Rockwell Automation 1769-OF2 Compact I/O Analog Modules - Manual (Page 74)

Publication 1769-UM002B-EN-P - July 2005
3-24 Module Data, Status, and Channel Configuration for the Input Modules
Channel Cut-Off Frequency
The -3 dB frequency is the filter cut-off frequency. The cut-off frequency is
defined as the point on the frequency response curve where frequency
components of the input signal are passed with 3 dB of attenuation. All input
frequency components at or below the cut-off frequency are passed by the
digital filter with less than 3 dB of attenuation. All frequency components
above the cut-off frequency are increasingly attenuated.
The cut-off frequency for each channel is defined by its filter frequency
selection and is equal to the filter frequency setting. Choose a filter frequency
so that your fastest changing signal is below that of the filter’s cut-off
frequency. The cut-off frequency should not be confused with the update
time. The cut-off frequency relates to how the digital filter attenuates
frequency components of the input signal. The update time defines the rate at
which an input channel is scanned and its channel data word is updated.
Module Update Time and Scanning Process
The module update time is defined as the time required for the module to
sample and convert the input signals of all enabled input channels and provide
the resulting data values to the processor. Module update time can be
calculated by adding the sum of all enabled channel times. Channel times
include channel scan time, channel switching time, and reconfiguration time.
The module sequentially samples the channels in a continuous loop.
The 1769-IF8 uses two parallel sampling loops as shown in Figure 3.5 to
update the entire module (all 8 channels) in an amount of time equal to only
four channel update times. The module performs parallel channel sampling on
pairs of inputs. Channels 0 and 4 are a pair. The other input channel pairs are
1 and 5, 2 and 6, and 3 and 7.