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TMUX136: TMUX136 for Switching TX Pair for 10/1000 Ethernet for RS-485

Mike Mrazik
Prodigy 10 points
Part Number: TMUX136

I am currently designing an Ethernet RMII circuit and would like to switch the TX+/TX- pair in and out with a RS-485 +/- pair depending on what my customer plugs in.  I have the Isolation Transformer for the Ethernet on the TX+/TX- and RX+/RX- pairs, so this switch I would like to happen on the Isolated side near the RJ45 connector.  However, the control signals for making the switch would be from our processor on the non-isolated side.  My mind says this should be a relay or an optocoupled switch, would this part allow that same functionality?

  • 0
    TI__Genius 15080 points

    Hello  Mike,

    So we actually have a app note that talks about the comparison between multiplexers and relays. 

    Any signals between 0 and VCC should be fine to control the mux.

    The low Ron and the low Con of this multiplexer  should be good for your application.

    Please take a look at the app note and let me know what you think.

    We can dig deeper and make sure the TMUX136 is the right device for you if needed!

    Regards,

    Kameron

  • 0 in reply to Kameron Hill
    Prodigy 10 points

    Hi Kameron,

    Thanks for the app note, good stuff.  What I'm trying to understand is what the voltage in comparison to the system ground (where this TMUX136 may be tied) will be one the other side of the ethernet transformer. 

    For example, I have a similar product a colleague did that used a different switch to switch TX+ (and then a second switch to switch TX-), its sitting on the output of the transformer, but its ground is referenced to the system on the primary side of the transformer.  With a scope, I measured between system ground and the TX+/TX- signals and got over 4V at its peak, while the switch was tied to 3.3V.  The TX+/TX- pins themselves differentially were only about 2V, but this voltage in comparison to the system ground exceeded the voltage of the switch (which is what you're saying I should not do on the mux).  

    So if I took the above scenario, and I had a MUX that can handle VCC up to 5V, then I would have been fine I assume.  But how do I know my signals I'm switching on the secondary side of the transformer will always stay at the same value and be consistent across PCBAs?  Think I'm just not fundamentally understanding how this works, but its 0-4V today, next power cycle how is it not 0-20V?  Its isolated, can't it be anything on the secondary side?  All the 1:1 windings do is tell me I'll have the same differential voltage on both sides. 

    Thanks again for the help

    Mike

  • 0 in reply to Mike Mrazik
    TI__Expert 8005 points

    Hello Mike,

    Kameron is out of office, please expect a reply from him sometime next week.

    I apologize for the inconvenience.

    Thanks,

    Nir 

  • 0 in reply to Nir Gilgur
    TI__Genius 15080 points

    Hello Mike,

    Thanks for the extra info and I think I understand your question. I wanted to clarify and state that this device is rated to handle io signals 0 to 3.6V

    So to answer part of your question ,TI multiplexers can handle your scenario which is called ground shifting.

    Essentially from what you are describing you have this mux in this conditions, VDD=3.3V  and the ground=-2V. (TX+/TX- pins  differentially 2V)

    First you can think about the recommended supply voltage of the device as  differential between ground and VDD. In stead of thinking that the supply needs to be 2.3V to 4.8V in this situation you can say the the difference between supply and ground is recommended not to exceed 4.8V. 

    Because in this example  The difference between VDD and ground is 5.3V this would exceed the recommended operating condition.

    In your example you would not necessarily need a 5V supply mux but instead just need a mux that can support the 2V differential input signal pair.( Which the TMUX136 can do) The TMUX136 having a 2.8V supply and -2V ground would be what you would need.

    I'm am not sure how to tell what voltage the ground will be at in your design across different PCBs but there are many other higher voltage multiplexers that can also support your application if that is tricky to determine!

    Please let me know what you think and I am happy to help further if needed!

    Regards,

    Kameron