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