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 thetime slot passes in an orderly sequence from one device to thenext, 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 deviceon the bus can be designated as the first to receive the timeslot, 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 onthe signal, cost, geographic layout, and the possibility of lineinterference.TWISTED COPPER PAIRTwisted pair copper conductors ranging from 16 to 24 gage arethe most commonly used and the best economic choice as thecommunications media for single building applications. Buslengths up to 4000 feet are common without use of extenders orrepeaters. When repeaters are used, extensions up to three or fourtimes this distance are possible. Serial bus and star wiringconfigurations (Fig. 2 and 3) permit efficient wiring layouts.BMSCENTRALFIBER OPTICFig. 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 LINESSYSTEM/ZONELEVELCONTROLLERCOMMUNICATIONSBUSSYSTEM/ZONELEVELCONTROLLERSYSTEM/ZONELEVELCONTROLLERC1863Fig. 2. Typical Serial Bus Wiring Configuration.COMMUNICATIONSBUSSYSTEM/ZONELEVELCONTROLLERSYSTEM/ZONELEVELCONTROLLERBMSCENTRALSYSTEM/ZONELEVELCONTROLLERSYSTEM/ZONELEVELCONTROLLERSYSTEM/ZONELEVELCONTROLLERC1862Common carrier telephone channels link distant buildings.Telephone line transmissions require either full time dedicatedphone lines or automatic dialing through modems (Fig. 4).REESTABLISHEDTWISTED PAIRCOMMUNICATIONSLINKOPERATIONS-LEVELPROCESSORPHONE LINEINTERFACEMODEMMODEMPHONE LINEINTERFACESYSTEM/ZONELEVELCONTROLLERSC1864Fig. 4. Dialup to Remote Controllers.ENGINEERING MANUAL OF AUTOMATIC CONTROL 175