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Original question:

SN65HVD33DR chip for RS485 half duplex operations.

SN65HVD78: Can a 485 network work if the R and D pin are combined?

Ryan Lee TW
Intellectual 626 points
Part Number: SN65HVD78

Hi Expert,

Is it possible to combine D and R pin together for half-duplex RS-485 application as below configuration?

The left side of SN65HVD78 is FPGA HR Bank, it is Bi-direction pin.

Thanks.

  • +1
    Guru 297970 points

    In the configuration shown, the D and R pins are separate. Only the DE and /RE pins are combined.

    Connecting D and R together is possible, as long as you ensure that the direction control signal (DE/RE) is correct.

  • +1
    TI__Mastermind 48377 points

    Hi Ryan,

    The diagram shows the enables tied together - not the R and D pins - so please see below for an answer to both configurations:

    1. The enables can be shorted together so that the combination of the pins acts as a single direction control pin. That is fine and very common use case. 

    2. R and D can be shorted together - but only the receiver or driver should be active at one time - so essentially if you were to short R and D together you would also need to short /RE and DE together to ensure that the receiver and driver are never active at the same time or else you are just creating a feedback loop where the R pin will drive the D pin which will drive the A/B pins which will go back to the R pin making communication impossible. 

    If you have any other questions please let me know.

    Best,

    Parker Dodson