An input/output (I/O) device for a distributed modular I/O system includes a base adapted to be connected to an associated support structure. A terminal block is connected to the base and includes a plurality of wiring connections adapted to be connected to field wiring of an associated controlled system. The I/O device further includes first and second I/O modules each including a plurality of removable single-channel I/O submodules that are each releasably connected to the base and each configured for a select I/O operation for input and output of data relative to the associated controlled system. One or more pairs of the single-channel I/O submodules can be configured to be redundant within or between the first and second I/O modules. Each of the single-channel I/O submodules is operatively connected to wiring connections of the terminal block through the base. The I/O device further includes first and second network switches connected to the base. The first and second network switches are adapted to be respectively connected to first and second backplane circuits. The I/O device further includes first and second system modules connected to the base and each respectively connected to both of the first and second network switches. The first and second system modules are also each respectively operatively connected to all of the removable single-channel I/O submodules of both of the first and second I/O modules such that the first and second system modules control communication of I/O data between the first and second network switches and the single-channel I/O submodules.