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Comparison Function
Via the parameter Condition for DO you can deactivate the output (never switching) or define a
comparison condition for the output's switching:
•
Counter value higher or equal comparison value
•
Counter value lower or equal comparison value
•
Counter value equal comparison value
To use the comparison function, you have to preset the comparison value in the first double word of
the process data outputs and release the digital output via the control word (bit QX8.1).
The bit IX9.1 Comparison condition met of the status word is activated as soon as the comparison
condition is met. The output switches and remains set as long as the comparison condition is met
accordingly to the parameterized hysteresis and pulse duration.
When using Counter value equal comparison value the output remains set during the pulse duration
parameterized. With pulse duration = 0, the output remains set until the comparison condition is not
met any more. When using a counting mode with primary direction, the output will be switched only
upon reaching the comparison value from the primary direction.
Pulse duration
Via the parameter Pulse duration you can determine how long the digital output should remain set.
The pulse duration can be preselected between 0 and 510 ms with an inaccuracy of less than 2.048
ms. With pulse duration = 0 the output behaves exclusively according to the comparison conditions.
If the comparison value is left during a pulse output and is reached again, there is no post-triggering
of the pulse duration.
Note: The bit Comparison condition met is activated together with the bit DO set of the status word.
In contrast to the DO set bit it remains active until it is reset with the bit Reset status bits of
the control word.
Hysteresis
It is possible to reduce frequent switching of the output and/ or triggering of a process alarm, e.g. if
the value of a sensor signal fluctuates around the comparison value, by setting the hysteresis.
Thereby you define a range above and below a reference value (zero crossing, overflow/underflow or
comparison value), within which the output will not be reset.
A limit value between 0 and 255 can be parameterised for the hysteresis. With hysteresis = 3 for
example, all values differing less than 3 from the reference value are smoothed. Hysteresis is
deactivated with the values 0 and 1.
The hysteresis is activated upon reaching the comparison condition. The comparison result remains
unchanged during active hysterersis until the counting value reaches the predetermined hysteresis
limit. After leaving the hysteresis range, hysteresis is reactivated only upon reaching the comparison
condition again.
After changing the hysteresis value, an active hysteresis remains active. The new hysteresis value is
actived during the next hysteresis event.
The behavior of the output for hysteresis = 0 (hysteresis deactivated) and hysteresis = 3 is shown in
the following diagrams (legends describe the behavior for hysteresis = 3):
GFK-2958E
Chapter 5 Detailed Descriptions of I/O Modules
RSTi-EP User Manual
176
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