CAN Termination Resistors

Termination reduces reflections on a CAN bus by matching the cable impedance at the two physical ends of the main bus. For a typical high-speed CAN network, the common starting point is one 120 Ω terminator at each end of the trunk, with short stubs between the trunk and nodes.

Placement

• Place terminators at the two physical ends of the bus, not at every node.
• Avoid long stubs and star wiring unless the network design and transceivers explicitly support them.
• Use the cable and termination value required by the network specification and project design.

Resistance Check

With the network powered down and isolated from circuits that can affect the reading, measure between CAN_H and CAN_L:

• approximately 60 Ω often indicates two 120 Ω terminators in parallel;
• approximately 120 Ω often indicates one 120 Ω terminator;
• a very high reading often indicates missing or disconnected termination;
• a value substantially below 60 Ω can indicate extra parallel termination or another load.

These are diagnostic clues, not pass/fail proof. Transceivers, switchable termination, gateways, and other connected circuitry can change the reading.

Switchable Termination

Some adapters or breakout boards provide a physical switch or software-controlled termination. Enable it only when that device is located at a physical end of the bus and the product documentation confirms the function for the exact model. After changing termination, repeat the resistance check where appropriate and inspect the signal with suitable test equipment if errors persist.

Troubleshooting Checklist

1. Verify that the bus is powered down before a resistance measurement unless the product procedure explicitly requires power.
2. Confirm there are exactly two intended end points on the main trunk.
3. Check connector pinout, CAN_H/CAN_L polarity, common reference, and shield strategy.
4. Compare bit timing across all active nodes.
5. Inspect error counters or error frames after reconnecting the bus.

Do not infer a hardware specification or supported termination-control method from this general guide; use the documentation for the exact adapter model.