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SN65HVD1785: Do we need to supply a dedicated 0V to all modules employing the RS485 Transceiver

Victor Songore
Prodigy 140 points
Part Number: SN65HVD1785

We have developed a system that employs the SN65HVD1785 to interface between several modules. Our modules are powered from a 24VDC source. The 24VDC  source can be common(shared) or independently supplied to the modules. Where the 24V is shared this implies a common ground to the modules.

My question is, when the units are powered from separate 24VDC, do I have to supply a dedicated 0V line linking the modules.

In other words when interfacing with SN65HVD1785, do we need to supply a dedicated 0V line.

  • 0
    Guru 276440 points

    The voltages on the A/B lines are measured with respect to GND.
    So you must use a common ground. (In general, a RS-485 bus needs three lines.)

    The SN65HVD1785 has a common-mode range of −20 V to 25 V. However, with independent supplies, the ground could float anywhere; you have to at least try to get a common ground.

  • 0
    TI__Intellectual 2090 points

    Hi Victor,

    Grounding schemes do differ across applications in RS-485.  Having separate device grounds between modules can introduce high ground-potential-differences (GPD) in the system, and, depending on the grounding scheme, can induce ground loop currents.  These can have a negative impact on the system performance.  High GPD can reach a point of exceeding the input common mode tolerance of the receivers on the bus.  See part a) of the figure below.

    However, directly connecting remote grounds through a common ground wire over a long distance is also not recommended.  This can cause large ground loop currents to couple into the data lines via common-mode noise.  See part b) of the figure below.

    The RS-485 standard instead recommends the separation of device ground and system ground by inserting resistors to limit the ground loop current.  See part c) of the figure below.  

    However, if high GPD still persists, galvanic isolation may be required for the system.  

    For your particular application, it looks like you will require a system ground to be in common across all RS-485 devices on the bus.  However, to connect each device to this system ground, the RS-485 standard recommends that resistors be inserted in order to limit ground loop current.  The potential benefits of this scheme will increase with the magnitude of ground potential difference (GPD) inherent in your system design.

    For more information, you can reference the grounding and isolation section of The RS-485 design guide.

    Best Regards,

    Max Megee

    TI Transceiver Applications

  • 0 in reply to Max Megee
    Prodigy 140 points
    Thank you all. Your solutions answered my issue completely.