When this structure is used for reading Raw Values (isReadModified is set to FALSE), it reads the values, qualities, and timestamps from the history database for the specified time domain for one or more HistoricalDataNodes . This parameter is intended for use by a Client that wants the actual data saved within the historian. The actual data may be compressed or may be all raw data collected for the item depending on the historian and the storage rules invoked when the item values were saved. When returnBounds is TRUE, the Bounding Values for the time domain are returned. The optional Bounding Values are provided to allow the Client to interpolate values for the start and end times when trending the actual data on a display.
The time domain of the request is defined by startTime, endTime, and numValuesPerNode; at least two of these shall be specified. If endTime is less than startTime, or endTime and numValuesPerNode alone are specified then the data will be returned in reverse order, with later data coming first as if time were flowing backward. If a startTime, endTime and numValuesPerNode are all provided and if more than numValuesPerNode values exist within that time range for a given Node then only numValuesPerNode values per Node shall be returned along with a continuationPoint. When a continuationPoint is returned, a Client wanting the next numValuesPerNode values shall call ReadRaw again with the continuationPoint set. If numValuesPerNode is 0, then all the values in the range are returned. A default value of DateTime.MinValue (see OPC 10000-6) is used to indicate when startTime or endTime is not specified.
It is specifically allowed for the startTime and the endTime to be identical. This allows the Client to request just one value. When the startTime and endTime are identical then time is presumed to be flowing forward. It is specifically not allowed for the Server to return a Bad_InvalidArgument StatusCode if the requested time domain is outside of the Server's range. Such a case shall be treated as an interval in which no data exists.
If the request takes a long time to process then the Server can return partial results with a ContinuationPoint. This might be done if the request is going to take more time than the Client timeout hint. It may take longer than the Client timeout hint to retrieve any results. In this case the Server may return zero results with a ContinuationPoint that allows the Server to resume the calculation on the next Client HistoryRead call.
If Bounding Values are requested and a non-zero numValuesPerNode was specified then any Bounding Values returned are included in the numValuesPerNode count. If numValuesPerNode is 1 then only the start bound is returned (the end bound if the reverse order is needed). If numValuesPerNode is 2 then the start bound and the first data point are returned (the end bound if reverse order is needed). When Bounding Values are requested and no bounding value is found then the corresponding StatusCode entry will be set to Bad_BoundNotFound, a timestamp equal to the start or end time as appropriate, and a value of null. How far back or forward to look in history for Bounding Values is Server dependent.
For an interval in which no data exists, if Bounding Values are not requested, then the corresponding StatusCode shall be Good_NoData. If Bounding Values are requested and one or both exist, then the result code returned is Success and the bounding value(s) are returned.
For cases where there are multiple values for a given timestamp, all but the most recent are considered to be Modified values and the Server shall return the most recent value. If the Server returns a value which hides other values at a timestamp then it shall set the ExtraData bit in the StatusCode associated with that value. If the Server contains additional information regarding a value then the ExtraData bit shall also be set. It indicates that ModifiedValues are available for retrieval, see 184.108.40.206.