Fieldbus connectivity is is usually modular/optional.
What fieldbus option is installed on your robot? What fieldbus options are available for that CNC?
KRCs can support a wide range of the major fieldbus options (Ethernet/IP, ProfiNet, EtherCat, etc), though you have to buy the appropriate option package. CNC machines tend to be much more limited, in my experience. Many don't support any fieldbus at all, and some don't even support discrete wired I/O.
So Panic is correct that Step One is to determine which fieldbusses both machines can support, followed by which ones on that list fit within your budget and technical support.
Discrete wired I/O is often the simplest, but is also the most limited. So one question is how much I/O you need for your application. KRCs don't actually have discrete wired I/O "naturally" -- instead, you connect a fieldbus I/O module via (EIP, Profi, ECat, etc) and then wire the discrete terminals to relays, sensors, etc. Even if you buy your KRC from KUKA with the discrete-I/O option, they just add a module like that and wire it up internally.
EIP and ProfiNet are probably the two easiest and most powerful (and popular) fieldbus options. Probably the 2nd-easiest option after discrete I/O, if the Haas supports it.
We should also mention safety -- usually machine tending doesn't involve Safety I/O, but if for any reason your application does (for example, you need the E-Stop on the robot to stop the Haas and vice-versa), then we have to get into Safety I/O, which is an entire topic of its own, and with very strict limitations and requirements.
