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. |
If times are not synchronized across the system components, for example by using an NTP server, its possible that the time at the PLC is older than other system components. If using MQTT engine, this can result in tag change events being ignored as the new event time is before current time for the tag at MQTT Engine. |
Let's assume an Edge Node has a single tag named Tag1