The Scaling Section

Scaling converts a data value from a raw value expressed in an arbitrary range of units, such as a number of counts, to one in engineering units, such as gallons per minute or pounds per square inch. The scaled data type can serve as a third form of data compression, in addition to collector compression and archive compression, if it converts a data value from a data type that uses a large number of bytes to one that uses fewer bytes.

To display or edit scaling parameters, select Scaling. The page shown in the following figure appears.

To modify the values, enter new values in the appropriate fields and then select the Update button at the bottom of the page to apply the changes. Until you select the Update button, entering a new value changes the display of the field name to blue. The fields in the Scaling section contain the following information:
Table 1. Engineering Unit Range
Hi Engineering UnitsDisplays the current value of the upper range limit of the span for this tag.
Lo Engineering UnitsDisplays the current value of the lower range limit of the span for this tag

Engineering Hi and Lo are retrieved automatically for F_CV fields for iFIX tags; all others are left at default settings. When adding tags from the server using an OPC Collector, the OPC Collector queries the server for the EGU units and EGU Hi/Lo limits. Not all OPC Servers make this information available, however. Therefore, if the server does not provide the limits when requested to do so, the collector automatically assigns an EGU range of 0 to 10,000.

Table 2. Input Scaling
Input ScalingSelect the appropriate option to enable or disable input scaling, which converts an input data point to an engineering units value.

For example, to rescale and save a 0 - 4096 input value to a scaled range of 0 - 100, you enter 0 and 4096 as the low and high input scale values and 0 and 100 as the low and high engineering units values, respectively.

If a data point exceeds the high or low end of the input scaling range, then Historian logs a bad data quality point with a ScaledOutOfRange subquality. In the previous example, if your input data is less than 0, or greater than 4096, then Historian records a bad data quality for the data point. For instance, a value of 4097, in this example, yields a bad data quality.

Hi Scale Value The upper limit of the span of the input value.
Lo Scale Value The lower limit of the span of the input value.

OPC Servers and TRUE Values

Some OPC Servers return a TRUE value as -1. If your OPC Server is returning TRUE values as -1, modify the following scaling settings in the Tag Maintenance page of Historian Administrator:
Hi Engineering Units = 0
Lo Engineering Units = 1 
Hi Scale Value = 0 
Lo Scale Value = - 1 
Input Scaling = Enabled