Abstract: A modular interface unit to serve as a building block in the framework of an Input-Output Subsystem of a digital data processing system. The unit, designated as a Base Module, manages a group of Intelligent I/O Interface Control units (Line Control Processors) and provides a Base Module distribution-control interface called a Distribution Card unit. A group of Line Control Processors have a common set of backplane connections to the distribution-control means and to a maintenance unit and termination control unit. The maintenance unit permits the exercising and diagnosis of any selected Line Control Processor on the Base Module. The termination control unit provides clocking and control functions for all of the Line Control Processors in the module and also common termination impedances for the interconnecting backplane lines. Means are provided for priority ranking of the various Line Control Processors within the Base Module unit.