TWControls

I didn't catch the decimal.  Probably switching the low tag to floating and lowering the trigger will work, but how many decimals to the left and the right will you need?

You are pretty much on the right track.  If it were me I would have two DINTs, LowDint and HighDint.  LowDint increments as normal, then have a GEQ source A being the LowDint and source B being 1,000,000,000 to add one to the to the HighDint and set the LowDint back to 0.  Then convert both to strings and Concatenate them into a string that represents the flow total.

I don't think it will have anything to do with the network switch since you data is coming over Devicenet.  If the data is coming over Devicenet then you Ethernet configuration isn't causing the problem, it is more  like it is helping expose the root problem.  See page 30 of the following manual.  Step 4 is what I believe is making it "fix itself" while you are online.  Servicing communications such is you PC affects the scan time.  Can you post your program?  I'm still leaning towards a data synchronization problem such as the trigger that starts the 526ms timer and the 2 digit divert code coming in at separate times. http://literature.rockwellautomation.com/idc/groups/literature/documents/um/1747-um011_-en-p.pdf

Yes, there is nothing that says you have to use the full range of an analog input.  Look at the SCP instruction to scale it into a real world value that you can understand.

Again without seeing you program it is hard to tell, but if there were a timing issue such as a hard wired trigger happening before the 2 digit divert code arriving over the devicenet network or implicit vs explicit messaging, then going online would slow down your scan time slightly which could change the timing of the input vs 2 digit arrival.

You may want to consider using a 250 ohm resistor which would give you 1-5VDC.  Many analog devices have a maximum impedance of 500 ohms so you will be on the edge of what they are capable of driving with a 500 ohm resistor.  See the bottom of this article  in the section titled "Measuring a 4-20ma Input With a Voltage Input Device" for more details https://www.theautomationstore.com/analog-circuits-voltage-0-10vdc-current-source-and-current-loop-4-20ma/

Without seeing your program I can only make wild guesses, but you being online with the controller does affect the controllers scan time.  Are these codes coming over explicit messaging or through mapped I/O points?  If explicit is the a corresponding input that tells you when to read the 2 digit code? What is the scan time when connected?  What type of PLC is it?

What type of instruction are you using to trigger your divert? How long is the input trigger duration?

You will be best to find someone with some RsLogix experience to help you out on this one.  It is going to be very tough for us to help without seeing your program.  What is your location?

We'll need a couple more details before we can be of any help. What type of PLC is it? Can you post your program? When do you notice they aren't right?...

Can you post your program so we can have a look?  Going to be tough guessing without a little insight.

There is no rule that states that in OTL instruction must have an OTU instruction. There are several cases that you might do this. The programmer may of had some glitch that he was trying to catch and so he put that in there so that he could come back later and see it. Probably the more common is that there is an HMI or other network device that is setting that bit to zero. Also don't forget to look for manipulation at the word level such as a move instruction to B3:0 affecting B3:0/5.

Being a 15 year old system I'm guessing it is a Standard Panelview.  Looking through the cobwebs in my head I don't think it had scripting.  I still wonder if the SLC's time isn't mapped to the Status Tags under Application Settings.

That's why I said "chances are" ;)

Can you post the Panelview program?