Why Are Termination Resistors Needed?

A Controller Area Network (CAN) requires termination resistors to ensure signal integrity, prevent reflections, and maintain proper bus impedance.

Here’s why they are needed and where to install them:

Why Are Termination Resistors Needed?

1. Prevent Signal Reflections:

   • CAN is a differential signaling system where signals travel along a twisted-pair cable (CAN_H and CAN_L).
   • If the network is not terminated correctly, signals can reflect at the ends of the bus, causing interference, data corruption, or unstable communication.

2. Maintain Impedance Matching:

   • A standard CAN bus has a characteristic impedance of 120 ohms.
   • Without termination, the impedance mismatch at the cable ends can cause signal distortions, leading to communication errors.

3. Stabilize the Recessive State:

   • CAN uses dominant (0) and recessive (1) bit states.
   • The termination resistors provide a path for current flow, ensuring that the bus returns to a proper recessive state when no device is transmitting.

Where to Install Termination Resistors?

• At Both Ends of the Bus:

  • The CAN bus is a linear topology (not a star or ring network).
  • A 120-ohm resistor should be placed at each physical end of the network.
  • This results in an effective parallel impedance of 60 ohms, which matches the bus impedance.

• Not at Individual Nodes:

  • Only the devices at the extreme ends of the CAN bus should have termination resistors.
  • Intermediate nodes should not have termination resistors, as this could lower the total impedance and cause communication issues.

Special Cases

• Stub Lengths Should Be Short:
  • Avoid long stubs branching off the main CAN bus, as they can create reflections and distort signals.
• High-Speed CAN (ISO 11898-2) and Low-Speed CAN (ISO 11898-3) Differences:
  • High-speed CAN (used in automotive and industrial applications) requires termination resistors at both ends.
  • Low-speed CAN (fault-tolerant CAN) uses internal termination resistors inside the nodes, and additional termination is not always required.

Conclusion

Termination resistors are crucial for ensuring reliable communication on a CAN network. They should always be 120 ohms and installed at each end of the CAN bus to prevent reflections and impedance mismatches.

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