Checking 100BASE-T1 Master and Slave Roles During Link Startup

A 100BASE-T1 link needs a compatible master and slave timing arrangement. These roles do not identify which application sends commands or which endpoint has higher processing authority. Confirm the physical layer roles before investigating a link that remains down despite plausible wiring and power.

Identify the role selection method

The TI DP83TC812 evaluation guide documents master and slave mode selection for its single pair Ethernet interface. The implementation uses device specific configuration, so use the corresponding data sheet and supported initialization sequence for the fitted PHY.

Record the intended role at each end and how it is established by hardware or software. Do not infer the active role solely from a jumper label if firmware can change the setting later.

Read the state after initialization

Capture role configuration, operating mode, reset state, and link status after normal startup completes. If a register change requires a restart or reset, follow the documented sequence and verify that the setting persists as expected.

Check both endpoints during the same trial. Two separately tested boards can each work with a laboratory partner yet fail when connected together because both were configured for the same role.

Separate line side and host interface issues

Establish whether the single pair link itself comes up before testing packet exchange through the MAC interface. A working line link does not prove correct host clocking or data interface configuration, while an absent line link should not initially be diagnosed through application software alone.

Include the required reference clock and power conditions in the startup record. Role configuration cannot compensate for a missing or unsuitable reference clock.

Exercise realistic startup order

Test simultaneous power up and each endpoint starting first. Include a reset or brief restart of one endpoint while the other remains operating. Measure link establishment and confirm that the recovered active roles remain compatible.

After link establishment, exchange traffic in both directions and inspect error counters. The master and slave distinction concerns timing behavior; both ends can participate in the intended bidirectional data connection.

Save endpoint identities, intended and read-back roles, initialization sequence, startup order, and link timing. Use those records when replacing one endpoint or updating its driver. A satisfactory result demonstrates repeatable startup with a deliberate role pairing. Relabeling one board as the system controller or observing successful traffic in only one boot sequence does not establish that the physical layer configuration is correct across the product’s supported power and reset scenarios.