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I'm creating a home automation system for condominiums. Practically there are a series of control units that communicate with remote sensors. Communication takes place via RS485 serial on an Ethernet cable that can be up to 100 meters long. The same Ethernet cable also passes power supply for the 12/24 VDC sensor.

The question is, does the RS485 serial interface between the various sensors and the control unit have to be isolated or, given that it is a domestic environment, is it not necessary?

Are there any other tricks I could use? for example magnets like those used in Ethernet interfaces (in my application I already use RJ45 connectors, so it would be enough to use those that have integrated magnets)

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  • \$\begingroup\$ I wouldn't expect RS485 signaling to work very well with Ethernet magnetics in the middle. \$\endgroup\$
    – brhans
    Jan 7 at 13:11
  • \$\begingroup\$ ethernet interfaces do not use magnets ... they use magnetics, in the form of transformers and chokes \$\endgroup\$
    – jsotola
    Jan 7 at 18:26

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In my experience the key question is whether the sensors are "floating" (isolated from local ground, and just connected to the ground wire from the master), or grounded at each location.

If they are floating, then non-isolated should be fine.

If they are locally grounded, then there is potentially a world of pain related to introducing ground loops, and the currents in the ground reference wire causing problems. In that case, having all the sensors use isolated bus transceivers, and isolated PSUs, will potentially say you a lot of grief.

There are a wide range of RS485 isolated transceivers, and some include isolated PSUs as well, for exactly this kind of setup.

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The question is, does the RS485 serial interface between the various sensors and the control unit have to be isolated or, given that it is a domestic environment, is it not necessary?

You might choose an isolated interface when there is a possibility that one of the outlying slaves might be galvanically connected to earth/ground. If the master device isn't earth isolated, you would make the RS485 interface an isolating type (thus preventing earth currents from any device in the home finding a path through the communications cable and disrupting your data).

Are there any other tricks I could use? for example magnets like those used in Ethernet interfaces (in my application I already use RJ45 connectors, so it would be enough to use those that have integrated magnets)

Sure, that would work too.

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