Always consult the official Owners Manuals and Guides first.
Information about the FM3 may be incomplete or wrong, it's being worked on.
What is FASLINK
FASLINK is a proprietary Fractal Audio communications link between the Axe-Fx II and the MFC-101 (FASLINK version 1), between the Axe-Fx III and FC controllers (FASLINK II), and between the FM3 and an FC controller (FASLINK II). FASLINK provides two-way communication and power via an XLR cable. XLR cables (microphone cables) are common, flexible and robust, supporting runs of hundreds of feet.
FASLINK replaces previous connection methods (MIDI and Ethernet).
There are two versions of FASLINK:
FASLINK (original) – connects the Axe-Fx II and MFC-101
FASLINK II – connects the Axe-Fx III and FC foot controller(s), and the FM3 and FC foot controllers. It features a new protocol. Length limit is 100 feet (source)
"FASLINK is not a balanced protocol. But it is robust because it uses high signaling levels. You can use any wire for FASLINK but ideally it should be shielded and a heavy enough gauge to carry up to 1A without significant voltage drop. If "I" were adapting FASLINK to CAT5 I would use each pair as a conductor. I.e. blue and blue/white tied together as one conductor, green and green/white tied together as another conducter. Then the drain wire (and maybe another conductor pair to ensure low resistance) connected to the XLR shield. FASLINK uses two conductors plus shield (standard mic cable). One conductor is power. The other conductor is data. The shield is the common return for the two conductors. We've tested FASLINK over 100m of cable. As long as the conductors are heavy enough so the voltage doesn't drop too much you can run it very long distances." source
"The FASLINK port is protected against shorts." source
"It's not standard MIDI. The Axe-Fx III uses a proprietary communication protocol. Devices can communicate over any physical layer by wrapping and unwrapping the comm protocol. Theoretically you can control it over MIDI, Ethernet, WiFi, RS-232, AppleTalk, Sneakernet, etc. The MIDI basics are there, of course, but to access the tuner, etc. you need to wrap and unwrap the protocol layer." source
Supported by which products
Axe-Fx III – supports FASLINK II
FM3 – supports FASLINK II
Axe-Fx II XL and XL+ – supports FASLINK
Axe-Fx II Mark I and II – requires FASLINK adapter to convert wiring from Ethernet/Ethercon to XLR
MFC-101 Mark III – supports FASLINK
MFC-101 Mark I and II – requires FASLINK adapter to convert wiring from Ethernet/Ethercon to XLR
FC controller – support FASLINK II
Set up a FASLINK connection
Axe-Fx III – connect the XLR cable between the Axe-Fx III and FC controller
FM3 – connect the XLR cable between the FM3 and FC controller
Axe-Fx II – connect the XLR cable. Turn on FASLINK in the I/O menu
MFC-101 – connect the XLR cable. Select the correct port in the MFC settings. Do NOT connect a power supply to the MFC-101 when using FASLINK
Provide power through FASLINK
A FASLINK II connection provides power (from the Axe-Fx III or FM3) to the first connected FC controller.
FASLINK adapters for older hardware
The FASLINK XA-1 and XA-2 adapters convert the wiring on older Axe-Fx II and MFC-101 hardware (see above) from Ethernet/Ethercon to XLR. These adapters are "active" (source). They can be bought directly from Fractal Audio's online shop and G66.
The adapters do not support FASLINK II.