Data retrieval modes

Cyclic

Retrieves one value per cycle. Whatever the value is when the cycle begins.

Delta

Retrieves a value each time the value changes from the previous value. For example, if the value of "4" followed an earlier value of "4", it would not be retrieved. But if "4’" followed "3", it would.

Full

Every value within a time period is retrieved.

Interpolated

Based on values before and after a certain point in time, Historian estimates the value for that time.

In this example, P2 is located exactly at the query start time. Because of this, P2 is returned at that time without need for interpolation. At the following cycle boundary, point PC1 is returned, which is the NULL value represented by P7 shifted forward to time TC1. At the last cycle boundary, point PC2 is returned, which has been interpolated using points P11 and P12.

Best Fit

"Best fit" retrieval allows for a compromise between delta retrieval and cyclic retrieval.

Delta retrieval can accurately represent a process over a long period of time, but requires a large number of data values. Cyclic retrieval is much more efficient, but less accurate, because of fewer values.

Best fit provides faster retrieval, like cyclic retrieval, plus the better representation, like delta retrieval.

Average

Uses a time-weighted average algorithm to calculate the value for each retrieval cycle.

For the following data values of a tag that uses linear interpolation, the time-weighted average is computed as:

Average = (((P1 + P2) / 2) x (T2 - T1)) + (((P2 + P3) / 2) x (T3 - T2)) + (((P3 + P4) / 2) x (T4 - T3)) / (T4 - T1)

Minimum

Returns the minimum value from the actual data values within a retrieval cycle. If there are no actual data points stored on the historian for a given cycle, nothing is returned. If there are NULL values in the cycle, NULL is returned for that cycle.

Maximum

Similarly, this mode returns the maximum value of actual data for the retrieval cycle.

Integral

Calculates the values at retrieval cycle boundaries by integrating the graph described by the points stored for the tag. In other words, it works much like average retrieval, but it additionally applies a scaling factor. This retrieval mode is useful for calculating volume for a particular tag (for example, gallons of water flowing through a valve over a certain period).

Integral retrieval works with analog tags only. For all other tags, normal cyclic results are returned.

Slope

Returns the slope of a line drawn through a given point and the point immediately before it, thus expressing the rate at which values change.

For example, two points P1 and P2 occur at times T1 and T2. The slope is calculated as:

(P2 - P1) / (T2 - T1)

The difference between T1 and T2 is measured in seconds, so the returned value represents the change in engineering units per second.

Counter

The change in a tag’s value from the beginning to the end of the period, factoring in any rollover value for the counter. This retrieval mode is useful for determining how much of an item was produced during a particular time period.

ValueState

Returns information on how long a tag has been in a particular value state during each retrieval cycle. That is, a time-in-state calculation is applied to the tag value.

RoundTrip

Like ValueState retrieval, this mode uses state occurrences within a period for its calculations. RoundTrip retrieval calculates the time between consecutive leading edges of the same state.

Bound Value

Retrieves either the start bound point or the end bound point for a requested point in time. For a start bound point, Historian retrieves the first value on or before the requested date/time. For an end bound point, Historian retrieves the first value after the requested date/time.