KSZ9031RNX Polarity Reversal Feature - What are the important things to keep in mind?

For 10BASE-T, 100BASE-TX, and 1000BASE-T, differential pin pairs A, and B are used for auto-negotiation.
Differential pin pairs C, and D are used for 1000BASE-T only.

For 100BASE-T (uses MLT-3 signals) and 1000BASE-T (uses PAM-5 signals) that are symmetric about +/- pin swing (i.e., the same signals are detected by the receiver regardless of the polarity), having matching polarity is irrelevant.

Receive +/- polarity detection and correction is a basic feature that is available with all latest PHYs and Switches with integrated PHYs, including KSZ9031RNX. The important thing to note is that the Legacy 10BASE-T PHYs and 10BASE-T Switches with integrated PHYs do not have received +/- polarity detection and correction feature. 10BASE-T PHY and Switches use differential pin pairs A, and B (as discussed above).

In summary:

  1. For KSZ9031RNX receive, all signals (Auto-negotiation, 10BASE-T, 100BASE-TX, and 1000BASE-T), regardless of whether the link partner transmitted +/- polarity is reversed or not, can be detected and received correctly.
  2. For KSZ9031RNX transmit, if the transmitted +/- polarity is reversed, it is a problem only for pairs A, and B when the link partner is a legacy 10BASE-T PHY or 10BASE-T Switch with integrated PHYs.
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Caution must be observed when implementing polarity reversals on pairs A, and B. The link partner (for example, legacy 10BASE-T PHYs, and Switches) may not have the feature of receive polarity detection and correction.

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