Retrieving Data from Historian

The sample has a time stamp, a value, and a quality. The time stamp returned is not typically the current time; it is the time stamp as sent by the collector. If you have a slow poll rate or if collector compression is enabled, the time stamp may be much earlier than the current time.

You must also specify a direction and number of samples when using this sampling mode.

The Interpolated sampling mode is also useful for data samples that haven't undergone archive compression. For example, you may want to plot data along an interval that doesn't match the collected raw samples. Using the Interpolated sampling mode would give you the most likely real-world values for the specified period.

This mode is best used when querying compressed data because the Data Archiver can switch to the more efficient raw data query.

Lab sampling is most often used to create a step chart rather than a smooth curve.

Use Lab sampling instead of interpolated if you only want true collected values returned. The Lab sampling mode is generally not useful on highly compressed data. Use interpolated sampling instead.

The trend sampling mode returns the maximum and minimum value collected during each interval. When plotted, this makes it possible to display an accurate representation of the data that wont miss any extrema, by only retrieving a minimum of points.

For example, a trend of one year of data with a one-day interval will return 730 values consisting of the 365 minimums and 365 maximums for each day of the year.

TrendtoRaw is used when the number of actual data samples are fewer than the requested number of samples. In that case, TrendtoRaw retrieves raw data in a given interval (between a selected raw minimum and raw maximum).

Trend2 sampling splits up a given time period into a number of intervals (using either a specified number of samples or specified interval length), and returns the minimum and maximum data values that occur within the range of each interval, together with the timestamps of the raw values.

Trend sampling mode is more suitable for plotting applications that prefer evenly-spaced data.

Trend2 sampling mode is more suitable for analysis of mins and maxes and for plotting programs that can handle unevenly spaced data.

The TrendtoRaw2 sampling mode almost always produces the same results as the Trend2 sampling mode. The exception is that, when more samples are requested than there are raw data points, the TrendtoRaw2 sampling mode returns all of the available raw data points with no further processing.

This sampling mode is used with the time range and filter tag conditions. The result starts with a starting time stamp and ends with an ending timestamp

The Count calculation mode is useful for analyzing the distribution of raw data samples. If you have a higher number of raw samples than expected, you may decide to implement collector or archive compression. If samples are missing, then you may want to slow your collection rates.

See the note in Minimum for additional information.

See the note in Maximum for additional information.

The OPCQAnd is a bit wise AND operation of all the 16 bit OPC qualities of the raw samples stored in the specified interval.

All the calculation modes such as minimum or average exclude bad values by default, so this modifier is not required with those.

This modifier is only useful with rawbytime or rawbynumber retrieval. Do not use it with any other sampling or calculation mode.

This modifier is only useful with rawbytime or rawbynumber retrieval. Do not use it with any other sampling or calculation mode.

ONLYIFUPTODATE

• Collector loses connection to the archiver
• Collector crashes
• Collector compression is used and no value exceeds the dead band

Normally, a data query for minimum value will interpolate a value at the start of each interval and use that together with any raw samples to determine the minimum value in the interval. Interpolation is necessary because some intervals may not have any raw samples stored.

Use this query modifier with calculation modes only, not with raw or sampled retrieval like interpolated modes.

A lab sampled average would be used when querying a tag that never ramps but changes in a step pattern such as a state value or setpoint.

Use this query modifier with calculation modes only, not raw or sampled retrieval like interpolated modes.

You can use ENUMNATIVEVALUE with any sampling or calculation mode.

You can use INCLUDEBAD with any sampling or calculation mode.

This query modifier is not always recommended.

In cases where you want to specify a time limit, you can do this by specifying an end time in your RawByNumber query and including the HONORENDTIME query modifier. Since RawByNumber has direction (backward or forward), the end time must be older than the start time for a backward direction or newer than the start time for a forward direction.

Use this query modifier only with the RawByNumber sampling mode.

When using FirstRawValue, FirstRawTime, LastRawValue, and LastRawTime calculation modes, we can use only the raw sample near each interval boundary and achieve the same result. The EXAMINEFEW query modifier enables this. If you are using one of these calculation modes, you may experience better read performance using the EXAMINEFEW query modifier.

Query performance varies depending on all of the above factors.

Use this query modifier only with FirstRawValue, FirstRawTime, LastRawValue, and LastRawTime calculation modes.

Stale tags are tags that have no new data samples within a specified period of time, and which have the potential to add to system overhead and slow down user queries.

The EXCLUDESTALE query modifier allows for exclusion of stale tags in data queries.

Unless permanently deleted, stale tags from the archiver are not removed but are simply marked as stale. Use the query without this query modifier to retrieve the sample values.

Data is not returned for stale tags. An ihSTATUS_STALED_TAG error is returned instead.

You can enter your filter conditions using Filter tag, Filter Comparison Mode, and Filter Comparison Value or you can put that all that information in a single FilterExpression.

• AND Condition
• OR Condition
• Combination of both AND and OR

Filter Expression can be used instead of FilterTag, FilterComparisonMode and FilterValue parameters. While using FilterExpression, the expression is passed within single quotes and for complex expressions we write the conditions within a parenthesis. There is no maximum length for a filter expression, but if it is called using OLEDB or Excel, they may have their own limitations.

The Filter Mode defines how time periods before and after transitions in the filter condition should be handled.

For example, AfterTime indicates that the filter condition should be True starting at the timestamp of the archive value that triggered the True condition and leading up to the timestamp of the archive value that triggered the False condition.

ExactTime

Retrieves data for the exact times that the filter condition is True (only True).

BeforeTime

Retrieves data from the time of the last False filter condition up until the time of the True condition (False until True).

AfterTime

Retrieves data from the time of the True filter condition up until the time of next False condition (True until False).

BeforeAndAfterTime

Retrieves data from the time of the last False filter condition up until the time of next False condition (While True).

The Filter Comparison Mode defines how archive values for the Filter Tag should be compared to the Filter Value to establish the state of the filter condition. If a Filter Tag and Filter Comparison Value are supplied, time periods are filtered from the results where the filter condition is False.

The value to compare the filter tag with when applying the appropriate filter to the data record set query (to determine the appropriate filter times).