Rockwell Automation Allen-Bradley 1734-IB8S, User Manual

Page: 38 / 212

38 Rockwell Automation Publication 1734-UM013J-EN-P - July 2014
Chapter 2 Safety Inputs, Safety Outputs, and Safety Data
Overfrequency Bit Operation
When the frequency exceeds 1 kHz, the module reports a data value of 1 kHz,
sets the Overfrequency status bit to 0, and latches it. While the Overfrequency
bit is set to 0, you must use an alternate method to monitor the frequency of the
system because the value reported by the module is latched at 1 kHz. Once you
have verified that the frequency is lower than 1 kHz, you may reset the
Overfrequency condition by setting the Reset Tach bit, which lets the module
begin measuring the frequency of field pulses again.
If you set the Reset Tach bit while the frequency is still above 1 kHz, the
Tachometer Overfrequency bit transitions to 1 (within range) momentarily.
However, as soon as the module begins measuring pulses, it will detect another
overfrequency condition and immediately set the Tachometer Overfrequency bit
to 0 again. The Reset Tach bit is edge-sensitive.
See Output Assemblies on page 192 for more information on resetting the
Overfrequency bit.
Figure 17 - Overfrequency Operation
In Figure 17 , the module reports a frequency of 0 Hz until the frequency of the
system reaches 1 Hz at A, when the module begins reporting the actual value. At
B, the frequency exceeds 1 kHz, the Overfrequency bit is set to 0, and the module
continues to report a data value of 1 kHz. Between B and C, you must monitor
the frequency by an alternate method because the value reported by the module
may not be accurate. After C, the Overfrequency condition can be cleared,
provided you have used an alternate method to verify that the actual frequency is
below 1 kHz.
ATTENTION: Before resetting the Overfrequency condition, you must use
another method to verify that the actual frequency is lower than 1 kHz.
A B C
1 kHz
1 Hz
Frequency = 0
Actual values are
reported.
Monitor frequency via an
alternate method.
Overfrequency
condition can be
cleared.
Frequency = 1 Hz
Overfrequency bit is set to 0.
Frequency = 1000 Hz