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Does the logger store quality as well as tag value?

Can the logger be configured to log on tag changes such that any change to value or quality is logged? If so, will just the value or just the quality be logged? or both?

When we talk of an item "change", we actually mean the item-as-a-whole. The OPC server, which is responsible for detecting the "change" must follow two criteria in determining if a change has occurred:

  • the value of the item is different from the previous reading
  • the quality of the item is different from the previous reading

Whether one or both criteria has changed, this constitutes a "change" which must be sent to the OPC client.

So, what is sent?

In OPC, a reference to the item is sent to the OPC Client along with its value, its quality and timestamp. So because the OPC Server sends ALL of this information in a "change" event, the OPC Client (in this case the OPC Data Logger) will receive all of this information and will consequently log it.

In essence, in OPC Data Logger you will simply specify that you want to log using DETAIL MODE and then you can specify what that looks like; i.e. if logging to a database, you will need to map item-attributes with table-fields; and that's it! When any "change" occurs for an item, its values (could be value, quality and timestamp -- but that is up to you) will be logged.