Why do RPG fans have two lines?

The early fans only had two lines, which were generally represented by red lines and black lines. Red indicates positive power supply and black line indicates negative power supply, that is, as long as the appropriate voltage is connected, the fan will rotate at full speed, and the speed is not adjustable. Although the fan is well controlled, there is no speed feedback. Whether the fan is rotating or not, and what is the speed of the fan, the computer CPU has no idea at all. Even if the fan is broken, the computer CPU can't detect it, and a closed loop can't be formed between temperature and speed. It can also be said that the state of the fan is uncontrollable and there is a hidden danger of heat dissipation.

In view of the above shortcomings of the two-wire fan, in order to improve the heat dissipation efficiency, the three-wire fan appeared later.