Rockwell Automation Publication 1444-UM001D-EN-P - June 2018 333
CIP Objects Appendix A
Reading FFT Data
Processing the returned data into the FFT is merely a matter of determining
the FMAX and the number of lines of the data. The data values themselves are
returned as floating point values that need no further processing. The
following describes how to calculate the FMAX and the number of lines.
Extended and Reduced FFT Lines
When requesting and reading FFT data the module can return either a fraction
of the lines of the common processed FFT, or an extended number of lines,
greater than the common processed FFT, from the raw FFT data.
Reading a fraction, either 37.5% or 62.5% of the processed FFT, is necessary to
account for the digital resampling and filtering that can be applied on the
primary or alternate signal paths. This reading makes sure that the presented
FFT is free of attenuation by the filters, or any potential aliasing effects due to
resampling if a reduced FMAX, or synchronous sampling, is selected for the
path. The difference in the fractions is due to the difference in the quality of
the digital filters available on the primary and alternate signal paths.
Reading an extended number of lines, 112.5%, of the processed FFT, is possible
and appropriate when no reduced FMAX, or synchronous sampling, is
selected on the primary or alternate path, so no resampling is being performed.
In this case, the highly efficient anti-alias filter that is applied within the 1444
hardware makes available a greater number of alias-free lines than the common
FFT function provides.
The SpecialRequest byte of the FFT and FFT(e) FFT data request record
includes a specification that the module return reduced or extended lines. It is
recommended that software always specify that the reduced or extended data
be returned.
The actual returned data size is derived by interrogating the applicable bits
returned in the ucDataSelect byte of the FFT header record.
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