Reviewing CAN Stub Reflections Against the Sampling Window

A CAN branch can create reflections even when the main bus has the intended termination. The effect depends on the branch’s electrical delay and the signal transition, not only on the nominal bit rate. Review stub geometry together with the transceivers and controller timing.

Describe the complete network

TI’s CAN physical-layer application report explains how unterminated stubs can reflect signals and why those reflections must settle before the relevant sampling interval. Its illustrative length calculations depend on stated edge and cable assumptions, so they are not universal limits for every CAN network.

Draw the main path, every branch, connector and end termination. Record cable type and approximate propagation delay. Include PCB routing between the connector and transceiver as part of the branch rather than measuring only the visible cable.

Compare time scales

Estimate the round-trip delay along each stub and compare it with the transceiver’s transition behavior. Consider the fastest relevant edges across permitted components and operating conditions. A slower communication rate does not necessarily slow those physical edges.

Then review the controller’s sample timing and the total propagation budget. For networks using different arbitration and data-phase rates, evaluate the applicable phases separately. Do not apply an old single-rate example to a faster data phase without checking the current devices and network requirements.

Find the demanding topology

Identify transmitters whose positions create the most unfavorable combination of branches and receiver locations. Include optional nodes and disconnected modules where the wiring remains attached. A short test cable with one nearby receiver can conceal behavior in the full installed arrangement.

Check whether the proposed repair changes the network architecture rather than only the symptom. Adding an extra terminating resistor to a branch can alter total loading and is not a generic remedy for a long stub. Review termination changes against the complete intended topology.

Measure at the receiving locations

Capture differential and individual line waveforms near relevant receiver pins using a suitable probing arrangement. Observe threshold crossings and settling around the controller’s sampling interval. Record communication error counters alongside the waveforms and test representative traffic patterns.

Keep the topology drawing, controller timing, transceiver models and cable configuration with the results. State the verified node and branch combinations. Repeat the assessment when adding a connector, extending a harness or substituting a faster-edge transceiver. The accepted branch arrangement should follow a timing and waveform review of the actual network, rather than a single length rule detached from the conditions that make it valid.