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Original question:

SUPPLY VOLTAGE OF SN65176

SN65176B: May you pls help check circuit?

Chen Bogey8
Mastermind 9990 points
Part Number: SN65176B

Hi Sir 

As attached file. 

my customer connect B PU resistor to Vcc and connect A PD resistor. is it ok for normal application?

                                                             Bogey

  • 0
    TI__Guru 62910 points

    Hi Bogey,

    This isn't a problem for the transceiver, but this is opposite of how most applications implement their RS-485 bus.  Most UARTs will use the "high" state during idle periods and then detect a falling edge as the start of a frame or word transmission.  Because of this, most RS-485 implementations will use a pull-up on A and pull-down on B so that the status of the line is high (VA > VB) when no transmitter circuits are active.

    If you used this pull-up/pull-down scheme with an application that used the "low" state to indicate a start bit, you would mistakenly see a start bit occur whenever a transmitter is disabled.  This would lead to the reception of incorrect data.  So, I think it would be best to check into the configuration of the MCU/UART used to interface to the D/R/DE/RE controls lines as well as the expected data formatting for the bus in general to make sure the failsafe biasing scheme aligns with it.

    Please let me know if this isn't clear or if you have further questions.

    Regards,
    Max