Rockwell Automation 1794-IJ2XT FLEX I/O Frequency Input Modules User Manual - Manual (Page 20)

Publication 1794-6.5.11 - September 2011
10 Overview of the Frequency Input Module
Termination on Number of Pulses
The normal mode is designed to provide wide bandwidth. However, it requires
the full 2 seconds to report a missing pulse. Another user parameter, Number
of Pulses to Terminate Sampling, is provided for scenarios where many pulses
are expected within the sampling window, such that early determination of
frequency is possible.
During normal operation, there is a sufficient number of pulses to prevent the
module from entering the 2-second extension mode. Once the user-defined
number of pulses is reached within the sampling window, the frequency is
immediately reported.
If the sampling window has been extended to 2 seconds while waiting for a
second pulse to occur, a situation which can occur during system startup, the
module does not wait for the number of pulses to be reached. Instead, as soon
as a second pulse occurs, the frequency is calculated based on the time
between the 2 pulses, and the procedure starts over.
Termination on Number of Pulses
Missing Pulse Multiplier
The last user parameter which is provided to modify the frequency algorithm
is the Missing Pulse Multiplier. In this case, the user can set the number of
Minimum Frequency Sampling Time windows allowed to extend the time to
capture a second pulse, before reporting a Missing Pulse Alarm. The intent
with this parameter is to allow a tradeoff of the bandwidth available with the
response time to report a missing pulse. This mechanism is shown in the
following figure.
Frequency reported at
number of pulses to
terminate reached
=
Pulses to terminate - 1
T2 - T1
Missing pulse alarm would be reported after 2 seconds, if no pulse occurs
pulses to terminate
Minimum sampling time
Frequency input
pulses
First pulse in window,
T1 time stored
T2, update as each
pulse occurs Number of pulses to
terminate reached
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