AB PLC5 Memory

  • Thread starter Hutchinson, Ben [LFS]
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Thread Starter

Hutchinson, Ben [LFS]

This one caught me off-guard today:

Prior to downloading a new program (Y), I uploaded existing program (X). Attempted download, but new program Y exceeded available memory, thereby clearing memory. Attempted to downloaded original program X, but it wouldn't fit either. Wait a minute, it was in there in the first place! Is there a threshold set lower than the actual memory that limits full downloads but may allow you to exceed threshold through on line changes? Or, is there some other explanation as to why what came out would not go back? All done in RSLogix 5 v 4.10.00

Ben Hutchinson

Anthony Kerstens

I don't think the program grew. I suspect that the processor is corrupted and needs to be completely cleared to its "shelf-state".

Alternatively, get a spare processor of the same part number and try loading to that. If the program came from the processor, and no offline modifications were made, then it should be able to
fit back in.

Anthony Kerstens P.Eng.

Michael Griffin

Was the original program written and loaded with the same version of programming software being used now? I have seen different versions of PLC software add different amounts of overhead to exactly the same ladder logic.

This is because there is not necessarily a one to one correspondence between what you see on the screen and what gets down loaded to the PLC. Some ladder instructions or patterns get constructed from several simpler instructions. A different version of software can "re-write" some of the hidden parts of the rungs without telling you what it is doing. This means it is possible for a program to "grow" without you doing anything other than loading it into your programming software.

I have seen this with other brands of PLC. Although I have worked with several models of PLC5, I'm not familiar enough with what goes on
inside it to say how possible this is. However, if the other suggestions you have received don't help, you might consider this one.

Michael Griffin
London, Ont. Canada
We did face this problem we took a spare processor card downloaded the old program and fitted it back to the back plane. After that we have made it a plan to take backups of the existing programs on devices every month. We did have a copy of the old prgram on a floppy. I think u are fortunate enough to have the backup. Go ahead!!!