Ethernet PCB layout - swapping signals or pairs for easier layout

[daloev]

[edited to correct some information]

I'm layout out a board with a Raspberry Pi CM4, so it is 1G Ethernet. A magjack with the tab on the bottom lays out easily, with no signals crossing, but I would like a tab on the top in my particular situation. Because the cable is oriented differently, the pinout on top-tab jack is the reverse of the pinout on bottom-tab jack, and now everything crosses. I believe that it is ok to swap certain signals to make the PCB layout easier, and I would like to review what is possible.

Swapping + and - within a pair is ok. A Microchip article (https://microchip.my.site.com/s/article/Can-the-Ethernet-transformer-pairs-be-swapped) says polarity detection and correction is standard in PHYs these days.

Is swapping entire pairs ok with certain limitations? Suppose a connector has this layout

PCB		Cable
Pin 1: TD1+		J1: TX1+
Pin 2: TD1-		J2: TX1-
Pin 3: TD2+		J3: TX2+
Pin 4: TD2-		J6: TX2-
Pin 5: TD3+		J4: TX3+
Pin 6: TD3-		J5: TX3-
Pin 7: TD4+		J7: TX4+
Pin 8: TD4-		J8: TX4-

A crossover cable exchanges TD1 and TD2, and may exchange TD3 and TD4, so those swaps are fine. Exchanges between those 2 groups (e.g. TD1 with TD3) will mess up auto negotiation. If you do such a swap and manually set the speed to 1G on both ends so you don't need auto negotiation, would gigabit Ethernet still work?

I don't need PoE, but how would PoE affect it? From Wikipedia (https://en.wikipedia.org/wiki/Power_over_Ethernet), it looks like swapping pairs or swapping signals within a pair would mess up 10/100 mode POE, exchanging DC+ and DC-. However, ajb says decent POE implementations have a diode network to handle any combination of pair polarities, so PoE isn't a consideration when swapping signals or pairs.

[ajb]

Link negotiation happens on the A (pins 1&2) and B (3&6) pairs, so those should not get swapped with the C and D pairs for that reason.  Swapping A with B and/or C with D should be fine. 

I don't need PoE, but how would PoE affect it? From Wikipedia (https://en.wikipedia.org/wiki/Power_over_Ethernet), it appears swapping pairs or swapping signals within a pair would mess up 10/100 mode POE, but 4-pair gigabit POE would work if you only exchange TD1 with TD3 and TD2 with TD4. Swapping + and - within a pair is fine for 4-pair gigabit POE because both wires in a pair are either DC+ and DC-

Any decent PoE PD implementation should use an input diode network (a bridge rectifier for two-pair versions, or a pair of bridge rectifiers for four-pair versions) to handle any combination of pair polarities.  That's independent of how the pairs are assigned at the PHY.