We just had an issue where Ignition didn’t show the current value of an OPC tag.
When looking in the tag browser in the designer, the string was empty. But when using the OPCUA Quick Client, we could read the string, and it contained a message.
We use this string for messaging. When the PLC has a message, it puts the message in that address. Then there’s a trigger on tag change, so Ignition can execute the message from the PLC. When done, we empty that message string from Ignition’s side as an acknowledgement, so the PLC knows it’s safe to write a new message.
In this case, the trigger wasn’t received (the string in Ignition was still empty), while the PLC was waiting for the acknowledgement. So the production eventually halted on the lack of messages (which is why we noticed it). But AFAICS, it can just as well happen on any other tag. It would just be less noticeable, as the production won’t halt on a wrong visualisation.
Here are the scan class settings we use for the messaging tags. I see anything wrong with it. The OPC connection settings are all on the default.
Would there be something we can fix to avoid this in the future?
This installation is using version 7.9.4, and is running for almost a year by now.