Duet 3 uses CAN-FD bus to communicate between the main board and expansion boards, and we plan to make a Smart Tool board using the same CAN-FD-bus.
For devices that are just sensors, CAN is likely to be overkill. Duet 3 uses a simpler interface with a 4-pin connector to connect sensors, including endstops sensors, Z probes and filament monitors. The benefit of CAN-FD is that it can handle high data rates and it can tolerate the ground noise that stepper motors and PWM'd heaters cause.
