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Questions realted to SN65HV1786 and SN65HV1781.

Oscar Gomez Fuente
Expert 2066 points
Other Parts Discussed in Thread: SN65HVD1781, SN65HVD1786, ISO3088

Hi,

I'm starting off a new design based on the RS485. I think the SN65HV1786 (5Vcc power supply) and (3.3V Power Supply) is going to be a good option because they have fault-protection. But I need some question before to read the datasheets.

Are these RS-485 transceiver fail-safe? Is necessary to put these resistors Rfs?

And I've got another question. If I use only a master and a slave in a bus RS-485. Can I eliminate the RT1 and RT2 resistors? I am using point to point connections of around 1 meter in length.

Thank you so much. Best regards.

SN65HV1781

  • 0
    TI__Mastermind 20820 points
    Hi Oscar,

    Yes, both the SN65HVD1786 and the SN65HVD1781 have on die fail safe circuits to protect the devices from open, short, and bus idle conditions. With these devices there is not need to implement external fail safe circuitry.

    I do not recommend eliminating the termination resistors in RS-485 when using point-to-point topology. It will work and communicate correctly but the RS-485 drivers are designed to support the TIA/EIA 485 standard which specifies that the driver needs to support a minimum differential voltage of 1.5V across a 54 ohm load. Without the 60 ohm load (2 120ohm resistors in parallel at each end of the bus) the voltage on the bus will not be in compliance with the 485 standard and over time may degrade the performance of the part.

    Please let me know if you have anymore questions?
  • 0 in reply to Michael Peffers
    Expert 2066 points
    Thank you Michael,


    This 120 ohm resistor, Could it be a typical 0603 1/10 Watts resistor?

    I've got other questions related to the transceiver ISO3088. In my desing I need to add 3 transceiver with isolation but I don't know If I can ask this question in this thread.

    Best regards.
  • 0 in reply to Oscar Gomez Fuente
    TI__Mastermind 20820 points
    Hi Oscar,

    Yes a typical 120ohm resistor can be used in an 0603 package. This is actually what we have installed on our EVMs. You could go to a 0402 if space was a concern in you design.

    I do not support the ISO products so any question you have regarding isolation should be posted in the industrial interface forum but you should start a new thread with ISOxxxx in the subject line so it is flagged by the ISO applications team.
  • 0 in reply to Michael Peffers
    Expert 2066 points

    Hi Michael,

    The last question: Where have to place the 120 Resistor, just before the A and B pins or after the R1(10 Ohm) and R2 (10 Ohm) resistors?

    I think just after R1 and R2 and before TVS diodes, but I would like to make sure.

    Best regards.

  • 0 in reply to Oscar Gomez Fuente
    TI__Mastermind 20820 points

    Hi Oscar,


    Below is a better layout example with instructions for 485:

    Please let me know what other questions you have?