data transfer bus...

These types of communications protocols are industry standards, and as with most industry standards, everyone has their own. The speeds range from hundreds of Hertz to hundreds of mega Hertz. Shielded and unshielded copper conductors are use, as well as glass and plastic optical fiber.

Worse, if the robot or CNC machine is manufactured as an integrated device the communication may adhere to no one's standard but the manufacturer's.

Common communications protocols that may be used would include RS-232 (which implies asynchronous serial), RS-485 (which is just an electrical spec, but in these cases is usually used for asynchronous serial), Ethernet (not terribly likely; it's overkill), CAN (getting more common, particularly in recently designed equipment).

Any of these communications methods would have their own proprietary or purchased protocols built on top -- just knowing it's one of the above still wouldn't help you much if you needed to analyze the system's behavior or (heaven forbid) modify it or use pieces.

Even if the thing is built piecemeal there's still no one standard -- but it's more likely that the interface between the two dissimilar parts will use some more standardized communication.