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)