It depends on what version im483's you have.
The 8P2 and PLG drivers don't have a seperate P1 ground. The P1 34 pin option does have a dedicated ground for the input side including Step/direction and opto. IMS told me that this was to isolate the additional step clock out /direction out on P1 and that this was a seperate ground from that on the P2 (power side) connector, but a common ground for chained drivers.
I have both types and was advised by IMS that for the 8ps and plg types the opto supply (pin 4) is grounded by the Power side (P2) ground (pin 3). This works for me but all my grounds are tied to a common, including Power and logic and case. If your motor power ground wasn't at 6v under I'd say ground it to case and logic also. No straight answer but hope that helps.
EDIT: I've just found the Product manual for the IM483's (not later programmable or IE versions) It shows a minimum connection option where the step or direction switch is to ground with the +ve 5v to opto supply... If you need the manual I'll post it.
PS - Are you the same gashmore guy building a yacht in his back yard?