Duet 0.8.5 and PWM fan

I noticed in the DWC, it has an option to show Fan RPM and Fan slider. But, I'm always seeing the RPM as 0, so it has me wondering why. I have a DC turbine blower fan in the PWM Fans 0 slot and the Hotend fan in the PWM Fan 1 location. And during my PLA prints with cooling, I can see the turbine blower turning on and running (I can't tell if its speed changes at all), and I see my hotend fan start once the temp goes <45C on either extruder nozzle. I'm assuming I must be missing something in my config.g to tell it how many RPMs are 100%, but I didn't see anything with Fan RPM besides M408 in the G-code Wiki.

The RPM display in DWC only works if you use a 4-wire PWM fan and connect it up in a certain way.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Any details to how it has to be connected somewhere?