Maintaining correlation for non-trivial program updates

Is there a proper procedure to downloading an updated program to a PLC while maintaining the existing tag values? Specifically, I need to add two new tanks to a tank farm currently in operation, and want to ensure that when I push my update, that the tag values for the existing tanks stay exactly the same - I believe the term is 'loss of correlation'. If I upload the current program, and make all of my changes offline, then I would expect the stale old tag values to overwrite the current.

I have done plenty of rung edits on running PLCs, but never adding additional programs. Can I create unscheduled programs, modify them while online, then schedule them when they're ready?

Thanks in advance for any insight.

What PLC brand and model are we talking about?

This is an AB CompactLogix 1769-L33ER, project version 20.13. All I/O is POINT I/O.

You can add all new programs, ladders, routines, and tags while online and do online edits, but needed to use caution that you don't over write something in use by mistake.


The biggest hassle i always have when doing large amounts of programming online is if you forget to change a tag from DINT to REAL, you have delete the tag and recreate. Also, you can usually add routines without issues while online, but cannot always delete them. As long as you use caution and understand the process, you should be able to do all programming via online edits.

Thanks for the advice. Any reason why this wouldn't work for an unscheduled program?

I'd like to copy one of the exiting tank's program (phase controlled) paste it as an unscheduled program while I update the mappings, new array indexes, etc. Then separately, update our HMI (we use ignition), before finally scheduling the program to begin operation.

If you must work offline, edit your program offline, export the programs/routines you need, then just import them online.