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questions about SN75HVD11D

Ikoma
Intellectual 940 points

Hi Experts,

Could you please support questions SN75HVD11D from my customer below?

1. In case of implementing ESD protection with referring the figure in datasheet below, where is the best place to implement the termination resister? Could you share its reason also?

 

2. In case of a RS-485 connection shown in the figure below, Will the communicatin performace be better if the branching point for slave #1 and slave #2 is closer to make the distance minimum between slave #1 and slave #2? Could you share its reason also?

Regards,

  • 0
    Guru 243180 points
    1. As shown in figure 29, the termination resistor should be between the series resistors and the TVS.
    (The series resistors form a voltage divider together with the termination resistor.)

    2. It is most important that the length of the branch between the cable and slave #2 is as short as possible. A long branch would introduce distortions, regardless of where it is in the cable. A very short branch is harmless, regardless of where it is in the cable. (RS-485 is designed to work with multidrop applications, and electrically, the master and slave #1 behave exactly the same.)
  • 0
    TI__Mastermind 20820 points
    Hi Ikoma,

    1. Clemens is right in his answer to you about the placement of the termination resistor. Ideally you would place the termination resistors as close to the transceiver bus pins as possible to minimize reflections because from the termination resistor to the bus pins is essentially an unterminated stub. The transient protection components should be placed close to the connector to minimize the distance a transient can travel on your board. Any series resistors can be place anywhere along the trace length.

    2. Clemens is right here as well in his description to you. I would just like to add that the distance between slave 1 and slave 2 is really not that important. Lets say you have a 10 meter of cable from mater to slave 1 with slave 1 holding the termination resistor. Slave 2 can be placed anywhere along the link and good performance can be realized. You need to ensure though that the length of the stub going to slave 2 is not longer than the distance from master to slave or slave 1 to slave 2 because then you may need to move your termination resistor as they are always placed at the farthest ends of the bus. The best approach is to follow Clemens recommendation and keep the length of the stub from the main trunk to slave 2 as short as possible.

    Please let us know if you have any more questions?
  • 0 in reply to Michael Peffers
    TI__Intellectual 940 points

    Hi Clemens, Hi Michael,

    Thank you very much for your prompt reply.

    1. Let me clarify the termination resister in the figure 29. It's the red circled resister in the figure below. Correct?

    2. I see. I'll tell the answer to the customer.

    Regards,

  • 0 in reply to Ikoma
    TI__Mastermind 20820 points

    Hi Ikoma,

    Yes the resistor that you circle is the termination resistor. RS-485 uses 120 ohm differential termination resistors at either end of the bus to ensure communication. Here is are a few links that may help your customer understand some of the details of RS-485 better:

    RS-485 Design Guide Application Note:

    http://www.ti.com/lit/pdf/slla272

    RS-485 reference design for protecting against IEC ESD, EFT, and surge:

  • 0 in reply to Michael Peffers
    TI__Intellectual 940 points
    Hi Michael,

    Thanks for your confirmation. Additional comment and the application note must help the customer also.

    Regards,