Exceptions to the store-forward algorithm

In some cases, the store-forward algorithm will fail due to expected behavior never occurring. The following two cases have been observed:

1. Bad quality and connection loss (10) quality detail always present.

2. The First-Store-Forward data value (248) is never seen.

   Note: The store forward transition timeout is not configurable.

To solve the first case, a timer runs during the store-forward data collection for each tag. If this timer exceeds a preset amount (currently 5 minutes) without going into a Store-Forward-Transition state, then the last good update time is moved to 1 millisecond beyond the update time of the value containing the bad quality and Connection Loss (10) detail. The tag then is moved into the Store-Forward-Transition state. The timer is reset and the store-forward data collection uses the new last good update time for its start time and the time of the transition as its end time.

Note: Since store-forward data collection is queried on multiple tags the tag may not move out of the store-forward state for several timer periods. This is due to the minimum of all the tag's last good update time used for the start time of the query and minimum is less than the timestamp of the Bad quality and Connection Loss (10) quality detail value of a particular tag in the query group.

Note: Item 1 usually occurs on slow or un-changing data values. The slow changing data values don't always have an update during the time the tag was in the store-forward state so the historian doesn't receive a replacement value for the bad quality and Connection Loss (10) data value.

To solve item 2 the same timer used in item 1 is employed. If this timer exceeds a preset amount (currently 5 minutes) without completing the Store-Forward-Transition state then the tag is moved to the normal data collection queue using the start time of the last good update and the end time of DateTime.Now.

Note: The timer is not started unless the RPC Reconnect (44) quality detail or a good data value is detected.