What is Namur how does it work?
NAMUR is a signaling standard, used primarily on proximity and limit switch devices.  It outputs a current signal depending on the state of the switch.  The switching spec is >2.1mA (typically but not always for NC) and &lt;1.2mA for the opposite switch state.  There is a 0.9mA deadband for noise immunity.  The output is usually characterized at an output of 8.2V when terminated with a 1k resistor.  Because of the nature of the signal, a conditioning circuit is needed to convert back to 0/5V or 0/24V logic levels.  

The standard was developed in Germany in the late 1940s for use in chemical plants.  "Normenarbeitsgemeinschaft für Mess- und Regeltechnik in der Chemischen Industrie" was shortened to NAMUR.  The standard is contained within EN 60947-5-6.