PLC Analog Input Conversion Formula

For any unit conversion on any PLC, you can use four function math with the following for PLC Analog Input Conversion Formula, which assumes integer math, as follows :

PLC Conversion Formula

Formulas:

EU = ((EUSpan x (DATA – DataOffset))/DataSpan) + EUOffset

ProcessValue = EU / FACTOR

DataOffset = Data in input register at 4 mA (or zero analog signal of whatever range – for example 0 VDC on a 0-10 VDC transmitter)

DataSPan = Data at 20 mA – Data @ 4 mA

FACTOR = arbitrary factor (multiple of 10) needed to get proper resolution and accuracy

EU = Engineering Units x FACTOR

EUOffset = Engineering Units @ 4 mA x FACTOR

EUSpan = (Engineering Units @ 20 mA – Engineering Units @ 4 mA) x FACTOR

DATA = Actual data reading in input register

For Example,

A Pressure transmitter of range 0 to 60 psig is connected to an PLC analog input card. The PLC Analog input card raw count starting from 6240 for 4mA and 31208 for 20mA. PLC reading 18975 raw counts and calculate equivalent pressure transmitter reading.

Note: PLC raw counts may vary from system to system.

0-60 psig from 4-20 mA with resolution of 0.1 psig:

FACTOR = 10

Data @ 4 mA = 6240

Data @ 20 mA = 31208

DataOffset = 6240

DataSpan = 31208 – 6240 = 24968

EUOffset = 0 x 10 = 0

EUSpan = (60 – 0) x 10 = 600

DATA = 18975

EU = ((EUSpan x (DATA – DataOffset))/DataSpan) + EUOffset

EU = ((600 x (18975 – 6240)) / 24968) + 0 = 306

ProcessValue = EU / FACTOR

Process Value = 306 / 10 = 30.6 psig

If you liked this article, then please subscribe to our YouTube Channel for PLC and SCADA video tutorials.

You can also follow us on Facebook and Twitter to receive daily updates.

Read Next:

Process Interlocks and Trips

Split Range Control Loop

Analyze a Cascade Control Loop

Ratio Controller Example

PID Tuning Procedure