Cyclic retrieval

Cyclic retrieval is the retrieval of stored data for the given time period based on a specified cyclic retrieval resolution, regardless of whether or not the value of the tag(s) has changed. It works with all types of tags. Cyclic retrieval produces a virtual rowset, which may or may not correspond to the actual data rows stored on the Historian.

In cyclic retrieval, one row is returned for each "cycle boundary." You specify the number of cycles either directly or by means of a time resolution, that is, the spacing of cycle boundaries in time. If you specify a number of cycles, the Historian returns that number of rows, evenly spaced in time over the requested period. The cyclic resolution is calculated by dividing the requested time period by the number of cycle boundaries. If you specify a resolution, the number of cycles is calculated by dividing the time period by the resolution.

If no data value is actually stored at a cycle boundary, the last value before the boundary is returned.

The default retrieval mode is cyclic for retrieval from analog tables, including analog and state summary tables.

Cyclic retrieval is fast and therefore consumes little server resources. However, it may not correctly reflect the stored data because important process values (gaps, spikes, etc.) might fall between cycle boundaries. For an alternative, see "Best fit" retrieval.