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 products.

Building a controls infrastructure

Infrastructure describes the basic facilities, services and installations needed for something to function. If that ‘something’ is a building, then its infrastructure includes plumbing,power,and telecommunications. Automated control systems are part of that infrastructure also, 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; ongoing maintenance and management are equally important. Owners and facility managers must consider first-cost operating costs (energy, maintenance, manpower), performance (consistency, reliability, predictability), and extensibility (flexibility, better use of resources).

Factoring interoperability into a HVAC system design lets the owner take advantage of the information-sharing capabilities available today.It also helps the building automation infrastructure keep pace with and benefit from changes in technology. Interoperability means owners can choose the best products, application, knowledge, and service from a variety of suppliers and it helps them reduce the cost of acquiring and then maintaining their BAS infrastructures. However, too often ‘interoperability’ is confused with ‘interchangeability’ or ‘plug-and-play’. Neither of these open, standard protocols includes criteria to govern the programming, setup and sequence of operation of compatible devices. Devices that are BACnet or LonTalk compatible can interoperate with each other, however setting up and commissioning them is likely to be unique and vendor-specific. Flexibility in the implementation of these protocols means that some setup is necessary to make them perform as intended.

Levels of interoperability

A unit-level controller provides standalone 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 LAN (local area network) 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, systemlevel 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 and commission and usually requires the services of a system integrator.

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 such as HVAC and lighting exchange information through a single user interface. BACnet also works well for providing centralised control over multiple, standalone buildings.

LonTalk protocol is particularly well-suited for unit-level, peer-topeer communication from equipment controllers. Pre-packaged in a microprocessor chip,LonTalk provides a cost-effective, easy-to implement means of connecting standalone devices (boiler controls or a factory-mounted, variable frequency drive,for example) within the scope of a HVAC subsystem. LonTalk's popular twisted-pair wiring option (FTT-10a/Free Topology) is twice as fast as the BACnet twisted-pair option and 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.

There are two approaches to specifying interoperable systems: performance-based and prescriptive. Performance-based specifications stipulate the required functions and performance without dictating the methods and equipment for achieving that performance. A prescriptive specification is more detailed in its stipulation of methods and equipment.A performance based specification with appropriate prescriptive language usually yields the most cost-effective solution.

Information in the specification

Stipulate the protocol and wiring media. Explicitly state which elements of the system must support a particular protocol, as well as how the elements will be connected. For example, ‘HVAC and lighting systems shall communicate using BACnet protocol over ARCNET or Ethernet LAN’, or ‘Terminal unit controllers shall reside on the LonTalk FTT-10a network’. BACnet and LonTalk are defined over a number of different wiring media. Base your selection on the cost and performance characteristics of each medium, the ease of future additions and the degree of compatibility with other media. (Twisted-pair wiring at 78 Kbaud, for example, is not directly compatible with Ethernet over the Internet at 10 Mbaud.) Identify data shared between systems. Be sure to state which LonTalk functional profile, BACnet device profile, or standard BACnet objects each controller must support. For example, ‘Terminal unit controllers shall support the LonMark Space Comfort Controller functional profile’, or ‘The status of all smoke detectors,pull stations and other alarm devices shall be described as BACnet standard binary input (BI) objects’.

(Note: LonTalk ‘functional profiles’ list the mandatory,optional and vendor-specific variable types appropriate for a given application.‘Device profiles’ are a new tool for specifying BACnet interoperability. They outline the basic requirements for standard types of interoperable controllers,for example Operator Workstation, Application-Specific Controller and Smart Actuator.)

Include a list of interoperable points for each controller and/or system.This may be simply a matter of adding an extra column to the points list to identify the BACnet and/or LonTalk points. Bear in mind that the number of points may impact the cost of the project.

Describe what each controller is expected to do with the interoperable data available via the network. For example,‘Display status of all points on graphics and trend status; add to standard life safety report; report changes in event log and other internal records’.

Both BACnet and LonTalk provide the means to deliver the following interoperable functions:

• Data sharing (controlling the status and monitoring the value of connected points)

• Scheduling (viewing and editing appointed system events)

• Alarm and event management (annunciation and acknowledgment of device alarms)

• Trending (collection of data on system performance and energy usage)

• Device and network management (for example, checking for loss of communication and coordinating time clocks in controllers)

Confirm the interoperability of each product. Suppliers should demonstrate how their products support interoperability. Request a BACnet ‘protocol implementation conformance statement’ (PICS) and a list of supported LonMark functional profiles including optional network variables. For example: ‘The terminal unitcontroller supplier shall provide a list confirming their support of all mandatory data included in the specified functional profile and identify which optional network variables and configuration properties they support. Any vendor-defined network variables or configuration properties shall be described via an XIF file supplied with the product’.

Remember that BACnet and LonTalk protocols do not provide interoperable programming or sequences of operation, nor do they yield interchangeable controllers.

Advances in technology and specification techniques suggest that BACnet and LonTalk will remain standard protocols for years to come.Applying them in combination establishes a BAS infrastructure that delivers interoperability in an efficient and economic manner. Our challenge is to educate ourselves and mentor our clients so that together we can make decisions that best serve their digital communication needs.

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