Rockwell Automation Logix5000 Controllers Function Block Diagram Programming Manual - Manual (Page 12)

Chapter 1 Program a Function Block Diagram
In this example, the value of tagA is stored at the beginning of the execution of the
routine. The stored value is used when Block_01 executes. The same stored value
is also used when Block_02 executes. If the value of tagA changes during execution
of the routine, the stored value of tagA in the IREF does not change until the next
execution of the routine.
This example is the same as the previous example. The value of tagA is stored only
once at the beginning of the execution of the routine. The routine uses this stored
value throughout the routine.
With version 11 and later of the application, use the same tag in multiple IREFs
and an OREF in the same routine. Because the values of tags in IREFs are latched
every scan through the routine, all IREFs use the same value, even if an OREF
obtains a different tag value during execution of the routine. In this example, if
tagA has a value of 25.4 when the routine starts executing this scan, and Block_01
changes the value of tagA to 50.9, the second IREF wired into Block_02 still uses a
value of 25.4 when Block_02 executes this scan.
12 Rockwell Automation Publication 1756-PM009E-EN-P - October 2014