Rockwell Automation 1746-FIO4V SLC 500 Fast Analog I/O/ User Manual - Manual (Page 45)

Publication 1746-UM009B-EN-P - September 2007
Write Ladder Logic 45
Calculate the Linear Relationship
Use the following equations to express the linear relationship between
the input value and the resulting scaled value.
Calculate the Out-of-range Limits
Use the following equation to compute low and high out-of-range
limits.
Ladder Logic
We present two examples for programming the processor.
The first example uses standard math instructions available in any
SLC 500 processor. This ladder logic prevents a processor fault by
unlatching the mathematical overflow bit S2:5/0 before the end of the
scan.
The second example uses the scaling instruction (SCL) available in
SLC 5/02 (and later) processors. The rate parameter is calculated by
multiplying the slope by 10,000. If the slope exceeds 3.2767, you
cannot use the SCL instruction.
Scaled value = (input value x slope) + offset
Slope = (scaled max – scaled min) / (input max – input min)
Offset = scaled min – (input min x slope)
(500 – 100) / (4095 – 0) = 400/4095 = 0.0977
(100 – (0 x [400/4095]) = 100
Scaled value = (input value x [0.0977]) + 100
Input value = (scaled value – offset) / slope
low limit: (275 – 100) / (0.0977) 1750 counts
~
–
high limit: (300 – 100) / (0.0977) 2750 counts
~
–
rate = (400/4095) x 10,000 = 977 (The slope is 0.0977 so you can use the SCL instruction.)