A 3D chip having at least one I/O layer connected to other 3D chip layers by a vertical bus such that the I/O layer(s) may accommodate protection and off-chip device drive circuits, customization circuits, translation circuits, conversions circuits and/or built-in self-test circuits capable of comprehensive chip or wafer level testing wherein the I/O layers function as a testhead. Substitution of I/O circuits or structures may be performed using E-fuses or the like responsive to such testing.