Reading Fan RPM At Two Places

[sabroe]

I wonder if anybody have any experience regarding the possibilities to attach a regular case or CPU fan in such a way that the RPM of the fan can be read at two places - both from a front display and through software from a motherboard connector - while the current to the fan is controlled from only one of the places.

Current front displays like the Aerocool V12XT or the NZXT Sentry LX are somewhat sad offerings in that they do not offer any possibilities for a paththrough connection to the motherboard fan connectors. It would be cool if the RPMs could be read from both the motherboard and still be visible on a front display.

Is it possible to e.g. modify a 3-pin y-splitter for this? Would the connectors on the front-display and the motherboard agree upon the ground level and in a sound way?

 

[Paperdoc]

Yes, you could try this, using a Y-splitter. Insert the splitter in the mobo SYS_FAN port, and take one set of wires to the fan. For the other set, cut the RED wire (should be Pin #2, the middle one) and plug the connector into the controller. This should feed the fan's speed signal (travels on the Yellow line from fan to speed sensor) to both mobo and controller, and ensure that the two devices have their Grounds connected together (the Black wires). But only the mobo will be able to feed +VDC power to the fan. I am not sure whether two speed pulse counter circuits connected in parallel to the same signal can work properly, but I think it should work.

This assumes you have a 3-pin fan. If yours is 4-pin, the color coding is different, and there is another line to cut.

 

[sabroe]

It may be worth trying. But still, whether two circuits listening in on the RPM signal in parallel can work together - and is a sound construction - is still an open question.

I realise that a normal 3-pin Y-splitter can not be used directly without a little bit of fiddling: A regular splitter for two fans has a female connector for the board/controller and two male connectors for the two fans. What is needed here is one male connector for the fan and two female connectors for connecting to 1) the front display fan controller and 2) the fan connector on the motherboard. Once the cable is made, the part which controls the fan speed can be changed by simply swapping the two female ends (disregarding possible issues with cable lengths).

Is it necessary to connect the black ground wire between the two fan controllers? -I have some older fan cables of different kinds and including some 3-pin, break-out cables with female connectors for a motherboard connector and with just the single, yellow wire and without ground and for redirecting the reading of the fan speed only (this has no parallel connectivity) ... so use cases do exist, where the ground level does not directly follow the signal on the yellow wire. Is it at all a good idea to connect the two ground wires; what if the connector on the front display fan controller and the connector on the motherboard through their different circuitry have _slightly_ different levels for the ground level ... could they put unwanted strain upon each other - however minor this may be - and possibly do any harm and over time?

 

[Paperdoc]

I understand what you're saying about slightly different "Ground" connections. The controller used to display the fan speed must have a Ground connection, of course. That serves both for the fan power circuit and the speed signal circuit - as you can see, any fan uses a common Ground lead for both circuits. However, I can understand your thought that the Ground from a mobo SYS_FAN port may not be the same as the Ground provided to the controller module from a 4-pin Molex power connector from the PSU. My guess, however, is that the PSU's ground connection is also good enough to serve as the speed signal Ground at the controller module.

So, IF you are using the fan controller for its normal functions anyway - that is, providing power to at least one fan, and fed from the PSU directly - then the controller module has a Ground connection. So I agree with you that it might be quite sufficient to feed ONLY the Yellow (speed pulse signal) line from the fan to the controller for reading it.