MCR (Master Control Reset) Ladder Logic Instruction

Definition

The MCR instruction is used as a soft emergency stop. Rungs placed between two MCR instructions can be disabled; or rather the outputs of the rungs are disabled when the MCR instruction is false and enabled when true.

There are a couple of rules to follow:

  •             Don’t jump into an MCR zone as unexpected action will occur.
  •             The zone must end with an unconditional MCR instruction usually (see last rule).
  •             Don’t nest one MCR zone within another MCR zone.
  •             If an MCR zone continues to the end of a routine it is not necessary to end the zone with an MCR instruction.

Note: The MCR is not a valid substitution for a hard-wired emergency stop circuit or master control relay. Use a hard-wired master control relay for emergency stop power shutdown.