220V Cooling Fan speed signal

Speed signal "220V Cooling Fan" circuit output from the usual form of pulses, each wave DC fan turned his head in a circle, so that a signal can be provided directly to the host for display via the data bus. Some DC fan speed signal is not output from the real DC fan speed, but the speed of a multiple, for example, 2 per revolution, four or six pulses, must be treated in order to form a true reflection of a DC fan speed signal ??? To distinguish "220V Cooling Fan" Speed is real or a multiple of speed, you can use the tachometer to measure the actual speed, then the data were compared. "220V Cooling Fan" circuit outputs an alarm signal from the "high" and "low" states, meaning two levels represented by the general system in accordance with positive logic, high indicates "Fault", " low "for" normal. "