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?

  • 3
    \$\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, 2018 at 15:01
  • 2
    \$\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\$ Mar 19, 2018 at 16:02
  • 2
    \$\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, 2018 at 20:25

2 Answers 2


midi is not like rs422.

In a midi input the incoming signals go straight to a LED. There is no other circuitry on the isolated side of a midi input (midi outputs are usually not isolated).

RS-422 uses differential signaling, but it feeds that into a bidirectional electronic transceiver, which has a limited tolerance for common mode voltage difference. So the "signal ground" of the RS-422 transceivers (which may be separate from the signal ground of the rest of the system, which in turn may or may not be separate from mains ground) need to be connected together.


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

Yes. Provided The Ground cable running From The isolated driver end at the source is connected to the remote end AND the ground is coupled weakly at the remote end to the power earth and not directly.

Run the ground cable along with the data lines and the galavanic isolation at the remote end is recommended.

Let us see some recommendations from TI.
The ground loop existence depends on the configuration of both driver and the receiver power and ground reference. By using isolated drover, you have successfully isolated earth ground as reference. But from the isolated end when you connect multiple systems together again (you have to make sure that the ground of the driver from isolated side is not coupled to the earth on the remote end. You can achieve this using digital isolators at the remote end.

If not the ground loop still exists.

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(Source of images above: Texas Instruments Application Report SLLA070D - RS-422 and RS-485 Standards Overview and System Configurations)


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