Practical troubleshooting of CAN Bus

CAN Bus is commonly used in automotive applications for communication between various VCUs and components within a vehicle.

Physical representation of CAN Bus -

Electrical characteristics of CAN Bus -

CAN transmits differentially over two lines, CAN High and CAN Low. There are two logic states:

• Dominant – Logic 0
• Recessive – Logic 1

Termination: Termination resistors are placed at both ends of the CAN bus line to match the characteristic impedance of the transmission line and minimize signal reflections. Common termination values are 120 ohms, and these resistors ensure that the signal propagates without distortion.

Stub Length: Stub length in a CAN bus refers to the length of any branches or extensions connected to the main bus line. Keeping stub lengths as short as possible is important for maintaining the signal integrity and overall reliability of the CAN bus network. Here's why,

1. Signal Reflections: Longer stub lengths can cause signal reflections, which occur when a signal encounters impedance mismatches at the junctions between the main bus line and the stubs. These reflections can distort the original signal, leading to errors in data transmission and reception. By keeping stub lengths short, the likelihood of signal reflections is reduced, improving communication reliability.
2. Signal Degradation: Longer stub lengths can increase signal attenuation, where the signal weakens as it travels along the stub.
3. Timing and Synchronization: Longer stub lengths can introduce delays in signal propagation, affecting the timing and synchronization of communication between nodes on the CAN bus network. Shorter stub lengths reduce propagation delays, ensuring that nodes can accurately synchronize with the bus and communicate within the specified timing constraints.
4. Electromagnetic Interference (EMI): Longer stub lengths can act as antennas, picking up electromagnetic interference (EMI) from the surrounding environment. This EMI can corrupt the CAN bus signals, leading to communication errors and data loss. Shorter stub lengths minimize the antenna effect, reducing the susceptibility to EMI and improving noise immunity.

Please click here for more about termination resistor and reflection.

How to check the CAN Bus is perfectly fine?

1. Visual Inspection: Start by visually inspecting the CAN bus wiring, connectors, and termination resistors for any signs of damage, corrosion, or loose connections.
2. Check Termination : Termination resistance of a CAN bus, which typically should be 120 ohms (60 ohms at each end). The resistance reading on the multimeter should ideally be around 60 ohms
3. Signal Voltage level : Check the voltage levels of the CAN High (CANH) and CAN Low (CANL) lines in a CAN bus, you can use a multimeter set to measure voltage. In idle case both CANH & CANL voltage is same as 2.5V.
4. A resistance reading of more than 20k ohms across the CAN High (CANH) and CAN Low (CANL) lines of a CAN bus without termination resistors suggests an open circuit or a high resistance fault in the bus line.
5. In a properly functioning CAN bus system, the CAN High (CANH) line should always have a higher voltage than the CAN Low (CANL) line. This voltage differential is necessary for proper signal transmission and reception within the CAN bus network.If you observe that the voltage on the CANH line is lower than the voltage on the CANL line, it suggests a potential wiring issue where the CANH and CANL lines may be reversed or improperly connected at some point in the network.

Errors in Controller Area Network (CAN) bus system:

1. Bus-Off Error: A bus-off error occurs when a node enters a bus-off state due to excessive error conditions. In this state, the node is unable to participate in further communication on the bus until it is reset or reinitialized. Bus-off errors typically result from severe communication disturbances or hardware malfunctions. The "bus-off" state is reached when the error counter is higher than 255 error points. In this state the CAN-controller is disconnected from the bus.
2. Bus Heavy Error: CAN bus network is experiencing a heavy load, leading to communication issues and errors. Check the baud rate.
3. Bus Light Error: Check the hardware adapters, Cables and software configuration. if we only have one CAN-Node (the PCAN-USB device), the CAN protocol will generate errors, because messages cannot be delivered, i.e. there is no CAN communication between nodes.
4. Passive errors : Passive error on CAN bus can be occur due to Bit Error, Stuff Error, CRC Error etc. Check terminating resistor and node lengths.