HVAC Control in the New Millennium.pdf - HVAC.Amickracing

HVAC Control in the New MillenniumOpen SystemsThe detailed concept of open systems for computers has developedfrom the Open System Interconnection (OSI) reference model for computercommunication standards. The OSI model has seven layers ofcommunication protocol and each of the layers has its own functionwith defined interfaces to higher and lower layers.These layers are used with programming object models to definethe meaning and format of data. This can provide a high level of multivendorinteroperability.In an ideal open systems architecture, devices communicate witheach other using a standard protocol as a native language, withouttranslation. The ideal system would have interoperability at all levels.Some so-called open systems interface workstations and controlsystem components from one manufacturer with other manufacturers’components using gateway hardware or software on the network segments.The native language may be a proprietary protocol in this system.The system is forced to be compatible, using translation in the formof the gateways implemented in hardware or software. The devices in acommunication segment must be from the same manufacturer. Thesedevices may include terminal units, chillers, fire alarm panels, securityaccess controls, generator controls, fume hoods and programmable logicalcontrollers.In a true open system, the communications between workstationsand field devices take place on a common bus using non-proprietaryprotocols. The field devices may be setup internally to perform the gatewayfunctions, or they can use external gateways to communicate withother manufacturers’ equipment using a proprietary protocol. Protocolsprovide for the data exchange of a defined set of programming objectsand services.In Europe, open systems development has produced Profibus andFND. In the United States, the Distributed Network Protocol (DNP) andUtility Communications Architecture (UCA) were developed for industrialand utility Supervisory Control and Data Acquisition (SCADA).Data communication protocols for building automation and controlnetworks include BACnet, sponsored by ASHRAE, and LonTalk,sponsored by the Echelon Corp. The BACnet protocol has been adoptedas an ANSI/ASHRAE standard.©2001 by The Fairmont Press, Inc. All rights reserved.