3 - frequency modes, Introduction, Frequency overview – Rockwell Automation 1756-HSC ControlLogix High Speed Counter Module User Manual
Page 29: Chapter 3, Frequency modes, Introduction frequency overview, Chapter
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Rockwell Automation Publication 1756-UM007C-EN-P - November 2011
Chapter
3
Frequency Modes
Introduction
This chapter describes the frequency modes that are available with the
1756-HSC/B module when using the HSC Data-extended
Comm Format.
The Frequency modes are:
• Frequency - number of input pulses per user-defined time interval.
• Period Rate - number of sampled, internal 4 MHz pulses per
user-defined number of incoming pulses, with outputs updated at the
end of the sample period with the Present Value, Totalizer, and
Stored Value tags.
• Continuous Rate - number of sampled, internal 4 MHz pulses per
user-defined number of incoming pulses, with outputs updated
throughout the sample period. The Present Value, Totalizer, and
Stored Value tags are updated only at the end of the sample period.
Frequency Overview
Each of the three Frequency modes use incoming pulse counts in a
user-defined interval to determine frequency values. The Stored Value tag
contains the calculated frequency and is always positive.
You can select one of three Frequency-operational modes based on the
frequency of the incoming signal. Frequency mode is best suited for
calculating higher frequencies because you define the sample period used to
count incoming pulses. At higher frequencies, there are a greater number of
pulses to be sampled that results in the ability to calculate frequency at a higher
resolution. The Stored Value tag is updated at the end of the selected sample
period.
Period Rate and Continuous Rate modes use an internal 4 MHz clock and a
user-defined number of incoming pulses configured by the Scaler value that
results in better performance at lower frequencies, where more 4 MHz pulses
are accumulated. Higher Scaler values also help to improve the calculation of
high frequency signals as longer pulse durations provide for more 4 MHz
pulses to be counted. Therefore, the combination of the Scaler and incoming
frequency determines the rate at which the frequency is updated in the
Stored Value tag.
Topic
Page
Period Rate and Continuous Rate Modes