Checking the LIN Commander Pullup and Recessive Voltage Path

A LIN transceiver may be suitable for more than one node role, but the external bus network is not necessarily identical for those roles. The commander pullup path establishes important recessive-state behavior. Review that path explicitly instead of assuming that the transceiver’s internal bias is sufficient for every installation.

Identify the intended node role

The TI TLIN1029-Q1 data sheet calls for an external pullup resistor and series diode to the supply when the device is used as a commander node. The selected network and device conditions should follow the applicable LIN implementation requirements.

Record which node provides that external path and which nodes are responders. Confirm that configurable boards do not accidentally populate the commander network at several locations.

Trace the recessive-current path

Draw the supply, diode, resistor, bus, and return path. Verify diode orientation and the supply used for the pullup. Include protection devices and any connector or harness resistance relevant to the voltage budget.

Measure the recessive bus voltage at both ends of the installed network. Compare it with each receiver’s applicable thresholds using its own supply and ground reference.

Check dominant loading

When a node drives dominant, the pullup network contributes current that the driver must sink. Calculate and measure that current under the highest relevant supply condition and compare it with the supported driver operation.

For example, an unintended second commander resistor can increase dominant current while leaving a no-traffic bus voltage looking normal. A recessive-only measurement would miss the additional loading.

Exercise supply and power states

Test the permitted combinations of powered and unpowered nodes. Observe whether the bus or pullup path feeds an inactive supply through an unintended route.

Include startup and wake transitions using the actual harness and node population. Record the transceiver mode controls and controller readiness so a mode error is not confused with an incorrect pullup.

Capture complete frames at the most demanding supply and loading conditions. Check receiving levels and timing rather than relying only on a controller’s successful frame count.

If the board supports selectable roles, verify the documented population or switch setting for each configuration. Make the role change visible in the circuit record so it is not left as an informal bench modification.

Save the role assignment, pullup schematic, voltage and current measurements, and frame captures. The completed review should establish a defined recessive path and acceptable dominant loading for the whole network. Installing a LIN transceiver alone does not establish the electrical configuration required for a commander node.