing by third-party labs.
In addition to getting the right products and systems, it’s important to get the right contractor and to make sure the installation is done correctly. “The devil is in the details,” says Jim Sinopoli, managing principal of Smart Buildings. “You have to be very clear about what you want.”
What’s more, it’s important to ensure that the contractor is providing what you asked for. To do that, facility managers should ask for one-line drawings showing what protocol will be used and how it will be implemented. The ideal time to do that is during bidding, but if that’s not possible those one line drawings should be required as part of the shop drawing review at the start of the construction process. The important thing is not to put it off. “If you wait until substantial completion, it’s too late,” says Szmulewicz.
Although it takes some work, facility managers can use open protocols to get important advantages for their systems. “If you set up your specs correctly, and you’re detailed as to what your requirements are, you’ll get what you want,” says Szmulewicz. To make that happen, facility managers have to educate themselves or bring in an expert. “If you leave it up to vendors,” says Szmulewicz, “they will implement it in the way that is cheapest for them.”
BACnet, LonWorks and Modbus: Getting What You Want
With Building Automation Protocols, Devil Is In The Details
