transmitter on. Each current level could close a set of contacts, or operate solid-state logic, at the other end of the circuit. That contact closure caused a change of state on the controlled device. Some remote control equipment could have options set to allow compatibility between manufacturers. That is, a base station that was configured to transmit with a +18 mA current could have options changed to (instead) make it transmit when +6 mA was present. In two-way radio use, AC signals were also present on the circuit pair. If the base station were idle, receive audio would be sent over the line from the base station to the dispatch office. In the presence of a transmit command current, the remote control console would send audio to be transmitted. The voice of the user in the dispatch office would be modulated and superimposed over the DC current that caused the transmitter to operate. See also Current source – a current loop transmitter Current-to-voltage converter Highway Addressable Remote Transducer Protocol NAMUR German industry standards body defining fault levels for 4–20 mA Piping and instrumentation diagram Gives the control scheme and associated piping and vessels. References Further reading Lipták, Béla G. Instrumentation Engineers' Handbook. Process Measurement and Analysis. CRC Press. 2003. HB. External links Fundamentals, System Design, and Setup for the 4 to 20 mA Current Loop What Voltage do I need to operate my 4...20 mA transducer? Current signal systems How to read current loop using arduino Communication circuits Control engineering Electronics standards Industrial automation Analog communication interfaces