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SN65HVD30: Pull up resistor connected to Pin R

xin zhao2
Prodigy 20 points
Part Number: SN65HVD30
Other Parts Discussed in Thread: SN65HVD33, SN75179B

Hi Team,

I found that in Fig. 30, a 10k Ohm pull up resistor is connected to Pin R for SN65HVD33,  may I ask the purpose of this resistor? And do I need to add this resistor for SN65HVD33? Previously, I was using SN75179B without this resistor, and it works fine for me.

Please let me know if you need some other information.

Thanks!
Sean

  • 0
    TI__Guru 62910 points

    Hi Sean,

    If the receiver is disabled (via pulling /RE high), the R output becomes high impedance.  In this case, if a high-level signal is expected by the MCU/UART it would be necessary to bias it externally using a pull-up resistance.  If this is not required or if the receiver is always enabled, then a pull-up is not needed (the R output uses a "push-pull" structure).  SN75179B has no way of disabling the receiver and so the pull-up is not recommended for that device.  Please let me know if that isn't clear.

    Max

  • 0 in reply to Miguel Robertson
    Prodigy 20 points

    Hi Max,

    Now I understand the reason. Thanks for your kind explanation.

    Sean