[SOLVED] 2 Fans connected to SYS_FAN - I would like to see RPMS for both

[SNek23]

Hi all,

I've been trying to find a proper answer for this for a while now. I get partial answers from older threads but could not find the stuff I need lol.

I have an Aorus B450 Pro WIFI mobo in a small ITX build. This mobo has only 2 fan headers : CPU and SYS. I would like to connect 2 fans (intake and exhaust) to the SYS_FAN header and be able to see the the RPMs for both fans in a software like HWMonitor. I'm not keen on being able to control them as I will rely on the SYS_FAN header to do that anyway.


What are the options?


Thanks!

 

[CountMike]

Hi all,

I've been trying to find a proper answer for this for a while now. I get partial answers from older threads but could not find the stuff I need lol.

I have an Aorus B450 Pro WIFI mobo in a small ITX build. This mobo has only 2 fan headers : CPU and SYS. I would like to connect 2 fans (intake and exhaust) to the SYS_FAN header and be able to see the the RPMs for both fans in a software like HWMonitor. I'm not keen on being able to control them as I will rely on the SYS_FAN header to do that anyway.


What are the options?


Thanks!

No options, you are getting report of fan speed that is somewhere in the middle of each one if they are different.

 

[SNek23]

No options, you are getting report of fan speed that is somewhere in the middle of each one if they are different.

Thanks for the info.

One last thing - assuming both fans are the same make and model, will the power distribution be the same for both fans if I use a PMW splitter?? i.e if fan 1 runs at 1400RPM will fan2 run at the same speed?

 

[CountMike]

Thanks for the info.

One last thing - assuming both fans are the same make and model, will the power distribution be the same for both fans if I use a PMW splitter?? i.e if fan 1 runs at 1400RPM will fan2 run at the same speed?

Fan manufacturers usually specify +/- 5-10% difference in fan speed for identical fans and with it power difference but that's small enough not to cause any problems.

 

[SNek23]

Fan manufacturers usually specify +/- 5-10% difference in fan speed for identical fans and with it power difference but that's small enough not to cause any problems.

Thanks!

 

[Paperdoc]

All correct above. Every fan generates a fan speed signal which is pulses 5 VDC high at 2 pulses per revolution, and sends that back to its header on Pin #3. The header counts those to show you a speed. BUT that header can NOT deal with TWO streams of pulses coming back, because they NEVER match up exactly - they always are different and drift in and out of synchonization. If you try to do that mix, the header gets wildy changing readings that are wrong, and may even cause errors to occur like triggering a fan failure shut-down. So any Splitter (or Hub) will send back to its mobo host header the speed of only ONE of its fans, and ignore any others. This does NOT affect ability to control the fan speeds - it only deprives you of "seeing" all the speeds.

There is one other impact, though. All mobo fan headers ALSO have another funtion. The monitor the fan speed signal for fan FAILURE - detected by a speed that is gone to zero, or in some cases is below a minimum setting. When that happens you get a warning so you can look into the problem. For CPU_FAN headers, the mobo MAY be designed to take stronger action, even to shutting down without waiting for the CPU chip's internal temperature sensor to show a high temp. (Few CHA_FAN or SYS_FAN headers will do this.) But when a Splitter cannot report ALL of its fans' speeds, that FAILURE detection job can't check ALL of the fans. So YOU should check all the fans from time to time to be sure they all still are working.

 

[SNek23]

All correct above. Every fan generates a fan speed signal which is pulses 5 VDC high at 2 pulses per revolution, and sends that back to its header on Pin #3. The header counts those to show you a speed. BUT that header can NOT deal with TWO streams of pulses coming back, because they NEVER match up exactly - they always are different and drift in and out of synchonization. If you try to do that mix, the header gets wildy changing readings that are wrong, and may even cause errors to occur like triggering a fan failure shut-down. So any Splitter (or Hub) will send back to its mobo host header the speed of only ONE of its fans, and ignore any others. This does NOT affect ability to control the fan speeds - it only deprives you of "seeing" all the speeds.

There is one other impact, though. All mobo fan headers ALSO have another funtion. The monitor the fan speed signal for fan FAILURE - detected by a speed that is gone to zero, or in some cases is below a minimum setting. When that happens you get a warning so you can look into the problem. For CPU_FAN headers, the mobo MAY be designed to take stronger action, even to shutting down without waiting for the CPU chip's internal temperature sensor to show a high temp. (Few CHA_FAN or SYS_FAN headers will do this.) But when a Splitter cannot report ALL of its fans' speeds, that FAILURE detection job can't check ALL of the fans. So YOU should check all the fans from time to time to be sure they all still are working.

Thanks for the explanation. If I understand conrrectly, the only thing that is "hindered" is my ability to see in software the additional fans and their RPMs. But in reality, they will be all running at roughly the same speeds which are dictated by the SYS_FAN.

 

[Paperdoc]

Correct.