Although the motor fan has high reliability, it is still a mechanical device. If it is used for a long time, its speed may decrease or even stop. So it is better to monitor the running state of the fan in real time, so as to detect problems in time. The alarm signal can be given when the speed of the fan is lower than a threshold value, while the speed signal output can realize the real-time monitoring of the speed of the fan. The alarm signal output from fan circuit has two states: "high level" and "low level". The meaning of two levels is generally represented by positive logic system, high level indicates "fault" and "low level" means "normal". The speed signal output from the fan circuit is usually pulse form. Each wave head indicates that the fan turns around. This signal can be displayed directly to the host through data bus. The speed signal of some fans is not the real speed of the fan, but the multiple of the rotational speed, for example, every turn to produce 2, 4 or 6 pulses. It must be processed to form the real speed signal of the fan. If you want to tell the real speed or a multiple of the speed of the fan, the tachometer can be used to measure the actual speed, and then compare with the displayed data. The speed signals of fans are usually output from three lead plugs, and three lead wires are yellow and black respectively, which are +12V power and grounding. The other color line is the speed signal output line. It should be noted that the third lead wires of some three lead fans are not speed signal output lines, but speed control signal lines, and input speed control signals to the fan motor through it.