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Custom TM4C1294 Board Ethernet Question

andrew blah
Prodigy 240 points
Other Parts Discussed in Thread: TM4C1294NCPDT

I am trying to design a custom board using the TM4C1294NCPDT Launchpad as reference and using TM4C1294 as my uC.

The application guide states that the uC+PHY should be > 1in away from the magnetics/transformer and the RJ45 connector should be <1in away from the magnetics/transformer.

The problem I am facing is that my custom board will be connecting to a backplane where the RJ45 connector should be. Will having a connector in the ethernet line be a fatal noise problem? This unit would be put in a server rack environment.

PHY <- 2inches -> connector to backplane <-2inches-> transformer <- 1inch -> RJ45

I have read on general forums that its better to put the transformer closer to the PHY (basically before the connector) and have the distance between the transformer and the RJ45 be longer because "that is the point of having a transformer." <-whatever that means.

Or does none of this matter as long as you have a GND plane under the PHY <-> transformer and an Earth GND plane under the transformer <-> RJ45.

Thanks,

Andrew

Application guide for reference (http://www.ti.com.cn/cn/lit/an/spma056/spma056.pdf ) 

  • 0
    TI__Guru**** 244400 points
    Hello Andrew,

    Are you using the on-chip PHY (based on the part number)? If yes and I read the statement right the PHY to Transformer is almost 4 inches. In this case make sure that the signal routing is length and impedance matched and is doubled up with SI Test on the PCB.

    Since the board now connects to the back plane, the GND may become discontinuous at the connector which would affect the quality of the signal. The only way around it is to check with SI tests to ensure that signal does not deteriorate,

    Regards
    Amit
  • 0 in reply to Amit Ashara
    Prodigy 240 points

    Hi Amit,

    Yes, I am using the on-chip PHY. Yes, its about 4 inches between the PHY and the transformer. I'll make sure the length and impedance match.

    So it would not be beneficial to move the transformer closer to the PHY then.

    Thanks Amit!

    -Andrew

  • 0 in reply to andrew blah
    TI__Guru**** 244400 points
    Hello Andrew

    In this case the Transformer is on the back plane, so a lot of the PCB layout would have to be based on the signal integrity. If you can close the gap it would be helpful.

    Regards
    Amit