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DP83849IF: Media converter application issue

Mike Brown24
Prodigy 170 points
Part Number: DP83849IF
Other Parts Discussed in Thread: TS3L501E

Hi 

We have designed a board to use DP83849IF  as a media converter. the configuration of our circuit is shown in the attached picture.

Our Device Under Test (DUT) has a built-in PHY and MAC chip which normally works with ethernet connection. We have added the DP83849IF to have the fiber functionality. Now the problem is that when I plug the ethernet cable the Link_LED is turned ON for 2 seconds and the activity LED blinks a few time but after that all the LEDs are turned off and DUT does not work.

Have you any idea what might be the problem? could it be a loading/driving issue? I mean because the ethernet port has to drive two magnetics at the same time the link is lost. or is it possible if the data between ethernet port and DUT interfere with the data from media converter.

I haven't tested the fiber functionality yet (input from SFP). but as in fiber configuration the path is only feedforward (ethernet port is NC) is it possible that we will have problems in that case too? 

Another point worth mentioning is that in case of connecting the ethernet port the conversion to fiber is not important for us. what is important in both cases is a working DUT.

  • 0
    TI__Mastermind 19780 points
    Hi Mike,

    Are you wired ORed between the RJ45 and the magnetics? If so, I would expect a loading problem here. What cable length are you using? Is the DP83849IF on at the same time as the DUT? Try powering off the 849IF, it will still present a 100R load but this will likely only affect your cable reach. Much like an old Ethernet hub.

    The DP83849IF will try to interact with the media if both DUT and 849IF are on the same copper and in auto-negotiation mode. This will be confusing for the entire bus. You could use a CSMA/CD approach with all 3 PHYs on the copper in 100base-tx half duplex. This will require forcing the link on DUT and 849IF.

    If your system is smart enough to know when to transition between DUT and media converter, you could also drop 1 of the magnetics and place a MUX routing the output of the magnetics to either the DUT or the media converter.

    See the TS3L501E as an example of the part used for the MUXing duty.

    Best Regards,
  • 0 in reply to Rob Rodrigues
    Prodigy 170 points

    Rob,

    Thanks for your answer and sorry for my late reply.

    I have attached the interface schematics to this post: 

    the upper picture is the SFP (green) side interface, and the lower picture is the DUT (blue) side. 

    I understand that in case of connecting an ethernet cable to the RJ45 port (JE1) the bus is confused as you said because both DUT and  849IF want to communicate on the bus. In this case we could have a switch to cut-off the 849IF as our only priority is reaching data to DUT.

    But, when an SFP module is connected to the circuit and no ethernet cable is connected to RJ45 port, I expect the 849IF to perform the conversion and communicate with DUT, because the path is feedforward and there is no device to confuse the bus anymore. Unfortunately my board does not function even with this scenario.

    p.s.: ETH_P1,P2,P3,P6 present in in both pictures are wired together. I tried removing R32, R33 in lower picture to solve the loading problem, but the system still does not work. 

  • 0 in reply to Mike Brown24
    TI__Mastermind 19780 points
    Hi Mike,

    Looking at your schematics, I assume port A of the DP83849IF is connected to the DUT and port B of the DP83849IF is connected to your SFP.

    Can you verify that the DP83849IF is strapped to media converter mode?

    Can you also verify that no link is present between the DP83849IF port A and the DUT?

    Please then provide a dump of the DP83849IF registers fort port A from register address 0x0 to 0x1f.

    Best Regards,
  • 0 in reply to Rob Rodrigues
    Prodigy 170 points

    Rob,

    Yes, Port A is in copper mode and connected to DUT and port B is in Fiber mode and connected to SFP.

    The straps are as follows:

    pulled-up: AN0_A, AN0_B, AN1_A, AN_EN_A , FX_EN_B, INT_B, INT_A, EXTENDER_EN

    pulled down: AN1_B, ANEN_B

    you mean by checking the LINK signal and its LED? it is turned off

    as we have produced the PCB with only strapping enabled in mind, we do not have access to registers.

    Do you suspect anything in the attached schematics? is the termination and loading of the two magnetics ok? (assume that no RJ-45 cable is inserted)

    and I have two other questions. Does 83849 work  with a 100base-EX SFP, or it only supports 100base-FX SFP? 

    the switch we are currently using is a Cisco 2960 POE switch. Is it necessary to use an attenuator on the way if we want to test the board with a 2 meter long fiber cable?

    Regards