Engineering Manual o.. - HVAC.Amickracing

BUILDING MANAGEMENT SYSTEM FUNDAMENTALSPEER COMMUNICATIONS PROTOCOLPeer communications protocol has the following advantagesover poll/response communications protocol:— Communication not dependent on a single device asthe master.— Direct communication between bus-connected deviceswithout going through the BMS central processor.— Global messages transmitted to all bus-connected devices.In peer communications a time slot is automatically passedfrom one bus-connected device to another as the means ofdesignating when a device has access to the bus. Since the timeslot passes in an orderly sequence from one device to the next,the communications network is sometimes termed a ring.However, the bus is not necessarily physically looped nor arethe devices physically connected to form a ring. Any device onthe bus can be designated as the first to receive the time slot,and any other device the next to receive it, and so on.COMMUNICATIONS MEDIAThe most common choices for BMS transmission trunks are:— Twisted copper pairs— Fiber optic cable— Common carrier telephone channelsThe media best suited for a given installation depends on thesignal, cost, geographic layout, and the possibility of lineinterference.TWISTED COPPER PAIRTwisted pair copper conductors ranging from 1.307 to 0.2051 mm 2are the most commonly used and the best economic choice as thecommunications media for single building applications. Buslengths up to 1200 meters are common without use of extendersor repeaters. When repeaters are used, extensions up to three orfour times this distance are possible. Serial bus and star wiringconfigurations (Fig. 2 and 3) permit efficient wiring layouts.SYSTEM/ZONELEVELCONTROLLERCOMMUNICATIONSBUSSYSTEM/ZONELEVELCONTROLLERSYSTEM/ZONELEVELCONTROLLERCOMMUNICATIONSBUSSYSTEM/ZONELEVELCONTROLLERFIBER OPTICSYSTEM/ZONELEVELCONTROLLERBMSCENTRALSYSTEM/ZONELEVELCONTROLLERSYSTEM/ZONELEVELCONTROLLERSYSTEM/ZONELEVELCONTROLLERC1862Fig. 3. Typical Star Wiring Configuration.Fiber optic transmission media is particularly suited toinstallation in an environment that interferes withcommunications, such as high electrical interference orfrequent electrical storms. The disadvantages of fiber optictransmission are the cost and lack of industry standards. Fiberoptic links are most often found between buildings.PHONE LINESCommon carrier telephone channels link distant buildings.Telephone line transmissions require either full time dedicatedphone lines or automatic dialing through modems (Fig. 4).A modem is required for signal compatibility with the phoneline. The BMS dial-up phone line interface providescompatibility with transmission rate and protocol of the phoneline and reduces the leased line operation to costs associatedonly with call frequency and connect time. As an example ofreduced costs, historical storage of alarm activity can be includedin the remote phone line interface so that the data istransmitted during periodic dialup by the central system.Critical alarms can be programmed at the remote to dial thecentral system immediately.BMSCENTRALC1863Fig. 2. Typical Serial Bus Wiring Configuration.REESTABLISHEDTWISTED PAIRCOMMUNICATIONSLINKOPERATIONS-LEVELPROCESSORPHONE LINEINTERFACEMODEMMODEMPHONE LINEINTERFACESYSTEM/ZONELEVELCONTROLLERSC1864Fig. 4. Dialup to Remote Controllers.188ENGINEERING MANUAL OF AUTOMATIC CONTROL