5000 online editing- Toggling between subroutines

lftmx3 · Oct 21, 2021

I have been asked to extensively modify a piece of machinery that is very mission critical to the operation, thus not allowed much downtime or time to debug online. I don't forsee this being a problem, as I have taken the existing code and downloaded it to a bench PLC, then developed and tested the new application there.

My idea is to simply copy the modified subroutines into the online PLC with a slightly different name, then if needed to revert to the old program, toggle between the old/new JSR's in the mainroutine schedule.

I am almost positive this won't be a problem- there will be identical OTE's in the routines, but as long as one routine is not being scanned, it should be OK, right?

Once the new process has been verified, I will be deleting the old routines from the processor.

Jim3846 · Oct 21, 2021

Only issue with that toggle bit, is if the plc loses power and that toggle bit is on, sometimes that toggle bit gets reset to zero.

So if you have your new code in just put a latch on bit to keep it on if you want it to run for awhile till you think you have the file debugged. Then delete later when you think its working as you want it.

RVaughan · Oct 21, 2021

You will want to remember the inertia of bits. That is, if a tag is writtten to in one routine, then you stop calling that routine, the tag stays the same value. It doesn't revert to zero just because it is no longer written to.
Make sure all of your tags are accounted for in both routines, or initialized outside of those routines when necessary.

"Bits only move when pushed."

PreLC · Oct 21, 2021

lftmx3 said:
I have been asked to extensively modify a piece of machinery that is very mission critical to the operation, thus not allowed much downtime or time to debug online. I don't forsee this being a problem, as I have taken the existing code and downloaded it to a bench PLC, then developed and tested the new application there.

My idea is to simply copy the modified subroutines into the online PLC with a slightly different name, then if needed to revert to the old program, toggle between the old/new JSR's in the mainroutine schedule.

I am almost positive this won't be a problem- there will be identical OTE's in the routines, but as long as one routine is not being scanned, it should be OK, right?

Once the new process has been verified, I will be deleting the old routines from the processor.

If you can program the code offline, why don't you try importing it online when the PLC is running? You can import while online and the tags are automatically created, and you can choose to import as "pending"/"Accepted" /"Finalized". If you are using AB processors, as indicated by the lingo.

This would allow you to test before compiling your code.

Jim3846 said:
Only issue with that toggle bit, is if the plc loses power and that toggle bit is on, sometimes that toggle bit gets reset to zero.

Can you elaborate a little on this?

MichaelG · Oct 24, 2021

PreLC said:
Can you elaborate a little on this?

It depends on how the bit is used as to what happens during "prescan"
for the OTE bit shows that the bit is reset
OTL there is no Action
OTU no Action

Extract from the RS Logix5000 manual below for OTE

5000 online editing- Toggling between subroutines

lftmx3

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Jim3846

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RVaughan

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PreLC

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MichaelG

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