I have 8 opto-isolated RS422 ports to install.

All of them are coming from a single PC card. It provides the opto-isolation for every RS422 port. Every signal is opto-isolated from the PC ground and from each other port.

Does every RS422 port require its ground signal to be connected to the distant device? I have read both answers, yes and no.

In this document, the "fully isolated" paragraph states :

A fully isolated RS-422/485 link generally requires a direct ground wire between the two ends. External surge protection is only required if large lightning related surges to ground are expected.

But in this post I understand that it is not necessary (maybe I didn't understand well) :

opto-isolator coupled differential signals

Systems like MIDI connect more-or-less differential signals to the LED of an opto-isolator at the receiver.

With proper design, similar systems can and sometimes do work just fine with kilovolts of offset between the system "ground" at one end of the cable and the system "ground" at the other end of the cable.

So, should I or shouldn't I use the proper ground for every RS422 port?


  • 2
    \$\begingroup\$ Optical isolation implies a galvanic isolation. Common ground is breaking the galvanic isolation. In that document the "full isolation" is happening on one side only, that's why it requires to pull a ground from the other side. \$\endgroup\$ – Eugene Sh. Mar 19 '18 at 15:01
  • 1
    \$\begingroup\$ @EugeneSh., The OP is not asking about a ground connection that crosses the isolation barrier. Only asking whether a ground connection is needed between the isolated transceiver and the remote equipment. \$\endgroup\$ – Solomon Slow Mar 19 '18 at 16:02
  • \$\begingroup\$ Ground is a required connection for RS-422 whether it is a dedicated wire or connected through earth ground. This looks like a good article on the subject: robustdc.com/download/index.php?file=AN005.pdf \$\endgroup\$ – EE_socal Mar 19 '18 at 20:25

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