RS-485 communication via two wires? 2

[KnicksJets]

We have an analog input module (Adam 4017) that needs to be connected to our data constrictor; they communicate via RS-485. The analog module has two pins (RS485+, RS485-) dedicated for communication.
Instead of buying RS-485 cable/converter to connect these two modules, I was wondering if I can just use two wires (say 16 gauge) to connect the two devices? I know ground shielding is needed, but I am testing it in a lab and the distance between the module is short (Practically next to each other). In an actual application I'd have a RS-485 cable.
Does anybody see any issue with using two wires here?
Thanks

 

[VEBill]

[URL unfurl="true"]http://en.wikipedia.org/wiki/RS-485#Three-wire_connection[/url]

 

[IRstuff]

Not only just for signal reference, but I've personally seen zapped receivers because of common mode voltages exceeding the max rating of the receivers, so GROUND!

TTFN
faq731-376

 

[djs]

RS-485 is not a true differential signal. A common ground is necessary. Also if the run is relatively long you will need to terminate each end with a resistor.

 

[LionelHutz]

It's good to see we got much better answers about this compared to one of the previous times it came up.

If the module only has 2 terminals then it should be using earth ground as the common wire.

 

[itsmoked]

You could avoid the third wire by using a 485 isolator. Then the two grounds can be unrelated.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[LionelHutz]

Keith, how does an isolator fix the ground potential difference issue? You just move the issue from the port on the device to the port on the isolator. You install an isolator and then run 3-wires so you can let the network side of the isolator float with respect to the local ground.

The OP did ask about the type of wire. Almost any type of wire would work for very short runs. The devices are likely connected to a common power source (like a power bar) so the devices using earth for the ground connection really shouldn't be an issue.

 

[IRstuff]

As in certain networks, receivers all have transformer coupling, which isolates the senders and receivers and eliminates the need for a reference ground. These would be considered true differential systems, as there is no common mode being transferred.

TTFN
faq731-376

 

[itsmoked]

Hi LionelHutz; It avoids the ground issue because the two sides are galvanically isolated. The grounds remain on their respective sides of the isolation. While technically you should be running the signal grounds normally you can ignore them with isolators. On either side the only requirement is that you not exceed the allowed common mode for the 485 drivers it's hard to exceed them if they're "floated".

Keith Cress
kcress -

http://www.flaminsystems.com

 

[DRWeig]

A good example is the LONWorks FT-10 network. The voltage levels and speead are very similar to RS-485. No third wire allowed, it's all differential without regard to common-mode. A shield grounded at one end is good -- but we've run an old-time twisted pair with no jacket across over 50 meters with no problem.

The FT-10 transceivers are transformer-isolated and very rugged.

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[LionelHutz]

Keith - The ports on an isolator can not float if you don't run the common wire. RS-485 is a 3-wire network. If you don't run the 3rd or common wire then you need to tie the common of the RS-485 port to the local ground. You must run the common wire to properly wire the network side of the isolator to allow the isolated port to float with respect to the local ground. If you ground the common of an isolated port then you've turned the isolated port into a non-isolated port and it has the exact same issues as the non-isolated device port. Then, the only remaining useful purpose of the isolator is to keep you from blowing up the port on the device.


Dave - good example of the wiring that will work but not good to compare no requirement for a common wire in that network to the common requirement of the RS-485 network.

 

[IRstuff]

I don't think that's a correct interpretation. Most isolators of this type have something on the order of 1000V common mode isolation.

TTFN
faq731-376

 

[LionelHutz]

OK, it seems this 3-wire RS-485 port stuff and the use of the common is harder to grasp then I though. Try reading this.

AN0005

The part under grounding and partially isolated RS-422/485 is what we are talking about when you install an isolator and then still run a 2-wire network. As I already posted, it doesn't eliminate any of the grounding problems and just protects the port on the device. The grounding issues are just moved to the isolated port on the isolator.

 

[IRstuff]

You need the LOCAL ground, of course, particularly if the interface is RS422/485, since the common-mode issue is still there. But, that does not mean that the local and remote ground are connected, which is what I thought we were talking about. In many EMI plans, cross platform ground connection is generally not allowed because of the desire to have single point grounds.

TTFN
faq731-376

 

[Noway2]

What about using a transformer for isolation, much like is done in Ethernet? Common mode becomes a non issue as you only pass the AC wave. You would still need a high enough bandwidth (Volt-Seconds) on the transformer to pass a sufficient envelope for the receiver to decode the signal, which may be harder to do at a lower clock rate / slower signal like RS-485.

 

[LionelHutz]

IRstuff - I'm not at all clear what you are referring to when you say you need the local ground. There multiple ways a RS-485 port can be connected. Which connection are you referring to?

1. The port is not isolated so you must only run 2-wires. The port is already using earth ground for the common of the RS-485 port so no further wiring is necessary.

2. The port is isolated and you decide to only run 2-wires. You must then connect the common of the RS-485 port to earth ground yourself.

3. The port is isolated and you run 3-wires, using the 3rd wire to tie the common of the RS-485 port to the common of the other RS-485 ports in the system. You must not connect the common of the RS-485 port to earth ground.



Norway2 - you need to do some reading on how RS-485 works.