First, need to understand the following concepts: current source (the output of a 4-wire transmitter is usually a current source signal), 3-wire transmitter, and 2-wire transmitter
l 4-wire transmitter：This type of transimitter has two wires for power supply and two wires for current output. The output of the 4-wire transmitter is usually a current source signal
l 3-wire transmitter：The current output share a ground wire with the power supply to save one wire.
l 2-wire transmitter：2-wire transmitters are mostly used in the industrial field. Only two wires are needed for signal transmission and power supply between the field transmitter and the control room. 4~20mA current as signal transmission also powers the field transmitter.
Taking GS8547-EX as an example, let’s first understand the signal acquisition loop and the power distribution loop
2-wire transmitter：The signal acquisition loop and the power distribution loop share two wires（7+,8-）
3-wire transmitter：power distribution loop（7+，10-），signal acquisition loop（9+，10-）
Current source（4-wire transmitter）:signal acquisition loop（9+，10-）；external power supply；
Definition of active signal and passive signal（According to whether the signal acquisition loop needs external power supply）
Active signal：In the 4~20mA signal acquisition loop, the signal generating end has its own voltage drive and no external power supply equipment is needed. This 4~20mA is an active signal. Generally, 4-wire transmitter and 3-wire transmitter output active signals.
Passive signal：In the 4~20mA signal acquisition loop, the signal generating end does not have its own voltage drive and external power supply equipment is needed. This 4~20mA is a passive signal. Generally, 2-wire transmitter outputs passive signals.
Method of judging whether external power supply is needed in the signal acquisition loop：
Disconnect the signal acquisition loop, and connect a voltmeter to the two lines of the field device (transmitter). If there is voltage, it means the field device outputs active signals. If there is no voltage, it means the field device outputs passive signals.
