Methods and apparatus for full I/O functionality and blocking of backdrive current

This invention relates generally to parallel processing of I/O ports in an electronic system and more specifically to the problems associated with electronic isolation between circuits when I/O ports of distinct, separate controllers are connected in common in an electronic system. An input/output (I/O) port designed for electrical interconnection with multiple similar ports includes an input read circuit, an output drive circuit, and a circuit to control the port for input or output mode by electrically disconnecting the output drive circuit from the port. Also included is a back drive current protection circuit placed in series in between an external I/O line and the input read and output drive circuits which blocks backdrive currents from other active processors when the processor that includes the protection circuit is in a powered down state while still allowing full input/output functionality in a powered up state.