RepRapFirmware (as at version 3.3) for Duet 3 with CAN-connected tool or expansion boards currently has the following limitations when tool boards or expansion boards are used.

We do not intend to remove these in future firmware versions.

• After upgrading firmware on an expansion or tool board, its configuration settings are lost. You must restart the main board, or at least re-run config.g. Duet Web Control usually offers a main board restart automatically after auto-installing firmware on an expansion or tool board..
• A heater on an expansion or tool board can only be controlled by a temperature sensor on the same expansion board. This is a safety precaution.
• Filament monitors must be connected to the same board as the corresponding extruder motor. This use so that the firmware can correlate the measured filament movement and the commanded extruder movement in real time.

We have no current plans to remove the following limitations, although removing them would be technically possible.

• Z probes connected to expansion or tool boards are limited to types 8 and 9.
• DHT temperature/humidity sensors connected to expansion boards are not supported

We plan to remove these in future firmware releases.

• Heater tuning is not yet supported on expansion or tool boards (M303). Support is included in release 3.3
• The main board does not react to heater faults on expansion boards by pausing the print. This restriction is planned to be removed in firmware 3.4.
• A thermostatically-controlled fan on an expansion board can only be controlled by a temperature sensor on the same expansion board. This restriction is planned to be removed in firmware 3.4.
• Endstop switches and Z probes connected to the main board cannot control motors on an expansion board. This is fixed in release 3.3.
• Due to CAN latency the motors connected to expansion boards may slightly overshoot the position at which the endstop or Z probe was triggered. This would not usually matter for an endstop switch, but it does mean that if the Z motor(s) is/are connected to an expansion board then repeated probing with a Z probe (e.g. for mesh bed compensation) is not advisable. This will be fixed in a future release
• Stalls of expansion board motors are not yet reported and cannot be used for homing.
• Change of stepper driver status on expansion boards are not proactively reported (but can be queried using M122). e.g. Overheat warnings, short to ground etc.
• MCU temperatures and monitored voltages on expansion boards are not yet reported in the object model (but can be queried using M122). This restriction is planned to be removed in firmware 3.4.
• When filament monitors are configured on expansion boards, the "calibrated" values in the object model are not updated; however they can be queried using M591 as usual.