Current loop

[1] In electrical signalling an analog current loop is used where a device must be monitored or controlled remotely over a pair of conductors. Only one current level can be present at any time.

A current loop is a industrial signaling standard used to send data from sensors to controllers (like 

PLCs or microcontrollers) in long distances. IO using varying voltage to transmit info (like 0–5V), it uses a varying electric current. The most common standard in engineering is the 4 to 20 mA current loop, a de facto standard where they're often for process control applications to carry signals from process instrumentation to proportional–integral–derivative (PID) controllers, SCADA systems, and PLCs. They are also used to transmit controller outputs to the modulating field devices like control valves.