Master Control Reset (MCR) in Ladder Logic

Use Master Control Reset (MCR) instructions in pairs to create program zones that turn off all the non-retentive outputs in the zone.

Rungs within the MCR zone are still scanned, but scan time is reduced due to the false state of non-retentive outputs

MCR zones let you enable or inhibit segments of your program, such as for 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 it activates the zone. – Always place the MCR instruction as the last instruction in a rung.

Master Control Reset (MCR) in Ladder Logic

Master%20Control%20Reset%20(MCR)%20in%20Ladder%20Logic

The MCR instruction is not a substitute for a hard–wired master control relay that provides emergency stop capability.

You should still install a hard–wired master control relay to provide emergency I/O power shutdown.

Do not jump (JMP) into an MCR zone.

Instructions that are programmed within the MCR zone starting at the LBL instruction and ending at the ‘END MCR’ instruction are always evaluated as though the MCR zone is true, regardless of the true state of the “Start MCR” instruction. If the zone is false, jumping into it activates the zone from the LBL to the end of the zone.

If you start instructions such as timers or counters in an MCR zone, instruction operation ceases when the zone is disabled.

Re-program critical operations outside the zone if necessary.

The TOF timer activates when placed inside of a false MCR zone.

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