Add-On Instructions - do your systems use these?
From a programmer's viewpoint, the ability to create custom-made Add-On Instructions is a powerful and exciting new feature.
From a maintenance technician's viewpoint, understanding the PLC's program was always challenging enough - even without a series of these new custom-made bumps in the road being thrown in.
In the best of circumstances it's usually a simple matter to access the Add-On Instruction's logic to see what's going on under the hood. Just Right-Click the instruction and select "Open Instruction Logic" from the popup menu. In other situations it might be completely impossible to do that. Consider that one of the main reasons that programmers love these things is the ability to put "source protection" security in place. In simplest terms, this allows the programmers to protect their intellectual property - and to ensure that all of their hard work can't be simply copied by their competitors.
You should also keep in mind that the original programmer might have decided to use either Structured Text or Function Block Diagrams when writing the code - as well as the "plain vanilla" Ladder Diagram logic that we've shown in these simple examples.
If your systems make use of Add-On Instructions we have hands-on exercises available to bring you up to speed on how to deal with them.
Reference material on Allen-Bradley's website
© Copyright - R.H.Beaufort - Charleston, SC – Updated: November 26, 2013