Ethernet PCB Devolopment Teensy 4.1


New member
Hi there,

I designed a PCB-Board to transfer informations from a USB-Scanner via MQTT to a Server. At first glance, this is not a complex task.
But: I gradually realized that Ethernet is not as trivial as I thought it was.
As a basis for my PCB design I used the Ethernet module available on the PJRC site:
Unfortunately, my design is not working and I just can't find the problem. Hopefully there are people here on the forum who are pro's in the subject and can help me! Regarding to my PCB-Design: I know that I should not have placed a ground area under the RJ45 jack. Furthermore I know that it is very suboptimal that the VCC line runs directly underneath the RJ45 jack.
I have attached the PBC design, the schematic and the datasheet of the used RJ45 jack (Sorry, its in german).

I am very happy about help! Thanks a lot in advance!



  • M100_T1D_32E4N_DB_DE.pdf
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  • PCBBack.png
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  • PCBFront.png
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  • SchematicScanStation.pdf
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Which ethernet magjack did you solder to your PCB?

Design looks correct for Cetus J1B12111CCD or Link-PP LPJ4012AHNL (the 2 possible parts used on the ethernet kit - PJRC has used both of these parts over the last 2 years - the PCB we ship today has both parts numbers printed).

But your message has a datasheet for Weidmüller M100_T1D_32E4N, which has different pinout.

Ethernet magjacks have many different pinouts. There is no official or de-facto standard.

During Teensy 4.1 development, we used UDE RB1-125BAG1A, which has a completely different pinout from Cetus and Weidmüller.

But the UDE part was unavailable when Teensy 4.1 started shipping, so the PCB was redesigned for the Cetus part.
Oh wow...that's not fair! I've spent hours for finding the problem and you are solving the problem in minutes. :p
Thanks a lot Paul! So my fault was the pin-mapping.
One more questing: In our case T+ of the magjack has to be connected to T+ of the Teensy-Board? Because in UART its Tx to RX.
Hello again, I have another question:
In your old version with UDE magjack you've implemented 4 capacitors. What are these for? And why did you reduce the capacitor number in the newer version?
The old PCB has 3 capacitors and 1 resistor. But it's really 2 pairs of parts in parallel.

The ones joining the grounds were completely unnecessary.

The other pair became just a single 0.1uF in the final version. But early on, it wasn't known (at least to me) whether a single capacitor would be good enough, or if a large cap and small higher frequency one would be needed.

And FWIW, I did spend several minutes following the wires on your PCB and coming back again and again to the conclusion it really ought to work. Wasn't until I closed the windows with copies of the images and a copy of the ethernet kit's PCB that I saw the datasheet with a different part number.