1766-RM001A-EN-P MicroLogix 1400 Programmable Controllers ...

Program Control Instructions 301While the rung state of the first MCR instruction is true, executionproceeds as if the zone were not present. When the rung state of the firstMCR instruction is false, the ladder logic within the MCR zone is executedas if the rung is false. All non-retentive outputs within the MCR zone arereset.MCR zones let you enable or inhibit segments of your program, such asfor recipe applications.When you program MCR instructions, note that:• You must end the zone with an unconditional MCR instruction.• You cannot nest one MCR zone within another.• Do not jump into an MCR zone. If the zone is false, jumping into itactivates the zone.TIPThe MCR instruction is not a substitute for a hard-wired master controlrelay that provides emergency stop capability. You still must install ahard-wired master control relay to provide emergency I/O power shutdown.ATTENTIONIf you start instructions such as timers or counters in an MCR zone,instruction operation ceases when the zone is disabled. Re-program criticaloperations outside the zone if necessary.Publication 1766-RM001A-EN-P - October 2008