PLC Analog Scaling Calculator
PLC analog input cards turn a 4 to 20 mA or 0 to 10 V signal into a raw integer. Convert that count to an engineering value, or work out the raw count the PLC should see for a given process value.
- Value = EUmin + (Raw − RawMin) × (EUmax − EUmin) ÷ (RawMax − RawMin)
- 0 + (13,824 − 0) × 10 ÷ 27,648 = 5
Scaling table
| Percent | Raw count | Engineering value | 4 to 20 mA |
|---|---|---|---|
| 0 % | 0 | 0 | 4.00 mA |
| 25 % | 6,912 | 2.5 | 8.00 mA |
| 50 % | 13,824 | 5 | 12.00 mA |
| 75 % | 20,736 | 7.5 | 16.00 mA |
| 100 % | 27,648 | 10 | 20.00 mA |
The Linear Scaling Formula
Analog scaling is a straight line between two points: the raw count at 0 % and the raw count at 100 %. The same equation is used by the Siemens SCALE and NORM_X/SCALE_X blocks, Allen Bradley SCP and Modicon scaling functions.
Worked example
A Siemens S7 card reads 13824 counts for a 0 to 10 bar transmitter. 13824 ÷ 27648 = 50 %, so the pressure is 5 bar.
Frequently Asked Questions
Why does Siemens use 27648 instead of 32767?
The nominal range ends at 27648 so that values up to 32511 can represent overrange, and 32767 can flag an overflow or wire break.
How do I scale 4 to 20 mA in a PLC?
Most cards map 4 mA to the raw minimum and 20 mA to the raw maximum. Enter that raw range and your transmitter range here; the result is the value your scaling block should produce.
What is the resolution of a 12 bit analog input?
A 12 bit card has 4096 steps, so each count is about 0.024 % of span. Use the ADC resolution calculator to see this in mA or engineering units.
Learn more on the blog
Why 24 volts is used in PLC systems
A practical, field focused guide on instrumentationblog.in that explains the theory behind this calculator and how engineers apply it on real plants.
Read Full Article