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Question
We’re designing a PCB using UbiSwitch’s 1000BASE-T ports and would like to swap Ethernet differential pairs to simplify routing.
Application note BB-AN-0004 describes pair-swapping support. Does this allow any arrangement of the four pairs? For example, could we change the pair order from ABCD to CABD?
Can we also reverse the + and − signals within an individual differential pair?
Answer
UbiSwitch supports automatic pair-swap correction for 1000BASE-T, but only within two specific groups: A/B and C/D.
You can swap A with B, and independently swap C with D. You cannot move pairs between those two groups.
Which pair orders are supported?
Starting from the standard order ABCD, the supported arrangements are:
| Pair order | Change | Supported |
|---|---|---|
ABCD |
No swap | Yes |
BACD |
Swap A and B | Yes |
ABDC |
Swap C and D | Yes |
BADC |
Swap A and B, and swap C and D | Yes |
CABD |
Move pairs between the A/B and C/D groups | No |
The two permitted swaps are independent, so you can apply either one or both to simplify PCB routing.
Why is ABCD → CABD not supported?
In CABD, pair C occupies a position belonging to the A/B group, and pair B occupies a position belonging to the C/D group.
UbiSwitch’s correction capability does not support this arrangement. It can correct swaps within each group, but it cannot automatically correct an arbitrary permutation of all four pairs.
How should the pairs connect to the MDI signals?
Keep the pair groups assigned as follows:
| Ethernet pair group | UbiSwitch MDI signals | Allowed flexibility |
|---|---|---|
| A and B | MDIP/N[0:1] |
A and B can exchange positions |
| C and D | MDIP/N[2:3] |
C and D can exchange positions |
When routing, keep A/B within MDI positions 0 and 1, and C/D within MDI positions 2 and 3.
Can I swap the positive and negative signals within a pair?
Yes. UbiSwitch supports polarity correction in 1000BASE-T, so you can reverse the + and − signals within an individual differential pair.
This is separate from swapping whole pairs. Reversing a pair’s polarity leaves it in the same MDI position; exchanging whole pairs changes which MDI position each pair connects to.
For routing flexibility, you can therefore use the supported A/B and C/D swaps together with polarity reversal within individual pairs. The A/B and C/D group assignments must still be preserved.