industrial//instrumentation//4-20 mA

The traditional analog signal of a field instrument: a current between 4 and 20 mA that represents the measured variable, for example 4 mA for 0 bar and 20 mA for 100 bar. The PLC's analog input converts the current into a digital value.

The traditional analog signal of a field instrument: a current between 4 and 20 mA that represents the measured variable, for example 4 mA for 0 bar and 20 mA for 100 bar. The PLC's analog input converts the current into a digital value.

The semantic intelligence is outside the sensor. The instrument sends one magnitude and nothing else; the PLC has to know what 4 mA and 20 mA mean, and a broken wire looks like 0 mA, which is how the live zero at 4 mA detects it.

Current, not voltage, because it survives long cables and noise; two wires can carry both the signal and the instrument's power.

It is the baseline every later step is measured against: HART keeps the loop and adds digital data on top; a smart sensor drops the loop for a digital protocol altogether.