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[SOLVED] does rs422 need ground?

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Member level 4
Feb 14, 2013
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Hi as I know RS485 full duplex, doesn't need ground but in some sites I saw that someone used "RS422" with 4 connection and some of them 5 connection(4data + ground).
which one is true?
all the best

RS-422 needs the two ends' grounds to remain within +/-7V
to meet input common mode range on the receiver. A hard
wired ground would enforce this, at the cost perhaps of
some longitudinal current or ground loop. In a fairly close
assembly perhaps an implicit ground relation can be assumed
but this assumption wants checking in each case.
RS485 is the same. A direct ground connection isn't essential as long as you can guarantee the differences between each end is small enough. I would always advise you DO use a ground to be safe. Even if the two (or more) ends measure low ground voltage differences on a test meter the link will still be prone to interference.

RS485 or RS422 with isolation don't need a ground wire, review the specification of respective interface standards like PROFIBUS. But if non-isolated, a ground connection is required to keep the common mode voltage within safe limits.
The problem with RS-485 in full duplex is cross-talk from CM coupling of signals. To get good performance a ground wire helps absorb stray CM noise and a good CM choke helps even more. Data wires should be balanced such as CAT5 or twisted pair with proper impedance terminator matched to cable impedance.

really thanks as I saw in application note and datasheet of non isolate part like ADM3490 analog devices, it doesn't use any ground connection even for it's evaluation boards. but I got that in some cases some manufactures used ground. so with your answered I understand using ground improved performance. also there is no difference between RS422 and RS485 in this case.

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