CAN Bus Termination: Why 120 Ω Goes at Both Ends

Learn the two-end 120 Ω CAN topology, the approximate 60 Ω unpowered check, switchable termination and common wiring faults.

Understand why a Classical CAN trunk is normally terminated at both physical ends, what an unpowered resistance check can tell you, and when a switchable 120 Ω resistor should stay off.

Why CAN bus termination exists

A CAN cable behaves as a transmission line. When the electrical energy in an edge reaches a cable end with a mismatched impedance, part of that energy reflects. The reflected edge can distort the differential waveform and reduce the receiver’s timing and voltage margin.

High-speed Classical CAN installations commonly use cable with a nominal 120 Ω characteristic impedance. A resistor close to that impedance at each far end of the trunk absorbs the travelling edge. Two 120 Ω end resistors appear in parallel to a DC resistance measurement, so an isolated, unpowered bus often measures close to 60 Ω between CANH and CANL.

[120 Ω] ───── trunk ───── node ───── trunk ───── [120 Ω]
                         │
                      short stub

Put termination at the physical ends—not every node

The key word is physical. The two terminators belong at the farthest electrical ends of the main cable. A node in the middle of the trunk normally connects through a short, unterminated stub.

Do not decide termination from node count. A two-node bench still has two cable ends. A ten-node trunk still normally has two end terminations. Adding 120 Ω at a third node lowers the equivalent load to roughly 40 Ω, increasing driver load and reducing differential amplitude.

If a removable node contains one of the two terminators, removing that node also removes the end termination. For field harnesses, place termination so the required two resistors remain present in every supported configuration.

Measure an unpowered bus carefully

Power down every node and discharge the network according to the equipment instructions.
Disconnect test equipment that could add another termination.
Measure resistance between CANH and CANL at an accessible connector.
Compare the result with the documented topology, not with a memorized number alone.

A result near 60 Ω is consistent with two 120 Ω resistors in parallel. Near 120 Ω can indicate one terminator. Near 40 Ω can indicate three. Very low or unstable readings can indicate a short or powered/active circuitry. Integrated bias, protection and other connected electronics can change the reading, so investigate rather than treating these values as an automatic pass/fail test.

When to use switchable 120 Ω termination

An analyzer with switchable termination is useful because the same adapter may be used in two different positions:

Adapter at a new bench endpoint: enable its 120 Ω resistor if it is intentionally one of the two physical ends.
Adapter tapped into an existing, correctly terminated trunk: leave its resistor off.

Mark the switch state in every capture record. “Termination available” is not the same as “termination should always be enabled.”

Symptoms that justify a termination check

Frames appear at a low bitrate but fail at a higher bitrate.
Error counters rise after a cable or node is added.
Oscilloscope traces show ringing or slow dominant-to-recessive recovery.
Communication works at one physical point but becomes unreliable at the far end.

These symptoms are not unique to termination. Wrong bit timing, long stubs, poor grounding, damaged transceivers, connector resistance and cable faults can look similar. Change one variable at a time and preserve the before/after trace.

Apply this to a pilot setup

The planned CAN Isolated interface includes a switchable-termination target, but the final switch behavior and electrical limits remain subject to EVT/DVT records. Join the pilot only if you can describe the topology you need to validate.