90 Rockwell Automation Publication 1756-UM058G-EN-P - November 2012
Chapter 5 Fast Module Features
Software Configurable Filter Times
To account for hard contact bounce, you can configure Off to On and On to Off
input filter times of 0…30, 000 µs in RSLogix 5000 software. These filters define
how long an input transition must remain in the new state before the module
considers the transition valid.
When an input transition occurs, the module timestamps the transition on the
edge of the transition and stores timestamp data for the transition. The module
then monitors the input for the duration of the filter time to verify that the input
remains in the new state:
• If the input remains in the new state for a time period equal to the filter
time, the input is recognized and recorded. The module sends timestamp
data for the transition and the input’s On/Off state to the controller.
• If the input changes state again before the duration of the filter time has
elapsed, the module continues to scan that input for up to 10x
the filter
time. During this continued scan period, one of the following events
occurs:
– In the time period that is 10x the duration of the filter time, the input
returns to the transitioned state for the duration of the filter time. In
this case, the module sends timestamp data from the initial transition to
the controller.
– In the time period that is 10x the duration of the filter time, the input
never remains in the transitioned state for the duration of the filter
time. In this case, the input is recognized, but the module does not
consider the original transition valid and drops the timestamp.
A 1756-IB16IF module is configured for a 2 ms filter time for Off to On
transitions. In this example, three possible scenarios can result after an input
transitions from Off to On:
•Scenario 1—The input turns On and remains On for the full 2 ms filter
time. The module considers the transition valid and sends the data
recorded at the transition to the controller (Figure 9 on page 91
).
•Scenario 2—The input turns On but turns Off before the 2 ms filter time
elapses. The module continues to monitor the input for 10x the duration of
the filter time. Within that time period, the input turns On again and
remains On for at least 2 ms. The module considers the transition valid and
sends the data timestamped at the original transition to the controller
(Figure 10 on page 91
).
•Scenario 3—The input turns On but turns Off before the 2 ms filter time
elapses. The module continues to monitor the input for 10x the duration of
the filter time. Within that time period, the input never remains On for at
least 2 ms. The module considers the transition invalid and drops the data
timestamped at the original transition (Figure 11 on page 91
).
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