Rockwell Automation Logix5000 Controllers Add-on Instructions Programming Manual - Manual (Page 26)

Chapter 1 Designing Add-On Instructions
Limits cannot be accessed inside Add-On Instruction logic.
The maximum data instance supported (which includes Inputs, Outputs, and
local tags) is two megabytes. The data type size is displayed on the bottom of the
Parameters and Local Tags tab in the Add-On Instruction Definition.
Runtime editing
Add-On Instructions can only be edited offline. If the intended functionality
needs to be changed in a running controller, consider carefully if an Add-On
Instruction is suitable.
Nesting Add-On Instructions
Add-On Instructions can call other Add-On Instructions in their routines. This
provides the ability to design more modular code by creating simpler instructions
that can be used to build more complex functionality by nesting instructions. The
instructions can be nested to seven levels deep.
Add-On Instructions cannot call other routines through a JSR instruction. You
must use nested instructions if you need complex functionality consisting of
multiple routines.
Tip: To nest Add-On Instructions, both the nested instruction and the instruction that calls it must be of the
same class type or the calling instruction will not verify. That is, standard Add-On Instructions may call only
standard Add-On Instructions and safety Add-On Instructions may call only safety Add-On Instructions.
Routines versus Add-On Instructions
You can write your code in three basic ways: to run in-line as a main routine, to use
subroutine calls, or as Add-On Instructions. The following table summarizes the
advantages and disadvantages of each.
26 Rockwell Automation Publication 1756-PM010F-EN-P - October 2014