Rockwell Automation 1794-IJ2XT FLEX I/O Frequency Input Modules User Manual - Manual (Page 18)

Publication 1794-6.5.11 - September 2011
8 Overview of the Frequency Input Module
processor to do other tasks and helps increase the overall throughput of your
controller system.
How Frequency Is
Calculated The following paragraphs explain the operation of the frequency input module
algorithm and its modifying parameters.
Frequency Calculation
Frequency is determined by a general algorithm which can be modified by
user-defined parameters. In its simplest form, the algorithm employs the
user-defined Minimum Sampling Time to set a window in which to count
pulses to determine the frequency. At least two pulses are required within the
sample window. Frequency is determined by storing the time the first pulse
occurred and then continually storing and updating the time of subsequent
pulses, along with the number of pulses encountered. When the window
expires, the frequency is calculated and the procedure repeats. The sequence
and formula for determining frequency are shown below.
Application Features
Feature Mode Purpose Alarm is ON when
Frequency alarm Frequency alarm
selected Activate alarm when calculated or scaled
frequency is higher than user-specified
frequency value. Frequency > user-specified value
Acceleration alarm Acceleration alarm
selected Activate acceleration alarm when
acceleration is greater than user-specified
acceleration value. | Acceleration | > user-specified value
Scaling % full scale and
acceleration Multiply and/or divide frequency by scalar Frequency > User-specified scaled
frequency value