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Deleted ControlLogix PLC tags still returning "Good" data

Something that we know is done on some customer sites is the deleting, adding or updating of tags online i.e. not a download. I have tried some simple adding and removing of tags online and found some peculiar behavior. If I remove a tag from the PLC that is still being accessed by TOP Server then I do not get any indication via OPC through the quality that the tag has gone bad. I can not write to the tag as expected but the quality remains as GOOD and retains the old value. The behavior is different if a tag that is configured in TOP Server but doesn't exist on the PLC is then added to the PLC. Adding and removing this tag now causes the quality to go from GOOD to BAD.

The behavior you are seeing has to do with the protocol your using vs. actual Symbolic tag addressing. We are reading memory addresses that are indexed to symbolic names. Deleting the Symbolic name in the index does not delete or clear the actual memory location. We are still reading data at that memory location even though the Symbolic name in the PLC index has been deleted.

  • If you restart the TOP Server (or stop and restart all client connections) after the tag has been deleted from the PLC index, you should find that the tag will no longer read as good.
  • If you actually download the program from RSLogix5000, the Server will also go into symbolic mode and back to physical mode and remap the index.

The is a development issue that people should be aware of, but should not see in normal production environment as deleting tags this way is very dangerous. We recognize that some users may try this, but the best we can do at this time is warn them of the dangers, as we have no way of knowing that the symbolic index was changed while using Physical type protocol reads.
If there are particular items that will be changed frequently (potentially I/O blocks), we suggest putting these tags under a different channel and device configuration in the server and setting this to Symbolic Mode. A small quantity (under 1000 for example) of tags configured this way can prevent this problem with no noticeable impact on overall performance.