b'Combining the use of clues from the characteristics of CAN signaling on theSo yes, it IS important to check for that Transport Layer with scope waveforms captured on the Physical Layer provides60 ohms, but the often-arduous process of de-powering the vehicle, putting the technicians a way to catch, identify, and isolate the fault quickly. Consideringnetwork to sleep, and takingthis measurement is often not needed when we can that when performing network diagnosis on Porsche vehicles of any era, oneobserve and measure certain characteristics of a CAN waveform such as in Figure 6, to must disassemble the vehicle at will just to isolate and test said modules, usingdetermine whether or not, with a fair degree of accuracy, that the bus lines are the waveform characteristics to guide our diagnostic direction proves to be hugeterminated and impedance balanced. when put into daily practice. First, some testing rules of thumb for CAN that we use on the hot line when conventional How Does the Physical Layer Work?diagnostics fail.The CAN bus and automotive networks are closed radio systems. Thats right, itRegardless of DTC chart steps, ALWAYS be sure to test the network is live, connected and working.is a digital logic defined radio system with a balanced antenna, which is the 120 ohm terminated twisted pair. The balanced bus lines form the antenna of theWe find that often the network diagnostic misses occur when we try to test multiple system, over which all of the individual CAN nodes broadcast their messages.communication codes through OEM DTC charts. Specifically, what is often done here, The CAN node inside the module is an actual digital radio transceiver, with aregardless of manufacturer, is to provide the technician with static tests (system is not transmitter and receiver built in.live and running) and, in many cases, we are asked to disconnect a modules connectors to check module supply voltage potential.The twisted pair wire scheme, balanced with the 120 ohm termination resistorsThe major diagnostic miss at this test step occurs because when we are testing for seen in Figure 2 is used primarily for two reasons; first, the balanced twistedvoltage potential with the connector disconnected, we are not loading the module, node, pair provides a high amount of common mode noise rejection from outsideor system, so we miss 90% of the main opportunity to find (catch) the fault occurring sources such as RFI and EMI interference, which makes it highly fault tolerant.while the car is in front of us. In our experience, this is because faults in communications Second, the electrical characteristics of the antenna are designed to work withnetworks will generally only occur when the system is fully connected, loaded the CAN transceivers to provide perfect differential signal timing and aelectrically, and pushing current. minimum of data collisions. The electrical design(60 ohm balanced antenna) prevents destructive magnetic reflection, which is the electrical mechanism thatIf we pin up the lab scope and watch the system actually working, we can see voltage can destroy the CAN transceiver over time. drops in supply voltage or anomalous data packet activity onthe bus lines when it is occurringand have a chance at tracking down the problem.In no way are we recommending here that you do circumvent OEM DTC charts, voltage checks, etc., but we have found that, hands down, relying solely on this pathway will inevitably lead to the big miss and a repeat visit most of the time. Nothing beats the speed and accuracy of testing network systems while the system is electrically loaded and broadcasting messages.Figure 7 shows a fantastic example of this exact point, load the system and test it live; this is when the system will fail.Hey Stop Hogging All the Seats on the Bus!We often see one single node that is having an issue, and when it does, that node can repeat its message over and over, refusing to let other modules broadcast on the bus. We call this hogging the bus. The electrical characteristics of this type of fault are shown in 1 2 Figure 8.was settingBut in order to understand this waveform characteristic, one must transmittedstudy a bit into the workings of the Transport Layer, to gain clues as inidcated datato whether or not this is happening. (greater than 10The structure of the data packet is shown back in Figure 5. Note that each bit space is identified and has a distinct purpose. To find the module that is faulting and hogging the bus, we need to focus on the CRC fields, the End of Frame bits, and the ACK bit, or acknowledgment bit. The ACK bit tells us that all nodes on the network that processed that message report that they are happy with the modules data, or not. 9 8'