Reason for 4-20mA current as a standard signal for transmitting data to and from PLCs

[Ken Roach]

That's an excellent diagram: thanks, Dan !

This morning's ControlGlobal newsletter featured a whitepaper from Acromag on exactly this topic:

http://www.controlglobal.com/wp_downloads/120925-acromag-two-wire-transmitters.html

You have to sign up (and probably get some spam...) but I found the paper easy to read and understand, and very applicable to beginners like our O.P.