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SN65HVD3082E: the benefit of external biasing in RS485 interface

Part Number: SN65HVD3082E

Hi team,


According to the RS485 training video in precision lab, it is said that “Should I provide external biasing even if my transceivers all have fail-safe biasing? If external biasing is used, the receivers may see the benefit of additional noise immunity.”

  1. Why does external biasing benefit the noise immunity with internal fail-safe biasing?

Best regards

Wesley Huang

  • In the worst case, the positive-going threshold is –10 mV. Assuming a 60 Ω load, a current of 167 µA would be able to go from 0 V to the threshold and to flip the bus into the other state.

    With external biasing, an idle bus would not be at 0 V, and more current would be required to switch.

  • Wesley,

    Another point is when the bus is short, idle, or open, the differential bus voltage is 0V. If the transceiver has the threshold of -200mV to 200mV per RS485 standard, the receiver output is undetermined and MCU could be mistakenly trigger by start of frame. The internal fail-safe circuit moves the Vih below 0V to avoid this. On top of this, the external fail-safe will make the bus voltage positive, making the receiver have less chance to generate unexpected signal due to noise.