Wiring, Termination, and Reliability
Welcome to Part 4 of our CAN Bus series. In this post, we take a closer look at the physical layer of a CAN network, which plays a vital role in ensuring reliable communication. While higher-level protocols often get the spotlight, the physical layer is what allows CAN to operate consistently in demanding environments. We’ll cover the basics of twisted-pair wiring, 120-ohm termination, common physical layer mistakes, how to diagnose them using CANLab, and key EMI considerations for automotive applications.
The Importance of the Physical Layer
CAN Bus is built to handle noisy, real-world environments like those found in vehicles, factories, and industrial machinery. A robust physical layer helps prevent corrupted frames, communication delays, and unpredictable failures. Most persistent CAN issues can be traced back to wiring faults, improper termination, or noise-related problems.
Twisted-Pair Wiring: Maintaining Signal Integrity
CAN Bus uses a differential signaling scheme. This means that the bit state (0/1) is based on the voltage difference between CAN_H and CANL. Twisting these wires together helps:
Cancel out electromagnetic interference (EMI)
Maintain balanced signal transmission
Minimize the effects of crosstalk
Recommendation: Use 22–24 AWG twisted pair wire. Keep the twist consistent (typically 33–50 twists per meter). In electrically noisy environments or long cable runs, consider using shielded twisted pair (STP) to further improve signal integrity.
120-Ohm Termination: Preventing Reflections
Termination resistors are critical for preventing signal reflections that can distort communication. According to the CAN standard (ISO 11898):
Install one 120-ohm resistor at each end of the main CAN bus segment
Do not add termination at intermediate nodes
The total load resistance between CAN_H and CAN_L should measure approximately 60 ohms
Signs of Improper Termination
Weak or distorted voltage levels on CAN_H and CAN_L
Communication errors or dropped frames
Devices failing to transmit or receive messages
Common Wiring Mistakes and How CANLab Can Help
Some common Physical layer issues:
| Issue | How to Detect It |
| Termination | This needs to be measured with the devices powered down. There should be approximately 60ohms between CAN_H and CAN_L. Over Terminated => 40ohoms Under Terminated => 120ohms |
| Open connections | This needs to be measured with the devices powered up. Measure the voltage between CAN_H and ground it should be around 2.5volts. Repeat for CAN_L it should be 2.5 as well. You may see the voltage jump a bit due to traffic on the CAN bus. |
| Incorrect wiring or swapped pins | Look for periodic Error Frames in the CAN trace window. This can be an indication that one device is wired incorrectly as Error Frames will occur when that device is attempting to transmit. |
| Unshielded or poorly routed wiring | Look for sporadic Error Frames in the CAN Trace window. |
Tip: CANLab’s physical layer monitoring tools make it easy to visualize bus voltage levels, detect signal reflections, and troubleshoot wiring faults that are hard to catch otherwise.
EMI in Automotive Environments
Vehicles are full of EMI sources including motors, solenoids, ignition systems, and switching power supplies. EMI can cause false edges, bit errors, and corrupted frames on a poorly protected CAN line.
EMI Mitigation Tips
Use twisted-pair or shielded cable, especially in long or sensitive segments
Avoid routing CAN lines parallel to high-current cables or near power converters
Keep stub lengths short (ideally under 30 cm)
Ground shielding at a single point to prevent ground loops
Proper cable routing and layout are just as important as correct electrical termination.
Conclusion
The physical layer is the unsung hero of the CAN Bus system. Correct wiring, termination, and EMI protection form the foundation for stable and predictable communication. With the help of tools like CANLab, engineers can catch subtle electrical issues early and build more robust CAN networks.
Next in the Series: Capturing and Debugging CAN Traffic
In Part 5, we’ll explore how to effectively capture and analyze CAN traffic during development and testing. Topics include:
Using ATI CANLab: Setup, filtering, and triggering
Comparison with free tools: SavvyCAN, Wireshark + SocketCAN
How engineers validate CAN during development and system integration
Stay tuned to learn how professionals approach real-world debugging and ensure system reliability through targeted CAN analysis.
