328 Rockwell Automation Publication 1756-RM006K-EN-P - November 2018
Execution
Function Block
Condition/State Action Taken
Prescan EnableIn and EnableOut bits are cleared to false.
Tag.EnableIn is false EnableIn and EnableOut bits are cleared to false.
Tag.EnableIn is true EnableIn and EnableOut are set to true.
The instruction executes.
Instruction first run N/A
Instruction first scan Recalculate coefficients.
Postscan EnableIn and EnableOut bits are cleared to false.
Structured Text
Condition/State Action Taken
Prescan See Prescan in the Function Block table.
Normal Execution See Tag.EnableIn is true in the Function Block table.
Postscan See Postscan in the Function Block table.
Example
The HPF instruction attenuates signals that occur below the configured cutoff
frequency. This instruction is typically used to filter low frequency "noise" or
disturbances that originate from either electrical or mechanical sources. You can
select a specific order of the filter to achieve various degrees of attenuation. Note
that higher orders increase the execution time for the filter instruction.
The following graphs illustrate the effect of the various orders of the filter for a
given cutoff frequency. For each graph, ideal asymptotic approximations are given
with gain and frequency in logarithmic scales. The actual response of the filter
approaches these curves but does not exactly match these curves.
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