Use the controller slot

When a programmable controller is installed in the ControlLogix chassis, it can act as a programmable controller directly attached to the DH+ network (like a PLC-5 processor does). The 1756-DHRIO module will forward any local messages it receives on the DH+ network to the programmable controller. Since there can be more than one programmable controller, and they can be in any slot in the ControlLogix chassis, the 1756-DHRIO module needs to know which slot the local programmable controller is in.