What this module is
The Mitsubishi RD78G8 is an 8-axis PLC expansion module sitting in the PLC Expansion Modules class, designed to drop into the iQ-R backplane alongside the CPU to drive servo and stepper axes from the motion control instruction set.
It carries the PLC Expansion Module role rather than a stand-alone controller — power, control, and field wiring all run back through the iQ-R base, so the panel-builder's job is slot assignment, axis-count budgeting, and not crowding the heat load next to the CPU.