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SN75LBC180: Communication with products with different power supply voltages

DDdoor
Expert 2355 points
Part Number: SN75LBC180
Other Parts Discussed in Thread: SN65HVD1473, THVD1450

Hello,

Is there any problem in connecting 3.3V drive product (SN65HVD1473) and 5V drive product (SN75LBC180D) for communication?

Please tell me about points to note.

Best regards,

DDdoor

  • 0
    TI__Guru 62920 points

    Hello,

    Both 3.3-V and 5-V powered RS-485 transceivers will meet the signaling requirements of the TIA/EIA-485 physical layer, and so they can communicate with one another without issue.  The only thing I can think of to be cautious of is use of external failsafe biasing resistances (i.e., to set idle-state differential biases on the bus).  These would ideally only be installed on one node only.  If there were a pull-up resistance to 5 V on one node and a pull-up to 3.3 V on an other node, current could flow from the higher voltage node to the lower voltage supply.  If this is a concern for the application, a diode can be placed in series with the pull-up resistance in order to block reverse current flow into the 3.3-V rail.

    By the way, we do have some newer RS-485 transceivers such as THVD1450 that you may want to consider for new applications. 

    Please let me know if this is not clear or if you have further questions.

    Regards,
    Max