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Thread: Teensy 4.1 ethernet header and library issues

  1. #1
    Senior Member
    Join Date
    Oct 2019

    Teensy 4.1 ethernet header and library issues

    I got a T41 breakout PCB made that has an RJ-45 header and needed capacitor configured. I'm having several issues:

    1. Without the Teensy attached, I am observing T+, T-, R+ and R- all shorted together. It's likely I have errors with the board...but I need to confirm what I should observe. In this configuration, I've read that Ts and Rs should appear as shorts to an ohm-meter. (Please confirm) If this is correct, my only issue is that the Ts got shorted to the Rs at some point.

    2. The library I tried to use failed and said that it could not detect the ethernet shield. Does this mean the RJ-45 jack is malfunctioning, or the Teensy can't communicate with its onboard hardware?

    3. The RJ-45 LED does not illuminate when a cable is plugged in. I verified continuity and verified that the LED illuminates when supplied with external voltage. I'm not sure if the Teensy has to run code for the LED to illuminate or if it supposed to happen based only on electircal connection. Please confirm.

    4. I'll take any hints on debug procedures to figure out what's happening.

    I'm using the below RJ-45 header. The pin numbering isn't the same as the PJRC site because the connector is different.

  2. #2
    Junior Member
    Join Date
    May 2021

    1. My ethernet breakout board works with Ts and Rs all shorted. I checked the pins on the breakout with a multimeter when the Teensy board was unplugged form the ethernet breakout. But if you're unsure about your own designed breakout board always double check every trace and pin connection with a schematic before you toast something

    2.Which library did you use? Do you maybe have errors in your code? Could you please show us your code.

    3. The Teensy has to establish a connection to a network device to light up the LED. Only plugging it into your switch, router or any other networking device doesn't necessarily light up you LED.

  3. #3
    Senior Member PaulStoffregen's Avatar
    Join Date
    Nov 2012
    Quote Originally Posted by kdharbert View Post
    1. Without the Teensy attached, I am observing T+, T-, R+ and R- all shorted together.
    That's normal, assuming you're measuring ohms with an ordinary multimeter. The ohms measurement is done by sending a DC current through the circuit and measuring the resulting DC (or average) voltage which results. This circuit is supposed to be nearly zero ohms at DC. It will only measure as a reasonable impedance if you make a very special measurement at the relatively high frequencies Ethernet uses.

    When you measure ohms from T+ to T-, the DC current your multimeter transmits flows along this path.

    Click image for larger version. 

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    Inside the magjack are little transformers which couple the ethernet signal. The transformer literally is just tiny wire wound around a piece of magnetic material. When you measure at DC or low frequencies, it appears to be merely the piece of wire it is. Like all transformers, it only works with AC signals above a certain frequency where the coil's inductance creates enough impedance.

    Both transmit and receive center taps are joined together on the PCB. So if you measure T+ to R-, your multimeter's DC current flows along this path.

    Click image for larger version. 

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    Using only a DC ohms measurement, it looks like everything is just shorted together. And indeed it is for DC & low frequency. Only high frequencies work with the magjack.

  4. #4
    Senior Member
    Join Date
    Oct 2019
    Wow thanks! Great news that there may not be anything wrong with the boards.
    I'll take another lap though the libraries and retry.
    Does Teensy make any electrical checks on the port when initializing?

  5. #5
    Senior Member
    Join Date
    Oct 2019
    I messed up library versions and usage. The NativeEthernet library works for DHCP, the LED illuminates and the Teensy returns pings.

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