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SN65HVD71: Interprocessor Communicator SN65HVD71DR is unclear about layout of pullups and pulldowns, and line resistors and caps, and their sizes

thomas wright
Prodigy 25 points

Part Number: SN65HVD71

Hello,

I read the reference doc and I do not know how to configure the lines for the SN65HVD71DR. Here is why I have a conflict. I cant determine if the spec sheets say to do pullups or pulldowns on the R and D pins. It literally says "or" in one of the reference figures; what do I choose and for what reasons?

Figure 10.2.2 of the device spec sheet, is the following image:

Omit RE and DE as my selected model does not have enables.

 

On page 17, last paragraph, it says the D pin has an internal pullup resistor to Vcc. Ok, do I need a pull down on it? A pull up would be redundant I assume, and I assume the microcontroller will pull down? I would think the micro would do the pull down, but in figure 41 below it visually suggests I can do a pullup or down. What do I do?



Pin R has an internal pullup resistor too. But in figure 37 above, it has a pullup resistor externally as well. Double pullup?

 

*I am suggesting this document be revised for some clarity, unless a more experienced person would just 'know' and this is written well for someone who knows the possibilities?

--------------------

 

So in short, can you give me clear instructions on all the external accoutrements for every pin to make this device run clean? Who gets pullups and pulldowns? Hilariously, I have nothing on my board right now and it works (no pull ups or downs, no line resistors between the two IPC's); I just want it to be ideal. I do not know if I am stressing the components.

 

Also, what do you suggest for my line resistors on the four output pins? They show these TVS high voltage transient defense items but I don't need that. This chip is simply an interprocessor communicator over a few feet of ethernet cable. Unless you recommend it?

 

Because there are two SN65HVD71DR talking to each other, one for each processor, if you suggest line resistors, should I put them on both sides of the ethernet next to each SN65HVD71DR? or is one set of line resistors/caps anywhere sufficient between the two devices? And what resistance and capacitance should those be?

 

Let me know if I need to clarify anything.

  • +1
    Guru 321790 points

    The choice between pull-up and pull-down depends on what you want the voltage to be. You are probably using a UART protocol, in which the idle state is high.

    A pull-up at the D pin makes sense if you expect much noise and want a stronger pull-up.

    A pull-up at the R pin makes sense if you want a defined voltage when the transceiver is not powered up.

    Transient voltage suppresion diodes make sense if you expect transient voltages. I guess there is no heavy industrial machinery near your ethernet cable?

    The "⑦" series resistors are necessary only if you expect transient voltages that are large enough so that the TVSes cannot clamp them completely.

  • 0 in reply to Clemens Ladisch
    Prodigy 25 points

    Hello!

    I do not expect any noise, I have a very clean board with respect to noise, so then I wouldnt need a D pin pullup.

    I my transceiver is always powered so an R pin pullup is not needed, nor do i need a defined level on that R pin .

    Ethernet cable is a few feet from a 10A 90V motor but that motor's noise emissions are quite small / boarderline undetectable (brushless permamag motor and we ramp it very gently with pwm). Never had an issue with that. So I don't need suppression.

    and with that said the '7' series resistors are really unecessary than.

    ---------
    so it sounds like it works without any pullups or downs on either D and R, and no line resistors or caps between the IPC's. Which is how it has been working, so you gave me confidence that it is configured properly and I'm not stressing anything.

    awesome C: less components.