The manual shows the motor power supply grounded and recomends that each amplifier NOT be daisy chained for both the + and common conductors from the DC supply, the 5v going into the IM483's is your 5v supply and is isolated by virtue of the opto's, this path is through the opto through your driver and to your 5v ground, so there should be no connection with your 5v supply and the amplifier apart from the fact that your 5v is grounded and the Motor supply would be grounded, which according to the manual is allowable.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.