Unreferenced Powersupply and RS485 network

Hello ,
I power RS485 from an Isolated power supply which is not referenced to the Earth ground. I use the same ground return wire from each node as my supply. I want to know if the communication will run smoothly without any reference to the ground. I have no galvanic Isolation here as I think there is no need for it as I have no different ground connections I run a GND cable to each node along with the twisted pair +Rs485 ,-Rs485 .I want to ask will MAX485 work well in this application .I have a network length of approximately 100 Feet .I do someting wrong kindly letme know .

Warm Regards

RS485 should work without common ground, just by differential signal +/-. Don't forget terminator resistors!!
It will be better if you use "E" version MAX485E - it has ESD protection.
MAX485E is rated up to 2.5Mbps on much longer distances.
Depending on you application you may use MAX481E, 483E, 485E, 487E.

Just want to add:
Profibus is an example of RS485 hardware and it uses 2 -wire connection without common.
Also look at MAX3088E: it works up to 10Mb
MAX3088 pdf

Hi Contr_Conn ,

Thanks for MAX485E . I will use proper termination resistances . Please tell me is there any requirement of the third conductor wire GND along with +485, -485 twisted pair wire . I wanna know about ground to ground potential that may develop between the two nodes grounds and will this be in a safe limit if we measure it in the 100 Feet run cable . I also want to know should I tie the chassis ground and the shield ground to the circuit Electical ground as the reference from Earth Ground is missing .

Thanks

I am sure, that you don't have to run a signal ground (GND) along with tisted pair differential signals.
You should terminate shield to the chassis ground on both(!) ends.

I think there is a requirement to potntial equalisation wire between devices in some cases (16 mm sq). But it shuld be run separately.

I am not sure if you have to ground a common wire (GND) of each PS to the chassis.
Look at Profibus-DP requirements, it is a true RS-485 line.
Give me your email and I will send you PDF file I have.

Hi,
Please send me any info to [email protected]
You said " You should terminate shield to the chassis ground on both(!) ends "

Should I tie the Electrical ground to it .My application doesnt have a Earth Ground it is totally floating .

thanks

I learned a lot about RS-485 from these guys:

http://www.robustdc.com

And here's a good tutorial that talks about several industrial implementations:

http://www.ccontrols.com/pdf/ExtV1N1.pdf


RS-485 ALWAYS needs a return path, whether it's a third wire (like A-B DH485) or a thick shield (Profibus DP) or the ground plane of the factory floor.

Why no earth ground

There is no ground connection if it doesnt do anything, EARTH is ground.

How do you ground without a ground connection..ie earth?

Personally I think its better to use either optic or galvanic isolation BUT you have to have an earth ground.