Item validation

When items are added, they are not initially validated. Since SIDirect symbolic addressing is in text format, the SIDirect Communication Driver can only determine whether an item is valid by querying the PLC to check whether the item is defined, and then returning the data type. Validation begins after items are added.

If you are adding many items, performance would be negatively impacted if the query was performed immediately. This delayed validation improves performance.

The SIDirect Communication Driver contains a tag database in its address space that closely mirrors the tag database in the S7 1200 or S7-1500 PLC. If the tag database in the PLC is not available, adding an item always succeeds and the data type VT_EMPTY is sent to the PLC. Once the SIDirect Communication Driver successfully queries the PLC, the data type is updated to the correct type.