Identify which arm is connected using digital inputs

[adeguet1]

There should be at least a "arm present" bit and maybe other digital inputs normally high or low based on which arm is connected. This could be used to make sure the arm is present but could also help detect:

• issues with software configuration, i.e. attempting to control a MTM with the PSM logic, etc.
• bad wiring with low level voltage. For example, SCSI cables loose or swapped inside the controllers