0------
in     | 
 0--   |      _____________________
   |   |      |
   *-- | --*--| transciever and '51
   |   *------|
   |   |   |  |_____________________
   |   |   |
   |   |   
   |   |   / 110 ohms
   |   |   
   |   |   /
   |   |   |
 0-    |   |
 out   |   |
 0-----*-- |
         ^-|  contact that opens when
              plug is inserted