Configuring Digital Input Filter Times - Siemens S7-1200 System Manual

Device configuration
5.5 Configuring the operation of the CPU
5.5.2

Configuring digital input filter times

The digital input filters protect your program from responding to unwanted fast changes in
the input signals, as may result from switch contact bounce or electrical noise. The default
filter time of 6.4 ms blocks unwanted transitions from typical mechanical contacts. Different
points in your application can require shorter filter times to detect and respond to inputs from
fast sensors, or longer filter times to block slow contact bounce or longer impulse noise.
An input filter time of 6.4 ms means that a single signal change, from '0' to '1' or from '1' to
'0', must continue for approximately 6.4 ms to be detected, and a single high or low pulse
shorter than approximately 6.4 ms is not detected. If an input signal switches between '0'
and '1' more rapidly than the filter time, the input point value can change in the user program
when the accumulated duration of new value pulses over old value pulses exceeds the filter
time.
The digital input filter works this way:
● When a "1" is input, it counts up, stopping at the filter time. The image register point
changes from "0" to "1" when the count reaches the filter time.
● When a "0" is input, it counts down, stopping at "0". The image register point changes
from "1" to "0" when the count reaches "0".
● If the input is changing back and forth, the counter will count up some and count down
some. The image register will change when the net accumulation of counts reaches
either the filter time or "0".
● A rapidly, changing signal with more "0's" than "1's" will eventually go to "0", and if there
are more "1's" than "0's", the image register will eventually change to "1".
Each input point has a single filter configuration that applies to all uses: process inputs,
interrupts, pulse catch, and HSC inputs. To configure input filter times, select "Digital Inputs".
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S7-1200 Programmable controller
System Manual, 03/2014, A5E02486680-AG