Siemens SIMATIC ET 200S Operating Instructions Manual page 148

1Count5V/500kHz
3.6 Count Modes
Hysteresis
An encoder can remain at a particular position and then fluctuate around this position. This
state causes the count to fluctuate around a particular value. If there is a comparison value
in this fluctuation range, for example, the associated output is switched on and off in
accordance with the rhythm of the fluctuations. To prevent switching occurring in the case of
small fluctuations, the 1Count5V/500kHz is equipped with an assignable hysteresis. You can
assign a range between 0 and 255 (0 means: hysteresis switched off).
Hysteresis also works with overflow and underflow.
Method of Operation with Count ≤ Comparison Value and Count ≥ Comparison Value
The diagram below provides an example of how hysteresis works. The figure shows the
differences in the behavior of an output when hysteresis of 0 (= switched off) is assigned as
opposed to hysteresis of 3. In the example, the comparison value is 5.
The following settings are assigned for the counter: "Main count direction" = "Up" and
"Switch on at count ≥ comparison value".
When the comparison condition is met, hysteresis becomes active. While the hysteresis is
active, the comparison result remains unchanged.
If the count value goes outside the hysteresis range, hysteresis is no longer active. The
comparator switches again according to its comparison conditions.
Figure 3-15
Note
If the count direction changes on the comparison value when hysteresis is active, the output
is reset.
148
Example of How Hysteresis Works
Technological Functions
Operating Instructions, 02/2007, A5E00124867-05