Rockwell Automation 1771-IL/B , D17716.5.91 ISOLATED ANALOG INPUT Module - Manual (Page 28)

Module Configuration
Chapter 4
4-4
You must also indicate what format will be used to read data from your
module. Typically, BCD is selected with PLC-2 processors, and 2’s
complement binary is selected with PLC-3 and PLC-5 processors. See
Appendix C for details on data format.
Table 4.D
Selecting the Data Format
Decimal Bit 10
(Octal Bit 12)
Decimal Bit 09
(Octal Bit 11)
Data Format
0 0 BCD
0 1 not used
1 0 two’s complement binary
1 1 signed magnitude binary
The analog input module has hardware-based high frequency filters on all
channels to reduce the effect of electrical noise on the input signal.
Software digital filtering is meant to reduce the effect of process noise on
the input signal.
The digital filter equation is a classic first order lag equation (Figure 4.1).
Using a step input change to illustrate the filter response (Figure 4.2), you
can see that when the digital filter constant time elapses, 63.2% of the total
response is reached. Each additional time constant achieves 63.2% of the
remaining response.
Figure 4.1
Digital Filter Equation
Y
n
= Y
n-1
+
t
t + TA
(X
n
- Y
n-1
)
Where: Yn = present output, filtered peak voltage (PV)
Y n –1 = previous output, filtered PV
t = module channel update time (seconds)
X n = present input, unfiltered PV
TA = digital filter time constant (seconds)
Data Format
Digital Filtering