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The timestamp for a tag read from a PLC will be the timestamp assigned by the PLC.

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This timestamp will be passed to the Ignition tag and delivered via MQTT Transmission to MQTT Engine.

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:

On a Transmission

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BIRTH message

• The BIRTH message for each Edge Node and Device will set the timestamp associated with each tag to the current time of the host MQTT Transmission server's OS.
• The timestamp at the Edge will show the last known timestamp from the PLC and Engine will display the timestamp included in the BIRTH message

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• When an OPC tag is restarted, the timestamp at Edge will reflect the last known change timestamp from the PLC whilst Engine will reflect the timestamp of the restart. 

Common Issue with tag timestamps

Time at PLC is older than Transmission or Engine leading to tag change events being ignored as the new event time is before current time for the tag at Engine