Instrumentation Signal

Instrumentation signals are signals with standardized ranges, such as DC 4–20 mA or DC 1–5 V, and are also called process signals. The minimum values, 4 mA or 1 V, can be clearly distinguished from 0 mA or 0 V, which indicate faults or disconnections.
For field devices and sensors where long wiring distances are required, DC 4–20 mA signals are mainstream because they are less susceptible to noise.
For wiring between terminal blocks and controllers inside control panels, DC 1–5 V signals are commonly used, as they are easier for measurement with test meters and for signal branching during inspections.