4 Pin Fan Tachometer - Why 2 pulses per revolution?

[bitshift]

Hi all,

I've been playing with a 4 pin PWM controlled fan (Here's a link to a specification for those interested).

The tachometer output signal is defined to output two pulses per revolution of the fan. I understand that there are small magnets attached to either side of the fan. A pulse is generated each time a small magnet passes a hall effect sensor.

Why are there two pulses per revolution and not just one?

TL;DR

In a 4 pin fan why does the tachometer output two pulses per revolution and not just one?

[dadler]

Maybe this will help explain:

http://electronics.stackexchange.com/questions/95301/what-exactly-is-the-purpose-of-a-hall-sensor-in-a-bldc

[bitshift]

Maybe this will help explain:

http://electronics.stackexchange.com/questions/95301/what-exactly-is-the-purpose-of-a-hall-sensor-in-a-bldc

Perfect, thank you