Understanding Smart Sensors - Nomads.usp

Communications for Smart Sensors 131Typical configuration of large open control systemOperatorworkstationsEngineers’workstationsSupervisorysystemsOperatorworkstationsControllers(DCS, PLC)Control network or high-speed fieldbusSensors andactuatorsSensors andactuatorsAC and DCdrivesHigh-speedfieldbusSensors andactuatorsLow-speedfieldbusLow-speedfieldbusFigure 6.5 Fieldbus control system architecture. (After: [10].)However, fieldbus is intended to handle a larger amount of data. All three networksare designed for real-time operation, but each handles a different amountof transmitted data.The CAN protocol is used to implement both SDS and Device-NET, but the two systems are not interoperable. The difference betweenthem is in the physical layer, which is not defined in the CAN specification,allowing users to implement different approaches.6.5.2 LonTalk ProtocolThe LonTalk protocol, developed by Echelon Corporation, has receivedconsiderable support for several industrial and consumer applications. The protocoldefines all seven layers of the OSI model. It uses differential Manchestercoding with data packet length of 256 bytes and can operate up to a maximumspeed of 1.25 Mbps. The arbitration is predictive carrier sense, multiple access(CSMA) and has collision detection and priority options. The LonTalk protocolcan be used to support sensor network to fieldbus requirements [11]. ALonWorks system works on a peer-to-peer basis and does not require a host