Expression items in transaction group executing late

This is a follow up to this thread. I started a new topic because this seems to be unrelated to the other issue and might affect other users as well.

It took me a while to make this reproduceable, it seems to be related to order of expression items in the group. After some testing, i found the following conditions necessary to trigger the issue:
[li]The group has to use the ‘Read’ mode instead ob subscribe. The example backup does not use any OPC items, but it still shows the issue.[/li]
[li]The expression item has to reference an Basic OPC/SQLTag Item.[/li]
[li]The expression items has to be in an certain order.[/li][/ol]

Under these circumstances, the expression item is always one trigger cycle behind. After the first trigger, it is ‘Stale’ (or shows an ‘TypeConversionError’ if the expression is more complex). See screenshot 1.
After any subsequent trigger, the Expression item shows the value from the previous trigger evaluation. (See screenshot 2).

Interestingly, the evaluation is ok, when the evaluation order is changed (Screenshot 3). One would expect the second expression to show the issue now, but this is not the case.

I hope you can find any system behind this, because this affects not only the first trigger cycle after a restart, but results in logging wrong data all the time.

The attached gateway backup contains the project, from which above screenshots were taken. I deleted some of the jdbc drivers from the backup to make it small enough to upload to the forum.
Ignition-backup-20121001-1256.gwbk (2.68 MB)


Sorry, I was out last week and didn’t see this. I’ll take a look at it and post back tomorrow.



We’ve tracked this down and fixed it. It was being brought out through Read mode, but has to do with the order of item execution, and how references execute. It would also be demonstrated by having a item that references another item further below it in the item list.

The fix will be in 7.5.4. Thanks for the detailed mockup!