There's at least one terminator on the planter. I'd look there first. When you unplug the planter from the tractor ISO connector then that connector is the last terminator in the CAN line and it sounds like the tractor steers and communicates fine. When you add the planter it sounds like you're having CAN communication issues. Check with your dealer service dept or your manual for CAN terminator locations and go from there. Are you using an implement receiver or rate controller? Or just have a plain old planter with no extra Deere goodies on it?