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SN65HVD1791: SN65HVD1791

Part Number: SN65HVD1791

SN65HVD1791 Fault-Protected RS-485 Transceiver (datasheet SLLS872I –JANUARY 2008–REVISED AUGUST 2015):

On the datasheet page 22, it says "Use 1-kΩ-to-10-kΩ pullup and pulldown resistors for enable lines to limit noise currents in these lines during transient events".

In one of our designs such resistors were not used, and we face issues indicating that the RS-422 device in the other end needs to be driven, even when not sending data. Thus it seems to not be according to the Idle bus conditions that occur when no driver on the bus is actively driving the lines.

Could the lack of such resistors explain what is described above?

  • Hi Erlend,

    What do you mean when you say the device on the other end needs to be driven? Do you mean it is receiving a "start bit" erroneously and it needs to respond? Or that it needs to transmit a level during idle periods to properly bias the bus? Or something else? I want to make sure we totally understand the behavior before trying to link it to a root cause.

  • Hi,
    I mean that the driver in the other end is active, setting a defined level, meaning that one avoids the idle bus conditions that occur when no driver on the bus is actively driving.
  • Hi Erlend,

    What is driving the enable pins for each device in your application (DE inputs)? We would expect the driver bus lines (Y and Z) to go idle only when both sides are set to DE=0V. The purpose of the resistor recommendation in the datasheet is to keep transient noise from unexpectedly enabling the driver if the MCU's drive strength is not high enough.
    In your application, are both DE inputs on either side being driven to 0V?

    Best Regards,
    Max Megee