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THVD1550: grounding method discussion

user021864
Intellectual 2145 points
Part Number: THVD1550

Hi I am confusing about what is the proper grounding for RS-485:

if we are using shielding for the cable, and there is GPD between two nodes, how should i connect the ground?

(building A with GNDA, building B with GNDB)

1. connect the shielding to GNDA at A building, and GNDB at B building.

2. connect the shielding to either GNDA or GNDB, and the other side left open.

3. connect neither GNDA nor GNDB, just left it open.

Please let me know your suggestions. Thanks for your help.

I also wanna know if this become difference when using isolated or non-isolated 458.

Thanks.

  • 0
    TI__Intellectual 1905 points

    Hi there,

    thanks for reaching out. In a nutshell: Typically the shield of the cable is connected to protective earth (PE) on both sides with a high-impedance parallel RC network to couple the protective earth (PE) to the signal GNDA/GNDB

    You can take a look at the following E2E post to get more insights:

    https://e2e.ti.com/support/applications/ti_designs/f/1009/t/635981?tisearch=e2e-sitesearch&keymatch=shield%20ground%20rs485

    Let me know if this is helping you out, don't hesitate to ask If you need any further assistance.

    Kind regards

    Dierk

  • 0 in reply to Dierk Bronner
    Prodigy 50 points
    Hi Dierk,
    If the RS485 cable goes from one building to another, the PE of both building might have different potential and cause ground current. This current might introduce interferences to the RS485 signals. Don't you think to let one of the shielding ends open will be better?
    Thanks
    Randy
  • 0 in reply to Randy Tu
    TI__Intellectual 1905 points

    Hi Randy,

    that is a really good question. Please let me discuss this with experts of my team and I'll come back to you tomorrow before EoB. Thanks for your patience 

    Kind regards 

    Dierk 

  • 0 in reply to Dierk Bronner
    TI__Intellectual 1905 points

    Hi Randy.

    if your application does not requires the GND shield connection on both sides then it is fine to keep the connection single ended.

    If you planning to go with a shield connection on both sides the THVD1550 can handle common mode shifts from -7 V to 12 V. As long as you potential difference is in that range you should not face issues. Keep in mind to use a resistor as shown in the other threat to limit the current.

    I hope this is helping you out, let me know if you need anything else.

    Kind regards 

    Dierk 

  • 0 in reply to Dierk Bronner
    Prodigy 50 points
    Thanks for the reply.
    Still confused by one thing: In which conditions the cable shielding should be connected to earth ground at both ends? Is there any benefit?
    Or, single ended earth is a must for communications between devices located at 2 buildings?
  • 0 in reply to Randy Tu
    TI__Expert 6955 points

    Randy,

    I'm jumping in on the thread in place of Dierk. Check out the figure 9 below for some grounding do's and don'ts.

    Figure 9. has different ways of grounding and their disadvantages. Grounding at both ends is generally not advised due to high ground loop current. One can ground on both sides through a resistor, but the system is more susceptible to noise. Figure 10. below as a good way of  singled-ended grounding also with the use of a signal isolator to help alleviate issues from the different ground potentials.

    These figures came out of TI's RS-485 design guide which has section addressing how to ground the transceivers. Check it out!

    Let me know if you have any more questions Randy!

    -Lucas