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In general, the Edge is the source of truth for tag timestamps although there are several events that will cause the timestamps to differ across the system:
In a BIRTH message
OPC Tag Disabled
OPC Tag Enabled
OPC Restart Tag
The timestamp for an Edge tag read from a PLC will be the timestamp assigned by the PLC and/or associated Ignition driver.
The timestamp for any other Edge tag will be the Ignition server OS time for change in Qualified Value (Value, Quality or Timestamp).
All timestamps published on the wire are in UTC and displayed converted to local time at the receiving application.
Lets assume an Edge Node has a single tag named Tag1
1. Transmission connects and publishes its BIRTH. The timestamp for Tag1 set at the Edge to 'now' per the Edge's system clock.
2. Tag change events occur naturally at the Edge - during this time the timestamp is always set using the tag change event time at the Edge. This results in Tag1's timestamp at Engine matching that of the Edge.
3. Transmission loses connection. At this point, Engine 'stales' the tag by setting the quality to BAD_STALE and sets the timestamp of Tag1 to 'now' per the Central Gateway's system clock.
At this point the tag may or may not be changing at the Edge - Engine won't know in real time this is happening which is why it sets the quality of tag1 to BAD_STALE. 5. Transmission reconnects and publishes its BIRTH. The timestamp for tag1 set at the Edge to 'now' per the Edge's system clock. Assuming the quality of tag1 at the Edge is GOOD - it will be set to GOOD at Engine. If the tag value had never changed at the Edge (per point 4), then you would see three events with the same value, three different timestamps, and the quality go from GOOD -> BAD_STALE -> GOOD.