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SN65HVD1781-Q1: Why there is need of biasing resistors in SN65HVD1781-Q1

Jagdish Karamchandani
Expert 1420 points
Part Number: SN65HVD1781-Q1

Hello TI Support,

We are using 2 devices with RS485 each having SN65HVD1781-Q1. We have problem communicating both devices & will appreciate your help.

1. When we try to communicate both devices through RS-485 it doesn't work. Supply voltage of both RS-485 in each device is 3.3V & we are communicating both devices through 2- wire mode that is A & B.

2. When we added 2.2K from A to 3.3V & B to GND then communication began to work. I am not sure if SN65HVD1781-Q1 is fail safe biased then why do it require any biasing resistors? Awaiting your reply.

Thanks,

jagdish

  • 0
    TI__Mastermind 25705 points

    Jagdish,

    1. What’s termination resistor value used, 54Ohm, 120Ohm or other value?

    2. HVD1781 can work with 3.3V to 5V power supply. But please be noted that when Vcc goes down, the output voltage decrease as well. I suggest for the first step of debug the customer check the voltages on bus for the common mode and differential values. If there is anything abnormal, we can start from there.

     

    Regards,

  • 0 in reply to Hao L
    Expert 1420 points
    Hi Hao L,

    Thanks for your response & sorry for delayed reply. I did verified that common mode voltages amounting to 35V were present because our boards were connected with USB & ethernet from other sources working on 230VAC, 50Hz.

    Making grounds common or having 100E resistor from GND of each RS-485 to third common wire resolved the issue. Actually some RS-485 literatures recommend using 100E series resistor (from GND to third wire) to limit loop current & simultaneously creating a good reference which is within common mode range of RS-485.

    Thanks,
    jagdish
  • 0 in reply to Jagdish Karamchandani
    TI__Mastermind 25705 points
    Jagdish,

    Great to know it works for you. Yes, connecting each node to real grounds with 100Ohm resistor could limit the loop current. But please be aware that if there is noise in the ground, it could be coupled to the signal. Please let me know if you have any questions.

    Regards,
  • 0 in reply to Hao L
    Expert 1420 points

    Hi Hao L,

    Thanks for the information, I will make sure about ground noise.