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?
Best Answer
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.