Controller output current loops

The simplest form of 4-20 mA current loop is the type used to represent the output of a process
controller, sending a command signal to a final control element. Here, the controller both supplies
the electrical power and regulates the DC current to the final control element, which acts as an
electrical load. To illustrate, consider the example of a controller sending a 4-20 mA signal to an
I/P (current-to-pressure) signal converter, which then pneumatically drives a control valve:

This particular controller has two digital displays, one for process variable (PV) and one for
setpoint (SP), with a bargraph for displaying the output value (Out). One pushbutton provides
the operator with a way to switch between Automatic and Manual modes (A/M), while two other
pushbuttons provide means to decrement and increment either the setpoint value (in Automatic
mode) or the Output value (in Manual mode).