May 2021

BY Damien Coleman, Product Manager/ EBI Specialist at Snap-on C ontroller Area Network (CAN) is a data transfer network designed to allow many nodes (modules) to communicate using a standard structure and format. This has the benefit of reducing vehicle wiring and allowing additional vehicle options to be added or removed easily. Troubleshooting such a complex system is reasonably straightforward once the technician has a basic understanding of the network and how data is transferred. This article will provide the technician with the necessary tools to successfully validate a CAN bus signal and help to identify faults where present. Please refer to Fig.1 to see a basic block diagram of a typical CAN Network. Multiple modules are connected to the bus in parallel. The data transfer rate is 500K bits/sec for high speed CAN. That’s 500,000 pieces of data or information per second! The signal is broadcast over two communication wires to ensure signal integrity is maintained, these wires are twisted together and are referred to as CAN High and CAN Low. To ensure messages of the highest priority are transmitted first, an arbitration field is part of the message structure. To ensure optimum signal quality, two fixed value resistors are connected to the bus to condition the signal. These are referred to as terminating resistors. All these characteristics will be described in greater detail as we go. Each module on the network has a CAN controller and a CAN transceiver (see Fig.2) on the CAN chip. A transceiver has the ability to both transmit and receive data. The CAN controller converts data (binary) sent from the microprocessor and sends it to the CAN transceiver. The CAN transceiver converts the binary data into a voltage range and this is the signal voltage observed on the network. Note: Inputs and outputs to a module are still analogue. A component such as a throttle motor will still require supplies, grounds and feedback signals as with a non-CAN vehicle. Referring to Fig.3, The diagram illustrates how a message received by the engine control module is 28 AFTERMARKET MAY 2021 TECHNICAL/SNAP-ON www.aftermarketonline.net Part two: Damain explains how CAN Bus works AUTOMOTIVE COMMUNICATION NETWORKS processed and transmitted on the network for other modules. In this example the engine speed input from the crankshaft position sensor to the engine ECM is also required by the Instrument cluster for the tachometer. The table below shows the expected voltage for both recessive and dominant states on the CAN High and CAN Low wires. State CAN High voltage CAN Low voltage Bit type Recessive 2.5 volts 2.5 volts 1 Dominant 3.5 volts 1.5 volts 0 Using an oscilloscope, the following voltage levels (see Fig.4) can be observed when connected to a network. As mentioned previously both CAN high and CAN low are twisted together to reduce external interference. Fig.5 shows the differential voltage between CAN high and CAN Low during data transfer. With a recessive bit, both CAN high and CAN low voltage levels are at 2.5V so the differential voltage is 0 volts. When a dominant bit is transmitted, CAN high rises to 3.5V whereas CAN low drops to 1.5V. This is a differential voltage of 2 volts. In the event of a transient condition occurring, such as an external voltage spike both CAN high and CAN low will be similarly affected. This means the differential voltage will remain at 2 volts (see Fig.6). This protects the integrity of the message. Looking at Fig.7 and Fig.8, the waveform shows both the signal when CAN high and CAN low are tested with respect to ground and the differential voltage between CAN high and Can low. Example shown from a Volvo XC – 90. Both traces mirror each other. Fig.7 is clean without glitches or spikes, and shows correct voltage, while Fig.8 shows correct differential voltage. Please note that a technician may observe an increased voltage on a trace at the end of a message. (see Fig.9). This is the End of Frame bit and is not a cause for concern. Due to the high data transfer rate a means to dampen signal reflections is required. To achieve this, manufacturers use two 120 Ohm resistors in parallel. Although some vehicle applications have the resistors located externally in the wiring loom, most locate the resistors in two separate modules, as shown in Fig.10. With both resistors equal in resistance and wired in parallel, the total circuit resistance (see Fig.11) is halved. Resistance in Parallel = (Product of resistors)/(Sum of resistors) (120*120)/(120+120)=60 Ohms With CAN bus it must be noted that the message carries the priority, not the module ID. A message with the lowest numerical identifier, most dominant bits (0), can transmit message. All modules listen until the bus is idle again before transmitting. See Fig.12 for an example. Module A: Loses the chance to send out its message first (Red arrow). Module C: Loses the chance to send out its message second (Blue arrow). Module B: Wins arbitration. Fig 1 Fig 2 Fig 3 Fig 4