Historian Point Type parameters
The
Point Type
tab displays a columnar grid of the parameters defined for the points of the Historian object.This table describes the parameters in the
Point Type
tab of the Historian object.Parameter | Description |
|---|---|
Name | The name of the point. Double-click to configure a unique name for the point. |
ScanClassNo | The scan class determines the frequency at which input points are scanned for new values. |
ExcDev | Exception Deviation. Specifies in engineering units how much a point's value must change before the interface considers it a significant value, and sends it to the server. As a general rule, you should set the exception slightly smaller than the precision of the instrument system. |
ExcDevPercent | Specifies the exception deviation as a percentage of span instead of in engineering units. For digital, string and Blob tags, ExcDev and ExcDevPercent are ignored and display by applications as zero. |
ExcMax | Exception Maximum. Specifies a limit on how long the interface can go without reporting a value to the Historian server. After the ExcMax time period elapses, the interface sends the next new value to the server, regardless of whether the new value is different from the last reported value. |
ExcMin | Exception Minimum. Specifies a limit on how frequently the interface can report values to the server. For example, for the interface to wait a full ten minutes before reporting a new value to the server, set the ExcMin attribute to ten minutes. ExcMin is typically set to zero. |
CompDev | Compression Deviation. Specifies in engineering units how much a value may differ from the previous value before it is considered to be a significant value. In most environments, set CompDev to the precision of the data source or hardware (instrument). Initially, set it to a lower value so that important data is not lost. After collecting data for a while, go back and check the data for your most important tags, and then adjust CompDev to a higher value, if necessary. Setting the CompDev attribute value too low causes too little data compression, and wastes space in the archive. Setting the value too high causes loss of useful data. For most flows, pressures,
and levels, use a deviation specification of 1% or 2% of span. For temperatures,
the deviation should usually be 1 degree or 2 degrees. |
CompDevPercent | Specifies the compression deviation as a percentage of span instead of in engineering units. Note : For non-numeric tags, CompDev and CompDevPercent are ignored. They
will be displayed by applications as zero. |
CompMax | Compression Maximum. A point is archived if the elapsed time since the previous time the point was saved is greater than the maximum time. The recommended maximum time specification is one work shift (for example, 8 hours). Duplicate values will be archived if the elapsed time exceeds CompMax. In most environments, set CompMax to the same value for all points in the system. |
CompMin | Compression Minimum. A point is archived if the elapsed time since the previous time the point was saved is greater than or equal to the minimum time, and the value has changed by more than the deviation. For data points associated with interfaces that send exception reports, set CompMin to 0. |
Compressing | Determines whether compression is turned on or off for the point. Compression should be turned on for all real-time points in the system. Set compression OFF for laboratory and manually entered tags so every value is recorded in the archive. To turn on compression, select the Compressing parameter for most points. With
compression off, every value sent is saved in the archive. Compression affects digital points, since a new value is recorded only when the current value changes. Points of types Blob and string have a similar behavior; new events pass compression only when the value changes. String values are compared ignoring case. For example, “VaLuE” and “valUe” are evaluated as equal. For Blob events, any change is significant. |
Step | The Step parameter affects only numeric points. It defines how numeric archived values are interpolated.
Data coming from continuous signals (such as signals from thermocouples, flow meters, and other devices that provide continuous measurement) should be archived in points with the step flag OFF. Data coming from discrete measurements (such as sampled lab data, batch charge weight, and other user-defined inputs) should be archived in points with the step flag ON. In addition, the step flag affects the compression calculation. When it is ON, a
linear change of value greater than or equal to CompDev passes compression. When
the step flag is OFF the complete compression algorithm is applied. |
DigitalSet | For digital points, the DigitalSet attribute specifies the name of the digital state set associated with the tag. The DigitalSet attribute has no meaning for non-digital tags. |
Scan | Toggling this checkbox, turns scanning ON or OFF for a point. By default scanning is turned ON (selected), which indicates that the program should be able to collect data for the point. Setting the Scan attribute to OFF (cleared) turns off data collection for that point. |
AdviseMode | Specifies that the point should run in the Advised data collection mode. In this mode, data is collected only when a value changes in the controller. It is not based on the scan rate. AdviseMode is selected by default as this mode is the most efficient because data is sent to the Historian server only when the value changes. |
SubObject Description | Descriptive text that distinguishes this point from other points. |
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