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SN65HVD3082E: connect with ISO7631F

Shinichi Inoue
Guru 20361 points
Part Number: SN65HVD3082E
Other Parts Discussed in Thread: SN65HVD3083E, ISO7631FC

Dear Specialists,

My customer is considering SN65HVD3082E and ISO7631F and has a question.

I would be grateful if you could advise.

---

The R pin of the SN65HVD3083E is connected to the ISO7631FCDWR isolator_IN pin.

According to the ISO7631FCDWR data sheet, a constant current source, that is, a high resistance, is connected between the INC pin and GND.

The SN65HVD3083E data sheet P.18 Figure 25 shows an external pull-up (10 kohm) between the R pin and Vcc(+5V).

In this case, the voltage is divided with the high resistance inside the isolator, and the isolator input voltage drops.

So is it possible to make the external pull-up of the RS485 transceiver_R pin to a pull-down resistor?

ｰｰｰ

I appreciate your great help in advance.

Best regards,

Shinichi

  • 0
    Guru 276230 points

    The sample circuit has a pull-up resistor because the idle state of a UART line is high.

    If you are using a different protocol where you want the default state to be low, then use a pull-down resistor (or rely on the ISO7631FC's internal pull-down current source).

  • 0 in reply to Clemens Ladisch
    TI__Guru 50795 points

    Thanks for the help Clemens!

    Hi Shinichi,

    There pull-up resistor in the datasheet is a suggestion, not a requirement. The transceiver itself is not affected by the default state of this signal while the receiver is inactive. 

    Let us know if you have any more questions.

    Regards,
    Eric Schott

  • 0
    Guru 20361 points

    Hi Clemens and Eric,

    Thank you for your reply.

    I understand pull-down resistor is able to use, it depends on the customer's application.

    I'll share this information with the customer.

    If they have an additional question, I consult you again.

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi