Corsairs RM, RMi, HXi and AXi, feature a USB and 4 pin header for theri intelligen PSU telemetry.
Roumors hoped that the 4 pin "legacy" interface is simply a PMBus header.
Noone did confirm this though and corsair did simply denie everything.
Looking at the pictures of my sadly defective HX850i, the silkscreen writing beside the connector does pretty clearly state what it is and where which thing needs to be connected.
Awesomly Corsair puts a legacy compatible cable into every box since you might have the older corsair commander stuff still in use. So to make it work, someone hopefully just needs to cut that cable and solder in a new plug for the PMBus connectors on most motherboards.