What this test proves

Inspect high-speed CAN physical signaling. A plausible voltage pattern does not prove message content or module function.

Tools

  • Two-channel automotive scope or approved differential probe
  • Matched probes and network topology
  • Optional CAN decoder

Preparation and precautions

  • Identify the correct high-speed CAN segment and safe reference.
  • Use high-impedance probes and avoid long added stubs or damaged terminals.

Connections

  • Observe CAN High and CAN Low relative to the approved common reference using suitable scope inputs, or measure their difference with a rated differential probe.

Equipment settings

  • DC coupling with a range that includes both line levels and possible disturbances. Start with a short microsecond-scale window to inspect bits, then expand for traffic. Set sample rate comfortably above the bus rate; CAN FD requires more bandwidth during its data phase.

Test sequence

  1. Wake the network in the specified manner.
  2. Observe both lines and the differential signal if available.
  3. Inspect edges, dominant/recessive states and disturbances.
  4. Compare another point on the same segment to localize deterioration.

Expected signal / readings

For typical high-speed CAN, recessive lines are near a common bias around 2.5 V. Dominant CAN High rises and CAN Low falls, commonly toward 3.5 V and 1.5 V. These are illustrative levels, not universal acceptance limits. Look for complementary transitions without severe ringing or clipping.

Interpreting results

  • A stuck dominant level, missing complement or excessive ringing warrants wiring, termination and node checks.
  • No traffic can mean a sleeping segment; validate wake state. Decoding and module scans are needed to assess message-level faults.

Common mistakes

  • Applying high-speed CAN levels to another physical layer
  • Using insufficient sample rate
  • Declaring the whole network healthy from one capture