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I'm in the process of designing a motion control system using RS485 and whilst we've generally made things work in the past with a more trial and error approach, in the interest of doing things right I'm trying to figure out what is the best practice exactly when it comes to ground connections.

I've been reading what I can find which seems relevant about RS485 ground connections but I'm uncertain about what exactly I should be connecting the signal ground to in the system I'm working on where there's multiple devices.

The motor drivers I'm using share a common signal and system (power input) ground, and the RS485 interfaces we're currently planning to use have shared chassis ground and signal ground.

  1. As both signal grounds connect to a common ground at the RS485 interface, will the stepper motor drivers (through their shared grounds back to the interface) not cause loads of noise on this ground?

  2. Where would be the appropriate place to connect these signal grounds (and so the connected DC neutral) to the AC ground in this system, if I presumably should do so?

You'll have to forgive me as I'm not an electrical engineer by training so am trying to figure out what I can, but any advice would be massively appreciated if there is anyone out there who would be able to advise!

And for reference all RS485 pairs are twisted from start to finish and all AC grounds can be assumed to be connected to the same.

And apologies if I am just completely of track / in over my head...

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More detail of each of the units to be controlled

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  • \$\begingroup\$ It is difficult to imagine the radiated and conducted ground noise from the DC-DC in the common mode relative to signal ground without more detail. Noise from port 2 may interfere with port 1 and the source using the same shield. I see you have two channels of data. Often separate CAT6 cables are needed with CM chokes but depend on the environmental motor noise and RLC impedance coupling and resulting SNR from CM noise.. I tied to simulate it but missing your specs on noise levels. tinyurl.com/2z4k9mol \$\endgroup\$
    – Tony Stewart EE75
    Dec 15, 2022 at 1:20
  • \$\begingroup\$ Blimey thanks - I've added a second diagram, does this help clarify? I was hoping that because cat6a cable has individually shielded twisted pairs then it would be ok? From measuring the RS485 easysync it looks like the signal ground is shared with the chassis ground on this. So presumably in this case both RS485 signal grounds would be connected to ground at this point? \$\endgroup\$
    – Will Laslett
    Dec 15, 2022 at 10:47

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