What are the advantages and disadvantages to using either in transaction groups?
Advantages of direct OPC items:
- Subscribed (or read) according to the rate of the group
- Alerts are attributed to the group
- Can have the same item multiple times (to write to multiple columns, do different things, etc).
SQLTags Reference, on the other hand, are a bit more limited. The #1 difference is that they are run externally (by the scan class), so you have to be careful about timing. That is, if you have a group that runs a 1 sec, but the tag is in a 10 sec scan class, you’ll only get a value change every 10 seconds. Beyond that, you can only have the tag in the group once, and you can’t configure any additional settings on it, like scaling or alerting.
The opc item technically leads to another subscription, but depending on rates this might just get paired up with the tag subscription, so the benefit of avoiding that would be extremely minimal.
Hope that answers your question,