expansion mainboard considerations

[Thorpydo]

I am in the process of designing and laying out the wiring for a printer that will have multiple Duet3 mainboard 6HC's (5 in total) used as expansion boards.

I would like to understand if any consideration should be given to which components are wired to which mainboards.
IE does it matter which mainboard is used for:

• bed heater output
• bed heater thermistor
• hot end outputs
• hot end thermistor
• homing switches

I would like to optimize for low CANFD bus load and therefore, my thinking is to keep as many components on the main mainboard as possible. Homing switches will only be used during startup when bus load will be load, so shouldn't matter which mainboard wired to.

I don't have a good grasp on what can be sent in a single CANFD message. If commands for all steppers and all heaters outputs can be sent in a single message, perhaps it's only important to keep the thermistors wired to the main mainboard to avoid a message from thermistor mainboards back to main mainboard.

Is my thinking correct here? Please ground my thinking and provide any insight you may have.
Thanks for your help and a great community!!

[dc42]

@Thorpydo the main load on the CAN bus is the movement commands. A single CAN message currently provides the movement details for all drivers on one board. Each expansion board needs a separate CAN message.

Heaters, thermistors and fans create very little CAN traffic. Heater temperature control is done locally on the expansion board, which is why RRF requires the heater and associated temperature sensor to be on the same board.

[Thorpydo]

@dc42 Great, thanks for the clarification!