Tag values reverting to previous states

AlexN2489

Member
Join Date
Jan 2018
Location
Wisconsin
Posts
1
Hi, im having an issue with programming on a 1756-L72 ControlLogix 5570 where every time I update a tag value it will save for that day, then the next day when I come in the tag value has gone back to its previous state. I am backing up the program in Asset Centre and the archived version matches the changes I make but the PLC is losing the changes daily. We have been using this system for our 2d label scanners for years and this issue started happening just a few months ago. Anyone have any experience with a similar issue?
 
Well, something is writing over the value. It doesn't just happen for no reason. Logic in the PLC, another PLC on the network, a HMI; ask to see what changed.
 
...Logic in the PLC, another PLC on the network, a HMI...

One way to sort this out is with "external access" in the tag properties. If it is set to "None" or "Read Only," the value must be changing by internal logic. If this is the case, do a cross-reference and examine all the destructive references for the unexpected changes.

If external access is "Read/Write" then it is open to changes from network-connected devices. This is where you will have to decide if changing it to Read Only is acceptable. If another device is writing to the tag, that functionality will get broken in some respect if you make the tag Read Only. It may at least help determine why it is changing, but with some risk that must be considered.

(With an -L72, you should have a new enough Logix rev with the external access setting in the tag properties.)
 
Correct me on this, but I thought External access don't include CIP messaging, only HMI/OPC access??

Well, I assumed it included CIP messaging, and in the following situation I tested it to be a good assumption.

I set up a "CIP Data Table Write" MSG from one Logix processor to another. With the receiving tag set to external access "Read/Write" the message works as expected. When changed to "Read Only" the sending message signals extended error code 16#0000_2100, access permission denied for requested service.

Interestingly, if the external access on the target of the write is set to "None" there is only primary error code 16#0004, IOI syntax error.

I'm sure this makes sense to those with more knowledge of the inner workings of CIP messaging. Also, I did not try other message types.

One last thing I tried is changing external access to "Read Only" on a consumed tag, and it had no effect. The producer was still able to update it.
 

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