Emerson PACSystems RSTi-EP User Manual page 195

User Manual
GFK-2958J
To use the comparison function, you must 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 510ms with an inaccuracy
of less than 2.048ms. 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:
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 parameterized 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 hysteresis 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 active 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):
Detailed Description of I/O Modules
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.
Chapter 5
Oct 2019
180