@ablack As @jay_s_uk has said and you use the "A" parameter in M954 to denote the CAN address you want to use. Plus you need to put a special IAP in the /firmware folder of the main board that you are going to use as an expansion board to facilitate firmware updates by the usual method. Fora 6HC, that file is Duet3_CANiap32_MB6HC.bin
Bear in mind that main boards only have a single CAN connector, so if you have other expansion boards, the 6HC must be the last one in the chain.
Here is a link to everything you need to know. https://docs.duet3d.com/en/User_manual/Machine_configuration/CAN_connection
You might need to scroll down to find the section "Mainboard as expansion board".
