Practical Industrial Data Networks:Design, Installation and ...

174 Practical Industrial Data Networks: Design, Installation and Troubleshooting12.11 The application layerThe CAN specification does not dictate how information within the CAN message framefields are to be interpreted – that was left up to the developers of the DeviceNetapplication software.Through the use of special identifier codes (bit patterns) in the identifier field, master isdifferentiated from slave. Also, sections of this field tell the slaves how to respond to themaster’s message. For example, slaves can be requested to respond with informationsimultaneously in which case the CAN bus arbitration scheme assures the timeliestconsecutive response from all slaves in decreasing order of priority. Or, slaves can bepolled individually, all through the selection of different identifier field codes. Thistechnique allows the system implementers more flexibility when establishing nodepriorities and device addresses.12.12 Troubleshooting12.12.1 IntroductionNetworks, in general, exhibit the following types of problems from time to time.The first type of problem is of an electronic nature, where a specific node (e.g. anetwork interface card) malfunctions. This can be due to a component failure or to anincorrect configuration of the device.The second type is related to the medium that interconnects the nodes. Here, theproblems are more often of an electromechanical nature and include open and shortcircuits, electrical noise, signal distortion and attenuation. Open and short circuits in thesignal path are caused by faulty connectors or cables. Electrical interference (noise) iscaused by incorrect grounding, broken shields or external sources of electromagnetic orradio frequency interference. Signal distortion and attenuation can be caused by incorrecttermination, failure to adhere to topology guidelines (e.g. drop cables too long), or faultyconnectors.Whereas these are general network-related problems, the following ones are veryspecific to DeviceNet:• Missing terminators• Excessive common mode voltage, caused by faulty connectors orexcessive cable length• Low power supply voltage caused by faulty connectors or excessive cablelength• Excessive signal propagation delays caused by excessive cable lengthThese problems will be discussed in more detail.12.12.2 Tools of the tradeThe following list is by no means complete, but is intended to give an overview of thetypes of tools available for commissioning and troubleshooting DeviceNet networks.Whereas some tools are sophisticated and expensive, many DeviceNet problems can besorted out with common sense and a multimeter.