Rockwell Automation 1790P-4R0 CompactBlock LDX I/O RTD/Resistance Input Module - Manual (Page 34)

Publication 1790-UM002A-EN-P
3-4 Module Data, Status, and Channel Configuration for DeviceNet
The module scales input data to the actual temperature values for the
selected RTD type per RTD standard. It expresses temperatures in 0.1
degree units, either degrees C or degrees F, depending on which
temperature scale is selected. For resistance inputs, the module expresses
resistance in 0.1
Ω
units for the
100
m
Ω
scale and in
0.01Ω
units for the
10
m
Ω
scale.
Negative temperatures are returned in 16-bit two’s complement binary
format. See Appendix B for a detailed explanation of two’s complement
binary numbers.
Filter Frequency The module supports filter selections corresponding to filter frequencies
of 10Hz, 25Hz, 50 Hz, 60 Hz, 100 Hz, 250 Hz, and 500 Hz. Your filter
frequency selection is determined by the desired range for the input type,
and the required effective resolution, which indicates the number of bits
in the input data that do not vary due to noise. Also consider the required
module update time when choosing a filter frequency. For example, the
10 Hz filter provides the greatest attenuation of 50 and 60 Hz noise and
the greatest resolution, but also provides the slowest response speed.
The choice that you make for filter frequency will affect:
•
noise rejection characteristics for module input
•
channel step response
•
channel cutoff frequency
•
effective resolution
•
module update time
Effects of Filter Frequency on Noise Rejection
The filter frequency that you choose for the module determines the
amount of noise rejection for the inputs. A smaller filter frequency (e.g.
10Hz) provides the best noise rejection and increases effective resolution,
but also increases channel update time. A larger filter frequency (e.g. 500
Hz) provides lower noise rejection, but also decreases the channel update
time and effective resolution.
When selecting a filter frequency, be sure to consider channel cutoff
frequency and channel step response to obtain acceptable noise rejection.
Choose a filter frequency so that your fastest-changing signal is below that
of the filter’s cutoff frequency.