I found a TTL converter box made custom made many years ago. It says Namur_to_TTL converter on it. What is Namur? I googled but couldn't find a clear explanation.
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 <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.
NAMUR signalling is based on a current-loop style interface instead of digital (on/off), voltage style. It is mainly used in explosive atmospheres and is part of an "Intrinsically Safe" connection strategy. The current and voltage are, by design, low enough to prevent a spark of sufficient energy to ignite an explosive atmosphere. It must therefore be used with an "Intrinsic Barrier" style amplifier before it can be used to signal a PLC or Process controller.