Two-wire system: DC power supply (DC 24V) supplies power to the data converter on-site through two wires, and at the same time, these two wires are the transmission lines for outputting dedicated signals (4～20mA). For DCS, this two-wire system is a passive signal.
Three-wire system: Two wires transmit signals, and the other one provides power. The three-wire system uses one wire for the positive pole of the power supply, one wire for the positive pole of the signal, and the same wire for the negative pole of the power supply and the negative pole of the signal. The power supply voltage is mostly DC 24V, and the output signal is DC4-20mA. For DCS, this three-wire system is an active signal.
Four-wire system: Two power cables and two signal cables are generally used for instruments with the higher power, such as electromagnetic flowmeters. For DCS, this signal is an active signal.
Generally, digital pressure transmitters are 2-wire systems, 24V power supply, and 4-20mA current signals output.
Open the back cover of the pressure transmitter and you will see four wiring terminals, OUT positive and negative and TEST positive and negative(TEST positive and negative are used by the manufacturer to calibrate the pressure transmitter), here we connect OUT positive and negative.
After power on, the digital pressure transmitter will display normally.