Chapter 4. Ladder Diagram (LD) Programming
Master Control Relay/End Master Control Relay
Nested form of the Master Control Relay an ENDMCRN instruction
Nested End Master Control Relay
An MCRN instruction marks the beginning of a section of logic that will be executed with no power
flow. The end of an MCRN section must be marked with an ENDMCRN having the same name as the
MCRN. ENDMCRNs must follow their corresponding MCRNs in the logic.
All rungs between an active MCRN and its corresponding ENDMCRN are executed with negative
power flow from the power rail. The ENDMCRN function associated with the MCRN causes normal
program execution to resume, with positive power flow coming from the power rail.
With a Master Control Relay, functions within the scope of the Master Control Relay are executed
without power flow, and coils are turned off.
Block calls within the scope of an active Master Control Relay will not execute. However, any timers
in the block will continue to accumulate time.
A rung may not contain anything after an MCRN.
Unlike JUMP instructions, MCRNs can only move forward. An ENDMCRN instruction must appear
after its corresponding MCRN instruction in a program.
The following controls are imposed by an MCRN:
Timers do not increment or decrement. TMR types are reset. For an ONDTR function, the
accumulator holds its value.
Normal outputs are off; negated outputs are on.
Note: When an MCRN is energized, the logic it controls is scanned and contact status is displayed,
but no outputs are energized. If you are not aware that an MCRN is controlling the logic being
observed, this might appear to be a faulty condition.
An MCRN and its associated ENDMCRN can be placed anywhere in a program, as long as the MCRN /
Is completely nested within another MCRN / ENDMCRN range, up to a maximum 255 levels of
nesting, or is completely outside of the range of another MCRN / ENDMCRN range.
Is completely nested within a FOR_LOOP / END_FOR range or is completely outside of the range
of a FOR_LOOP / END_FOR.
PACSystems* RX7i, RX3i and RSTi-EP CPU Programmer's Reference Manual
Always associated with... Mnemonics
an MCRN instruction