PLC5/20 At Memory Limits - Being Cautious

Hello everyone,

This is my first post at this forum, although I don't know why I took so long to join. I've been working in this field for a few years now and have frequented this forum as a reference in the past. I'm looking forward to getting involved!

My problem:

I'm working with a nearly at-capacity PLC5/20. In the controller properties, there are the following details:

Program Files: 18, Words: 7949
Data Files: 41, Words: 7734
Overhead: 360
Memory Used: 16043 Words
Free Memory: 341 Words

I am attempting to address (in existing data tables) about 3 or 4 rungs of logic, including 2 timers, 1 additional memory bit, and a few compare instructions. It's a relatively minor change, but I'm concerned about adding logic into the PLC with this little memory left.

My question is; do I need to be concerned about "breaking the camel's back" so to speak, and faulting the PLC if I exceed the remaining memory during an online edit? Assuming there is space in the data table and that it's not being overrun, should I be concerned about this type of fault for an on-the-run change? Or will there be some kind of warning that prevents me from disrupting the running of the PLC?

Many thanks in advance for your time.

In my experience RS-Logix 5 will pop-up an error message when you attempt to accept the edits. The message does not indicate low memory, just something like "cannot accept edits." I have never faulted a processor in this situation, and have run into it several times over the years.

Welcome to the world of posting...

The PLC5/20.....ah yes...

When you make an online edit, the PLC makes a copy of the rung that you are going to edit, so adding to the overhead. So do not edit too large a rung...

Can you do this exercise offline first to see what effect you are going to have on the PLC memory?

At least then you will see what the overall total is, although with that little memory left, you may well be struggling - if not for this change, but for anything in the future.

Is there any code you can remove to create more memory, or delete data files that are not required? - appreciate deleting data files will require you to be in program mode....

Welcome to the forum!

I've had the same experience as Mispeld. The lower your free memory gets and the more complex the rung being edited, the more likely you'll get that scenario. With some of our processors, I'd be happy to have 341 words remaining!

If you're in a bind, the quick (but not necessarily cheap) fix would be to upgrade to a 5/40, 5/60 or 5/80. The not so quick (but possibly a little cheaper) fix would be to migrate to a Logix based platform.

You should get a popup message when you reach the max memory
I would make a copy of the program and edit it off line first and when finished check the memory again if all is well either upload or make the changes on line. Remember when you upload the program will stop you will need to consider this when you upload.

Thanks for the responses, everyone. The code has been added without issue.

I checked ahead of time in an offline copy and the free memory dropped by 47 words to 294 remaining afterwards.

I take all of your points that there's some cleanup work that can be done; at this point it's just a matter of keeping the PLC running until it can be replaced.

Cheers,

RedKingRory said:

Thanks for the responses, everyone. The code has been added without issue.

I checked ahead of time in an offline copy and the free memory dropped by 47 words to 294 remaining afterwards.

I take all of your points that there's some cleanup work that can be done; at this point it's just a matter of keeping the PLC running until it can be replaced.

Cheers,

Click to expand...

One warning - you need enough spare memory to hold a new copy of the rung that you are editing. So a really large rung will give you the error mentioned by GaryS, RonJon, and Mispeld.