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SN65HVS885 Grounding Issue

Varban Metodiev
Intellectual 640 points
Other Parts Discussed in Thread: SN65HVS885

Hello Team,

We have connected 4 x SN65HVS885 in cascade mode. We read them successfully and correctly only when all of the RE (return) pins are connected to a common ground. Also, we needed to connect the free IPs to this common ground.


Since our project includes digital inputs from different field devices, we need a disctinct connection for every IP/RE couple. However, in that case, a logical level "1" is randomly generated on some of the free input pins (IPs).


So, my question is, how is it possible to have the SN65HVS885 work correctly without using a common ground? Instead, each input field pair should use its own IP-RE couple.

Kind regards,

Varban

  • 0
    TI__Expert 5400 points

    Varban,

    Allow me to look into this and I will get back to you.  A schematic and waveforms of the erroneous bits would be useful, if you can provide this information.

    Thanks -- 

  • 0 in reply to Bart Stiller
    Intellectual 640 points

    Hello Bart,

    The wave forms look like normal 5V square impulses, jumping around on the free IPx inputs. I will try to provide you a screenshot from the oscilloscope.

    The schematic we have used is the referential - from the datasheet. We have changed only the IPx/REx connectivity - instead of a common ground we want to use each IP/RE pair as an independent one.

    Thank you for your help!

    Varban

  • 0 in reply to Varban Metodiev
    TI__Expert 5400 points
    Varban,

    Each of the digital inputs are referenced to the local ground of the HVS885. Thus it is not surprising that erroneous transitions occur when multiple grounds are connected to the device via RE0 ... RE7. The simplest solutions would be to either 1) connect the ground of the HVS885 to the ground of the digital input sources, or 2) isolate the digital input sources from IP0 ... IP7, so that each IP/RE pair is referenced the the local ground of the HVS885.
  • 0 in reply to Bart Stiller
    Intellectual 640 points

    Bart,

    Thank you very much. Your answer was quite helpful to us.

    Regards,

    Varban