Abstract: A modular input/output system has a power supply unit, a central processing unit, and at least a pair of input/output module units. Each input/output unit has a base and an input/output module. The base includes a backplane printed circuit board having an input/output module interface. The base has a pair of connectors connected to the printed circuit board and capable of connecting to other units. The base has a field connection terminal block for connecting to external sensors and controls. The input/output module includes a circuit for interfacing with external sensors and controls and the CPU unit. The modular input/output system allows installation and removal of input/output modules of one of the input/output module units without affecting other input/output modules. The system can have multiple power supply units for powering the I/O modules.