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SN65HVD3082E: SN65HVD3082ED Design Question

GaryC
Expert 3285 points
Part Number: SN65HVD3082E

Tool/software:

  1. Can the level of this 485 input be directly pulled out to 5V? Is this current limiting resistor unnecessary?
  2. We now use the red box part of the TX logic. Is this internal high resistance OC, or is it a real internal impedance value?
  3. If you pull up 5V, how can you ensure that the positive and negative potentials of AB reach the 5V level?

BR,

Gary

  • 0
    Guru 316470 points

    These are not current-limiting resistors. They are called fail-safe resistors, and they ensure that the two bus lines have a valid differential voltage even when no driver is active.

    Modern transceivers like the SN65HVD3082E already have fail-safe inputs that work with a differential voltage of 0 V. If all receivers on the bus have fail-safe inputs, then you do not need fail-safe resistors.

    Voltages on the RS-485 bus are not logic levels; what matters is the differential voltage (typically ±2 V); the common-mode voltage is limited to a certain range, but does not matter otherwise.

  • +1
    TI__Guru 72191 points

    Thanks Clemens,

    Gary do you have any additional questions or did Clemens answer everything for you?

    -Bobby