Logix 5000 Online Edit Error: Out of memory in the controller

harryting

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The tech onsite got the above message during an online edit and he had to back out the edit. This is a first I seen this, I did search RA KB and also here. The only relevant KB I found is #40461. I guess I'm asking here because I still see quite a bit of memory left. there's a lot of I/O memory left but under "data and Logic Memory". The one that says "Largest Block Free" is a concern though.

Total: 2,097,152 bytes
Free: 527,140 bytes
Used: 1,570,012 bytes

Max Used: 1,667,196 bytes

Largest Block Free: 71,560 bytes




Processor: 1756-L61/B
Version: 19 or 20

-I'm not at the site, so everything is 2nd hand info. I guess I can do an offline edit than a download but I'm just curious about you guys thought on how to handle this.
 
Reading a bit more into the KB I referenced above, memory fragmentation seems to be the problem. the largest Block free is only 71,560 bytes campared with a total of 527,140 byte free. This fragmentation is probably caused by online edit over the years.

Also, this PLC in question got constant arithmetic overflow error, which doesn't stop the PLC but could this also a contributing cause of the memory fragmentation?
 
I have only seen this when doing multiple online edits of a fairly large structured text routine during development. Doing a download defrags the memory and allows another 10 or so online edits. With ST the compiler has to create a copy of the entire routine somewhere else in memory and then change the reference to that routine, deleting the old. This leaves a hole in memory the size of the older (often smaller) routine.

Workarounds:
Bigger processor
Upload current data, make changes, download program. Obviously requires downtime.
Break up into smaller routines.
Neater ST program
Use ladder, where it only needs to make space for one ladder worth of online edits.
Different brand of PLC. (Which(?))

Not sure what the rules are for FBD or SFC.
 
The tech onsite got the above message during an online edit and he had to back out the edit. ... The one that says "Largest Block Free" is a concern though.

Total: 2,097,152 bytes
Free: 527,140 bytes
Used: 1,570,012 bytes

Max Used: 1,667,196 bytes

Largest Block Free: 71,560 bytes


Processor: 1756-L61/B
Version: 19 or 20

I have not seen this one on a ControlLogix so far. I saw similar on an old PLC 5/60 many times.

The program that is being edited is larger than the 71,560 memory block. The new block, as you figured out, is larger than that so the new code cannot be loaded into one contiguous block, then the ControlLogix switched to look at the new block instead of the old.

In the PLC 5/60, this could happen in ladder logic. The whole subroutine (ladder file) is switched as a block, not just the rung that is changed or added.

Since I have not run into this in ControlLogix, I have not investigated what the exact conditions are for each of the languages.

Loading the program into the ControlLogix will, as mentioned, fix the fragmentation. Or you can make the edit offline and load in the program.

If this happens down the road, you may want to split up, or segment, the large program file into smaller ones so that you can perform more online edits before you get the message again. This should not be an issue if it is the first time you have seen it.
 

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