Understanding Smart Sensors - Nomads.usp

128 Understanding Smart SensorsInterframespaceData frameInterframespaceStart of frameArbitration fieldControl field Data field CRC fieldACK fieldEnd of frameFigure 6.3 CAN data frame.remote transmission request (RTR) bit is sent recessive. Otherwise, the remoteframe is identical to the data frame [8].Error detection and error signaling are possible because CAN has a varietyof built-in error counters that help contain errors and prevent nodes that havefailures from restricting communications on the bus. A faulty transmittingnode always increases its error counter more than other nodes. Therefore, thefaulty node becomes the first to go “bus off.” Automatic retransmission of corruptedmessages minimizes the possibility of an undetected message. CAN alsohas the ability to distinguish between temporary errors and permanent nodefailures, which makes it ideal for the high-noise environments that occur inautomotive and industrial applications [7]. The error message frame is shownin Figure 6.4. Bit error, stuff error, CRC error, form error, and acknowledgeerror can be detected. The error frame contains the superposition of error flagssent by various nodes on the network and the error delimiter. Both passive andactive error codes are possible in CAN, depending on the status of the node [8].The CAN protocol does not address the physical layer requirements.The transceiver is not specified, but rather it is left to the user’s networkDataframeError frameInterframe space oroverload frameError flagSuperpositionof error flagsError delimiterFigure 6.4 CAN error frame.