Ensuring your building's systems can talk to one another is vital to its smooth operation. But a tailored approach to protocol and set up is also needed to get the most from each application
FOR commercial building control systems, interoperability represents one of the most promising developments since direct digital controls (DDC) were invented.
But by the time that open, standard protocols like BACnet and LonTalk were available nearly a decade later, rather than be released from the single supplier chain, building owners and facility managers found it was still impossible to choose the best suppliers for their building.
The specialised DDC controllers that managed individual pieces of equipment so well were largely incompatible with each other. System upgrades, whether to accommodate facility expansion or simply to keep pace with technology, meant changing the entire building management system (BMS), or more likely, adding dedicated interfaces for each new system.
Today, however, interoperability is a requirement within the building controls industry. It means that independently designed DDC products work together using the same communication standard and a single user interface monitors and controls independently designed DDC product.
Infrastructure describes the basic facilities, services and installations needed for something to function. If that something is a building, then its infrastructure will include plumbing, power, and telecommunications. Automated control systems are part of that infrastructure, particularly in large applications with multiple buildings and/or systems provided by different manufacturers. Careful infrastructure planning, design, and installation are required to protect and enhance the value of the facility, and ongoing maintenance and management are equally important. Owners and facility managers must consider operating costs (energy, maintenance, manpower), performance (consistency, reliability, predictability), and extensibility (flexibility, better use of resources).
Factoring interoperability into an HVAC system design lets the owner take advantage of the information-sharing capabilities available. It also helps the building automation infrastructure benefit from changes in technology. Interoperability means owners can choose the best products, application, knowledge and service from a variety of suppliers. Interoperability helps them reduce the cost of acquiring and then maintaining their BMS infrastructures.
Too often interoperability is confused with interchangeability or plug-and-play. Neither of these open, standard protocols includes criteria to govern the programming, set up and sequence of operation of compatible devices.
Devices that are BACnet - or LonTalk - compatible can interoperate with each other, but setting up and commissioning them is likely to be vendor-specific. Flexibility in the implementation of these protocols means that some set up is necessary to make them perform as intended.
Interoperability levels
A unit-level controller provides stand-alone control of a device such as an air handler or a cooling tower. Networks of unit-level controllers are co-ordinated by an intermediate, system-level controller.
A system-level controller resides on the main local area network (LAN) of a building control system, along with system controllers that co-ordinate other building control functions, such as hvac, lighting, or fire-and-life safety. Interoperability at the unit (equipment controller) and system (building controller) levels is important to the overall flexibility of the BAS infrastructure, as well as for competitive procurement.
Within the conventional approach to BAS architecture, the hierarchy delineates subsystems by function and scope. Distributed, unit-level controllers are attached to a central, system-level controller. This design reduces the number of interface points and isolates each subsystem for commissioning and diagnostics without compromising interoperability.
An alternative approach uses an open, standard protocol to flatten the BAS architecture by connecting the unit-level controllers directly to the main LAN. This approach reduces hardware first-cost by eliminating the system-level controllers, but requires scattering the system co-ordination logic generally found in system controllers (scheduling, trending, and optimisation routines) throughout the controls network. This can make the building control system more difficult to install.
Which protocol?
BACnet's strength is its ability to exchange data over common (and in many cases, existing) high-speed, local and wide area networks such as ARCNET, Ethernet and Ethernet/IP (the internet). BACnet also has well defined standards for typical system level BAS applications such as scheduling, trending and alarm management. It works well for communications between system-level controllers, letting subsystems exchange information through a single user interface.
LonTalk protocol is particularly well suited for unit-level, peer-to-peer communication from equipment controllers. Pre-packaged in a microprocessor chip, LonTalk provides a cost-effective, easy-to-implement means of connecting stand-alone devices (boiler controls or a factory-mounted, variable frequency drive, for example) within the scope of an HVAC subsystem.
LonTalk's popular twisted-pair wiring option (FTT-10a/Free Topology) is twice as fast as the BACnet twisted-pair option and has similar proprietary offerings. Also, the LonMark Interoperability Association has created standard definitions of typical control devices.
These functional profiles greatly enhance the capacity for interoperability among LonTalk unit controllers.
Although open, standard protocols make it easier to deliver interoperability, design details must be explicitly acknowledged and communicated. An effective specification defines required functions and performance, and addresses methods and equipment, that will achieve the required performance.