 It is also used for dmx512 digital stage controls.First used in the 1970s by Japanese radio manufacturers, for microphone connectors on Citizens Band and amateur radio equipment, 
these screw-on circular connectors were available with 2, 3, 4, 5, 6, 7, or 8 pins.They often were used for other purposes as well, such as 1970s amateur radio transceivers (microphone), 1970s Kenwood mobile radios (2-pin, DC power), Eastern Asian built soldering stations in 1990s.