Why 4–20 mA became common
A current loop represents a process range with current rather than voltage. In a conventional linear mapping, 4 mA is the lower-range value and 20 mA is the upper-range value. Current transmission is useful over industrial cable runs because the same loop current flows through series elements within the circuit's operating limits.
The value of a live zero
Using 4 mA rather than 0 mA for the normal zero point provides headroom below the measurement range that can help distinguish certain abnormal conditions from a legitimate zero process value. Exact diagnostic current conventions depend on device and system configuration.
Voltage signals
Voltage ranges such as 0–10 V are also common, especially in building automation and shorter-distance applications. Voltage signals are more directly affected by voltage drop and reference/grounding arrangements, so system design matters.
HART
FieldComm Group defines HART as a bidirectional communication protocol providing data access between intelligent field instruments and host systems. It can add digital configuration and diagnostic information while the primary process value remains available through the conventional loop architecture.
Digital field communication
Fully digital networks can carry multiple process variables, device status and diagnostics over shared communication infrastructure. They reduce some analog-scaling issues but add network configuration, interoperability and lifecycle considerations.