ANALOG SCALING FORMULA IN PLC: Convert Raw Input Into Engineering Units! 
A PLC analog input usually does not directly give you the actual process value.
For example, a pressure transmitter may measure:
0–10 bar
while the PLC analog input module may provide a raw digital value such as:
0–27648
So we need to scale the raw PLC value into engineering units.
The Standard Analog Scaling Formula
Where:
EU = Engineering Unit value
Raw = Actual PLC raw input
Rawmin = Minimum raw input
Rawmax = Maximum raw input
EUmin = Minimum engineering value
EUmax = Maximum engineering value
Example: 4–20 mA Pressure Transmitter
Suppose:
Transmitter range = 0–10 bar
PLC raw range:
4 mA = 0
20 mA = 27648
If the PLC receives:
Raw = 13824
Then:
PLC displays = 5 bar
Another Example
Suppose a temperature transmitter has:
Range = −50 to 150°C
PLC raw range:
0–27648
Raw input:
13824
Then:
Raw 13824 = 50°C
4–20 mA Scaling Formula
For a 4–20 mA signal, a commonly used direct formula is:
Example:
Transmitter = 0–100 bar
Input = 12 mA
12 mA = 50 bar
Don’t Forget the Difference
Raw Scaling:
PLC raw value → Engineering Units
Current Scaling:
4–20 mA → Engineering Units
Both ultimately perform the same job:
Easy Formula to Remember
Engineering Value = Low Limit + [(Input − Input Low) × Range] ÷ Input Range
This formula is widely applicable to:
PLC
DCS
SCADA
Analog Input Modules
4–20 mA Systems
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