According to IEC 1131 - 3, it is possible to create Ladder Diagram nets that contain feedback loops, i.e. where an element is used as both contact(s) and coil(s) in the same net.
In Ladder Diagram, all external input values such as those associated with contacts are gathered before each net is evaluated.
In the above example:
Where the net uses the state of its own output, the value of FAN (MB 7) coil associated with an inverted contact of MB 7 is always the value resulting from the previous evaluation.
However, if the value of FAN (MB 7) is used in any following nets, the latest evaluated state is used.