The run-time history service processes are started automatically when DeltaV services are started, and the Continuous Historian subsystem has been configured to be active on that workstation. History collection and historical data services for client applications are not available until these processes are started.
These run-time processes terminate when DeltaV services terminate.
When history tags are configured for collection, the DeltaV workstation hosting the active historian periodically samples the parameter field and Ff status field for storage in the active historical data set. The time stamp of the data sample is attached by the workstation during the sampling/storage process. Heavy ACN communications loading and/or historian workstation loading can decrease the accuracy of the time stamp.
Users who are especially concerned with having accurately time-stamped historical data can purchase an NTP server to ensure accurate time bases in all nodes in the DeltaV system and very small system time adjustment amounts. If an NTP server is not used, a system clock adjustment, either forward or backward in time, could occur on a node hosting a historian.
If the system time on a historian node is set forward (time increased), the history clients will see historical data as if it had not changed for the time span the clock was set forward. There will be flat lines or gaps between samples plotted. No unusual status values will be inserted indicating the time shift.
If the system time is set backward (time decreased), the new data from the same time period is added to the historical database. If you plot this in Process History View, you will see that the line jumps back in time on the chart. No unusual status values will be inserted indicating the time shift.