differential signal current return

Its a twisted pair, a balanced line. There is no referencing to ground.

Having had to debug an RS-422 system between 2 buildings on different pole pigs, the ground is important from the standpoint of staying in the common mode range of the receiver. RS-485 has expanded common mode range but that setup between the buildings required optical isolation to get it running reliably.

GG

Hi, Ran. You're in the right place to get an answer.

In RS-422 differential transmission, the current in both lines should be balanced across a 100 ohm load resistance at the receiving end. One wire sources current into one side of the resistor, the other sinks the same current from the other side of the resistor. When the logic changes state, the source and sink switch sides. And since it's twisted pair, any noise pickup should be impressed on both lines equally, making signal decoding easy even in the presence of significant electrical noise.

The fly in the ointment occurs when there's a potential difference between signal grounds of two transceivers. A prior post mentioned that Ethernet lines are transformer-coupled. RS-422 is *not* transformer-coupled, so this ground potential difference can impress itself across the pair. Not only that, but the ground loop current will probably not flow equally in both sides, unbalancing the pair. In fact, this is a main cause of data transmission problems in RS-422 communications.

It's considered good practice to include a signal ground line providing a low impedance connection between the two circuit grounds in RS-422, so any ground loop current will flow there instead of the signal lines.

Here's a Circuit Cellar tutorial on RS-485 (a similar balanced differential pair serial transmission method which has pretty much superceded RS-422) which describes the issue at some length, and is easy to follow:

This link describes the basics of RS-422 and RS-485 communication:

Maxim App Note 723: Selecting and Using RS-232, RS-422, and RS-485 Serial Data Standards

Once you're done, you'll have a good idea why it's important to have a signal ground wire in RS-422, and what it's for.

So as long as there's no difference in potential between the two transceivers in RS-422, no current will flow in the signal ground wire, and the current going out the + side will be matched by current coming in the -, or vice versa. If there is a ground loop and there is a signal ground or drain wire, the current flow in the pair will remain balanced. But without a signal ground or drain wire, no guarantees.

Good luck Chris

When there are significant ground loop currents which cause significant voltages across the signal ground line, this is the best way to get a reliable drop. As GG says, sometimes it's just about the only way.

Cheers Chris

twisted pairs are used for the + and - side of the signal. they are balanced lines and do not depend or used any form of common ground for its signal transmissions. differential input and output devices are used to send and receive the data. basically when data is being sent both lines will alternate with each other. the input of the devices uses a differential + and - inputs. think of it as a transformer output going into a twisted pair line and ending at another transformer as input. both the output windings of the sending device and input windings of the receiving device do not have their windings associated with the common/ground of either device. in Dc couple devices you normally have dif amps.

Thanks GG, Jamie and ofcourse Chris- this is a great help for me! Best of the week for you all, Ran