Best Practices Retained Variables


    • Do not use retained variables as outputs of Function Blocks.
      • Doing so may cause problems with retaining the value if the FB is executed with the Enable input off.
      • The output structure can cause it to be conditionally updated to zero or the old value.

A work-around solution is to use a copy (1 gain) function to selectively enable updating the retain variable from an intermediate variable, which is the output of a function block. (Figure 1)

Figure 1: Example: copy (1 gain)

In Figure 2:

  • The ReadActualPositionX is continually reading position feedback from an axis.
    • The copy block is saving a specific value to a retain variable.
  • ActualPositionX is the intermediate variable and SavedPositionX is the retain variable.

Figure 2: Example: Workaround