Skip to main content
Most transmitters send their measurement as an analog signal: 4–20 mA, 0–20 mA or 0–10 V. The PLC’s analog input turns that signal into a raw number, and your program scales the raw number into engineering units such as bar, °C or litres.

​
The formula

Every analog scaling is the same straight line between two points:
Example: a pressure transmitter with a 0–10 bar range sends 12 mA.
Siemens S7-1200 and S7-1500 analog inputs use 0 to 27648 for the nominal range. In TIA Portal you do the same calculation with NORM_X (raw count to 0.0–1.0) followed by SCALE_X (0.0–1.0 to engineering units). Other PLC brands use other raw ranges, so set Raw count at 100 % to match your module’s manual.

​
Calculator

Pick the signal type, enter the transmitter range, and move the slider to see the engineering value and the raw count the PLC reads.

​
Why 4–20 mA starts at 4 mA

A 4–20 mA loop has a live zero: 0 % of the measuring range is 4 mA, not 0 mA. If the current drops close to 0 mA, the PLC knows the wire is broken or the sensor has failed, instead of reading a valid “0 bar”. Many transmitters follow the NAMUR NE 43 recommendation: the normal signal stays between 3.8 and 20.5 mA, and a current of 3.6 mA or less, or 21 mA or more, signals a fault. The calculator shows a warning in those zones.

Modbus float converter

Turn two Modbus registers into a float in all four byte orders.

Get my courses – 20% off

Learn PLC programming step by step. Use code WIKI for 20% off.