meloon
Posts: 1
Joined: Mon May 17, 2021 1:57 pm

CM4 ETH pairs swap

Tue May 18, 2021 12:19 pm

I maked custom board based on RPi CM4. ETH connector is on top of PCB and SOM is on bottom side, so I decided to swap pairs to make track routing easier . BCM54210 datasheet is not available, but CM4 datasheet specified that PHY can detect pair swap and line swap. First prototype have ETH pairs swapped as follows:
CM4 - RJ45 socket
TRD0 - TRD2
TRD1 - TRD3
TRD2 - TRD0
TRD3 - TRD1
Ethernet doesn't work on straight cable, but when I swapped pairs in RJ45 jack to reverse changes on PCB it works fine.
Is there any limitation regarding to pair swap or maybe I made something wrong? What about polarity swap in single pair?
Attachments
RPI4CM_ETH.PNG
RPI4CM_ETH.PNG (189.29 KiB) Viewed 200 times

dp11
Raspberry Pi Engineer & Forum Moderator
Raspberry Pi Engineer & Forum Moderator
Posts: 520
Joined: Thu Dec 29, 2011 5:46 pm

Re: CM4 ETH pairs swap

Tue May 18, 2021 5:24 pm

I've had a quick go in the lab swapping pairs over in a cable ( not quite the same as on the PCB) and it appears to work. e.g orange to green and brown to blue.

LakshmiN
Posts: 52
Joined: Mon May 13, 2019 7:50 am

Re: CM4 ETH pairs swap

Wed May 19, 2021 10:05 am

I've also had a similar issue where,
CM4 RJ45 jack
TRD0 TRD0
TRD1 TRD1
TRD2 TRD3
TRD3 TRD2.
.
I should expect it to work.

but Ethernet doesn't work on straight cable

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