Rockwell Automation 1794-IJ2XT FLEX I/O Frequency Input Modules User Manual - Manual (Page 19)

Publication 1794-6.5.11 - September 2011
Overview of the Frequency Input Module 9
Normal Mode Frequency Determination, at Least Two Pulses
in Sampling Time
If only one pulse occurs within the sampling window, when the sample time
has expired, the window is extended to 2 s to allow for a second pulse to occur.
As soon as a second pulse occurs, the frequency is calculated and the
procedures starts over. If no second pulse is detected, zero frequency is
reported and a Missing Pulse Alarm is generated. The following figure depicts
this scenario.
Normal Mode, Only One Pulse in Sampling Time,
Sampling Time Extended
Missing pulse alarm would be
reported after 2 s, if no pulse occurs
Frequency reported
at end sampling time Pulses counted
T2 - T1
pulses counted
First pulse in window,
T1 time stored Last pulse in window T2, update as each
pulse occurs
Minimum sampling time
Frequency input
pulses
45340
=
45341
Frequency reported at
first pulse in extended Time
2 - 1
T2 - T1
=
Missing pulse alarm is reported after 2 s, if no pulse occurs
Minimum
sampling time
Frequency input
pulses 2 = pulses counted
First pulse in window,
T1 time stored First pulse after
window extended,
T2 time stored
Sampling time
extended to 2 s