4-20mA Output / Input PLC to PLC

How far away is the station from your PLC?

If the distance is less than a kilometer, you can, theoretically, just lay out your shielded instrumentation cable and interface the station and PLC directly through isolation modules. If the distance is more than a kilometer, you'll need a repeater somewhere in between them.

Alternatively, you can go wireless. B&B Electronics has some products along these lines.

I'm not sure I understand your question correctly, but if you want to convert your 4-20mA signal to something friendlier for the PLC, simply connect the signal to a shunt resistor, say 500Ω, to convert to a 2-10V signal.

You probably already know this, but it is important that you ground your communication wire's shield at one end, but not both ends. Weird things will happen.

Here is an example of a loop isolater. There are other varieties, but this one provides true isolation. You shouldn't have any problems finding a vendor on Globalspec.

Thanks for the comments, just a note in respect to these

Distance is about 800m from the PLC, and yes i will be using line isolators at the remote end to interface with there equipment, in the past when using isolators i have found that they are not reliable in converting 4-20 to 4-20 (Passive) in a true sense, can anybody help with a reliable manufacturer for these isolator's. In the passed Ive used murr with poor results.

There is an interesting solution out there where you can convert the 4-20mA signal to a fiber optic one and then back again to 4-20mA on the receiver end.

http://weedinstrument.com/product_groups/fiber/analog_system.html

These are not real common but the datasheet said they have 0.1% accuracy. I have not used them before, but they caught my interest and I thought of your application.