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Implementing a MODBus network using Two-wire RS-485 as required by drives. Drives communicate fine on the bench using two wire twisted pair connecting data +, -. Wire purchased from Belden specifically for use in RS-485 comes with two conductors (twisted pair) and shield. MODBUS Standard states that a third conductor must also interconnect all the devices. I interpret this as an additional conductor as a signal common and not the shield. My drive connections are just =.-and shield. Could someone please explain or tell me a source that will clear the fog?

Right on CKloman!

The "two wire" RS485 connection is a myth. It never has been that way. You need a third ground wire to make sure that no one on the network violates the common mode voltage specification. And you should never use the shield as a third conductor. A good choice is to use a two pair, individually shielded, twisted pair cable such as Belden 3107A or equivalent or Belden's 3106A that has an extra ground conductor wire. If you choose the 3107A then hook up your RS485 + and - connections to one pair, and then just twist the other pair together to use for the GND connection. You still need to pay attention to the shield connection in either case. It should be connected to earth ground at one end only and not connected to any other ground points along the way.

There are several good tutorials on the net regarding RS485 connections. Just do a google search.

Jerry Miille

I would add that for temporary connections (e.g. controllers' programming cables) that are necessarily done when devices are already powered-up, it is advisable to connect your "third wire" between the devices *before* plugging-in the programming cable. I damaged a couple of communication cards just because of poor grounding or floating voltages (and when we're operating at customers' factories, we never know...)

regards
Luca Gallina

We typically use Belden's 'blue hose' for most of our RS485/2-wire runs. It has a foil and braided shield, as well as a third 'gnd' wire that is electrically connected to them both. We use this as we have it in abundance for our AB equipment, it's a very hardy wire, and the impedance is about the same.

I do also always use the recommended terminating resistors. RS485 spec calls for 120-ohm, but I generally use the 150-ohm units that come standard with our AB PLCs.

I have, as well, successfully connected devices at distances of 1000-ft away from our 'master' on shielded signal wire (typical 4-20ma stuff) and they have worked fine. If you're looking at some extreme distances like this though, you may want to look into RF'ing it using a pair of Phoenix 900mhz modbus radios, or something from Prosoft.

Hope this is of help.

But OK, what if the device I am using has only two points available? I am using Micromotion 050 and it has only two terminals, there ain't any ground?
Any ideas?

You can use an RS-485 isolator near your "two points" device, that will support a two wire local connection on one side, and three wire connection to your other device(s) on the other side.

The missing 3rd conductor is the most common reason that when you take your 'staged' project from a lab to an installation the comms go bad.

The full article and explnation is provided here:
http://www.chipkin.com/articles/rs485-cables-why-you-need-3-w ires-for-2-two-wire-rs485