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Scale an Analog Input and
Detect an Out-of-range
Condition
The following example shows input range checking and scaling the analog
input to engineering units for an FIO4V analog input module.
We are making the following assumptions:
The FIO4V is located slot 3 of a modular system.
A pressure sensor with a 0-10V dc output is wired to input channel 1.
The sensor signal voltage is proportional to a range of 100-500 PSI.
The process pressure must stay between 275 and 300 PSI.
(If the pressure deviates from this range, your logic sets an alarm bit.)
Data is presented in PSI for monitoring and display purposes.
Input Scaling
The scaling operation is displayed in the following graph. It displays the
linear relationship between the input and the resulting scaled values.
500 PSI
(Scaled max.)
Scaled
Value
300 PSI
275 PSI
100 PSI
(Scaled min.)
0 = 0V dc
(input min.)
Calculating the Linear Relationship
Use the following equations to express the linear relationship between the
input value and the resulting scaled value.
Scaled value = (input value x slope) + offset
Slope = (scaled max. – scaled min.) / (input max. – input min.)
(500 – 100) / (4095 – 0) = 400/4095 = 0.0977
Offset = scaled min. – (input min. x slope)
100 – (0 x (400/4095) = 100
Scaled value = [input value x (0.0977)] + 100
AB Parts
1 1 1
Chapter 5
Writing Ladder Logic
Process operating range
Low
High
4095 = 10V dc
Limit
Limit
(input max.)
Input Value
5–5
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