When talking about Programmable Logic Controllers and/or industrial controls – current loop usually refers to the standard analog interface defined by the Instrument Society of America (ISA) in ANSI/ISA-S50-1. The signal level is 4mA to 20mA – where 4mA represents a sensor’s 0% reading and 20mA represents 100%. (while 0mA may indicate an open wire). Transmitter outputs may have various isolation among loop power source, input sensor, and 4-20mA output current. The isolation of the transmitter may impact the type of PLC input required. Three types of signals are defined, which correspond to the number of wires used:
Type 2 – require a single pair of wires which provide the 4-20mA signal and excitation for the sensor circuits.
Type 3 – use three wires to carry the signal and provide excitation.
Type 4 transducers provide separate connections for power and current. Two wires are provided for power and two wires are needed for the 4-20mA signal.