According to whether the signal acquisition loop needs an 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, a 2 wire temperature transmitter outputs passive signals. Method of judging whether the 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 (voltage 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.