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SN65HVD11: 1Hz signal transfer through Ethernet cable

David Zhao
Intellectual 1820 points
Part Number: SN65HVD11

Hi Supporter

My customer transfer 1Hz signal between two boards. One piece SN65HVD11 on each board. Two boards are connected by Ethernet cable through RJ45. If GND of two boards are not connected well, could SN65HVD11 still work normal? Is there any risk SN65HVD11 broken? Thanks!

Regards

David

  • 0
    TI__Guru 62910 points
    Hi David,

    SN65HVD11 will still be able to work normally as long as the voltage difference between the grounds of each node is not larger than 7 V. (RS-485 devices are generally designed to operate with slight mismatches in ground potentials. These ground potential differences result in offsets in the common-mode voltages that appear on the differential bus, and the TIA/EIA-485 standard specifies that transceivers should work over a -7V to +12 V common mode range.)

    If the ground potential difference were very large, it is possible that damage could occur. For example, if Node 1 has a ground that is much higher than Node 2, then the voltages output by Node 1's driver when it is active would also be very high compared to the ground reference of Node 2. This could cause the absolute maximum ratings of Node 2's transceiver to be exceeded. Note that this would be less of a concern if the grounds are floating (or isolated). Since there is typically some (high-valued) resistance from the RS-485 pins of a transceiver to its ground pin, if there are no external factors forcing a voltage difference between grounds then the RS-485 connection between them will help to pull their grounds closer together.

    FYI, you can read more about RS-485 grounding in Section 11 of this application note:

    www.ti.com/.../slla272c.pdf

    Regards,
    Max