Cyclic retrieval

Cyclic retrieval mode returns stored data over a trend duration at equal length cyclic intervals. In Cyclic retrieval mode, a single value is returned at the beginning of each cyclic interval.

If no data is stored at the start of a cyclic interval, the last recorded value before the start of an interval is returned. If two consecutive intervals have the same value, no value is returned for the second interval.

Cyclic retrieval works with all types of tags and produces a virtual rowset, which may or may not correspond to the actual data rows stored on the Historian.

The length of a cyclic interval is dynamic and determined by the duration of a trend and the resolution of a trend in pixels: 1 cyclic interval for every 2 pixels.

The following graph shows how tag values are retrieved from the Historian using Cyclic retrieval mode.

Data is retrieved over a period starting at TC0 and an ending at TC2. Each dot in the graph represents an actual data point stored in the Historian. The Historian can return data from three cyclic intervals at TC0, TC1, and TC2.

The following data points are returned by Cyclic retrieval mode:

• At TC0: P2, because it falls on the start boundary of an interval

• At TC1: P7, because it is the last data point before the start of the next cyclic interval

• At TC2: none because P7 and P11 are the same value

Cyclic retrieval mode is fast and consumes little Historian resources. However, cyclic retrieval mode may not accurately reflect stored data because important process values (data gaps, value spikes) may fall outside of the retrieval intervals and not appear as values shown in the trend.