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CAN Bus Troubleshooting


CAN Bus Troubleshooting Guide

Debugging ECUs with Automated Decode
& Trigger

What is the CAN bus?

The Controller Area Network (CAN) is a bus structure originally designed for automotive applications, but it has also found its way into other areas. The CAN bus is a balanced (differential) two-wire interface running over different types of cables depending on the application. Several different data rates are defined by the standard with 1 Mb/s being the fastest for CAN. The more recent variant, CAN FD (Flexible Datarate), has increased transmission speeds up to 10 Mb/s, while also increasing the package payloads data field max length. To maintain signal quality, the bus requires 120Ω termination resistance at each end of the cable to ensure impedance matching.

Since its introduction in the 1980’s CAN bus has been used to transfer data between electronic control units (ECUs) and with vehicle sensors.

CAN bus in the connected car

The amount of data, generated, transmitted, and received in automobiles has increased significantly in recent years, and will continue to rise in the future. Today, many automobiles contain more than 80 Electronic Control Units (ECUs), which are connected via different bus networks. In the years to come, we expect this number to exceed 100 as some luxury cars are already using 150 ECUs.

Greater integration between vehicle subsystems means that, in addition to handling sensor and actuator signals, many ECUs communicate over more serial buses at a time. For example, it is very common for an ECU to communicate over both CAN (for critical systems) and LIN (for lower-priority controls, such as windows and mirrors).

CAN bus troubleshooting

Because the safety of the driver relies on these systems communicating properly, it’s essential to ensure a CAN bus is configured properly. Fortunately, troubleshooting CAN bus issues is easy when you have the right tools. In this short guide, we’ll cover troubleshooting tips and the instruments you need to accomplish the task quickly.

Debug the decoded CAN bus protocol

Decoding serial bus protocols manually is time-consuming work, and it’s easy to make mistakes. Tektronix’s automotive bus decoding and triggering packages provide straightforward, automated decoding and triggering for popular ECU buses like CAN, CAN FD, LIN and FlexRay.

Decoding a CAN FD signal

CAN FD signal on Channel 1 being decoded and displayed as a bus waveform (bottom) and event table (top)

Troubleshooting Signal Faults

CAN bus protocol decoding is just the beginning. Troubleshooting problems when a bus doesn’t work, or worse, works intermittently, goes beyond the bus traffic and into the realm of signal integrity.

Often these problems, emanating from signal integrity issues in the physical layer like crosstalk, noise, and improper termination, are most effectively detected through waveform analysis.

An oscilloscope allows the engineer to examine the analog bus waveforms to evaluate signal quality and noise, as well as study multiple signals to look for interactions and identify crosstalk.

CAN bus protocol decoding

The CAN bus requires 120 Ω termination. An unterminated bus will result in poor signal quality (upper signal).

Visualizing Multiple Channels/Sensors/Actuators

An ECU module showing communication with CAN buses

ECUs communicate via CAN, LIN, and other buses, and directly with sensors and actuators.

With the complexity of the multi-bus and multi-sensor/actuator systems it’s often difficult to get an overview of the working environment.

Most of Tektronix’s scopes allow the user to view multiple buses and control signals simultaneously. Mixed signal oscilloscopes enable the use of digital channels for bus decoding, freeing up analog channels for evaluating signal quality. The 5 Series MSO is especially good at providing visibility into complex ECUs, thanks to its high channel-count, FlexChannel inputs, and large HD display.

Facing challenges in other connected car applications?