Rockwell Automation 1771-PD PID MODULE (+DU) - Manual (Page 121)

Programming
Chapter 3
3Ć78
Values are read and displayed in engineering units of gallons/minute. The
flow transmitter outputs a signal from +4 to +20mA. The range of the
flow transmitter is 10 gallons/minute at +4mA and 70 gallons/minute at
+20mA. A 10 gallons/minute rate is reported as 0000 when scaling is not
used
Table 3.M
Scaling
Analog
Signal
Actual
Flow Rate
W65 READ PV1
Without Scaling
W65 READ PV1
With Scaling
+20mA 70 gallons/minute 4095 70
+16mA 55 gallons/minute 3072 55
+12mA 40 gallons/minute 2048 40
+ 8mA 25 gallons/minute 1024 25
+ 4mA 10 gallons/minute 0000 10
1 part in 4096 gallons/minute
no scaling scaling
W30 B17=0 W30 B17 = 1
W30 B04, B03 = 0
SMIN1 W31 = 10
SMAX1 W32 = 70
Next assume that the process variable and the set point are scaled but the
error term is not. SP1 is 40 gallons/minute. The flow rate at the PV1
input is 25 gallons/minute. The unscaled values are SP1 = 2048 and PV 1
= 1024. ERROR = SP - PV. Since ERROR1 is reported unscaled, the
value will be 1024. If the error term had been scaled, the value of 15
would have been reported. This was obtained by subtracting the PV value
from the SP value.
Scaling Example 2
In example 2, the lower limit of the process variable range SMIN is a
negative number. Selected values of the scaled and unscaled process
variable as read in word W65(W72) are listed in table 3.N. The current
input varies from +4 to +20ma for a temperature range of -20
0 C to +60 0 C.
The scale limits SMAX and SMIN are stored in words W32(W52) and