The DvCH status value indicates the validity of historical information
involving historian behavior not covered by the Ff status value.
Historian values associated with raw history samples usually have at
most one historian status bit set. Historian values returned for history
aggregates involving several raw data samples are quite likely to have several
historian status bits set. Composite historian status values for interpolated
history samples or history aggregates are obtained by bit-ORing the individual
historian status values that were involved in computing the interpolated sample
or aggregate.
The individual historian status bits defined are:
-
History data unavailable
(** No Data **) – When set, indicates a type of hole in the
historical data record. This status would let history clients know that the
historical data server has no historical data set mounted that includes this
history tag for this point in time. Situations would include the period of time
before the oldest history sample currently online for a history tag or gaps in
the online historical data record due to historical data sets being removed.
- History collection not
configured
(Not Configured) – When set, indicates a type of hole in the
historical data record. This lets history clients know that the hole is
intentional, that is, caused by the configuration engineer deciding to no
longer have historical data captured for this tag on this node, or configuring
the historian to be disabled.
- Historian/scanner
shutdown (Historian shut down) – When set, indicates a type of hole in the
historical data record. This status lets history clients know that the hole was
a temporary outage due to the historian not operating or due to planned or
unplanned DeltaV shutdowns. It is likely that no other history tags have data
available from this server at this point in time.
- History data
unreadable
(Not Readable) – When set, indicates a type of hole in the
historical data record. This lets history clients know that the history tags
became unreadable for whatever reason: source node crash, communication
problems, configuration changes partially distributed, etc.
- Aggregate value invalid
(Bad Aggregate) – This status bit is set for aggregate values requested
using
read processed calls that could not be produced. Situations
where it is impossible to compute a requested aggregate value might include:
- A minimum, maximum, or
average is requested for an interval that is entirely within a hole in raw data
samples, so there are no useful values that can be used in computing the
aggregate.
- An aggregate is
requested that is not supported on a history tag of that type; for example,
requesting a time-weighted average for a string type history tag.