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SN65HVD09: SN65HVD09 unused channels

Part Number: SN65HVD09

Hi, 

I am using "SN65HVD09" as RS 422 receiver. But I am not using few channels. Should I keep it as floating ? or any other recommendation is there ?

  • Hi Brijesh,

    If you take a look at the ELECTRICAL CHARACTERISTICS at page 5 you'll find the differential input thresholds voltages VIT+ and VIT-. Voltages from -200 mV to +200 mV at the differential input are not defined and the result could be an undefined or toggling output signal.

    conclusion: If you use the device as a receiver and leave the differential inputs floating, the respective output A signal is undefined or maybe toggling. In order to obtain a stable condition, connect unused B+ with pull up resistor to Vcc and the B- to respective GND. Since the SN65HVD09 is positive logic your output signal will be a defined HIGH.

    Let me know if this is helping you out.

    Kind regards
    Dierk

  • Hi Dierk,

    Thanks for the reply. I got your point, but there are internal pull up and pull down resistors are shown in B+ and B-, so is it ok to keep unused channel as floating ?
  • The internal 100 kΩ resistors have almost no effect because the 2 kΩ/4 kΩ resistor divider is stronger. (I guess the latter are actually part of the output driver; a full-duplex device would not have them at the inputs.)

    Without external resistors, the difference is only 50 mV. You need to add pull-ups/-downs that are strong enough to result in a 200 mV difference; 10 kΩ should work.
  • Hi Clemens,

    thanks for pointing this out.

    You are correct a 10 kΩ resistor is suitable. It is resulting in 0.5 mA current which is in the range of IBus input current also shown in the ELECTRICAL CHARACTERISTICS at page 5 of the datasheet.

    Kind regards

    Dierk