Thank you for your reply.
In my case all the transducers get there supply from the system 12V / 24V.
They then return a proportional 4-20mA loop current back to the controller. the controller then either processes the signal or displays the value of the processed (4-20mA) signal value.
The signals are reference to Length, Angle, pressure, temperature rotational speed, etc.
When fault finding in the field, it will make it so much easier to be able to plug in my 4-20mA "simulator" and thereby eliminate {pin point the cause of the fault.)
Transducer, wiring or the controller itself.