Power supply voltage of a PCI or a similar communication bus interface is separated from one or more other power supply voltages on a backplane, on boards insertable into the backplane, and on the bus interface components of the boards. The power supply of the bus interface (VIO) is provided to cold spare boards inserted into the backplane, while the other voltages are not provided to the cold spare boards. Availability of the VIO on the cold spare boards prevents the VIO clamping diodes on the PCI I/O lines from grounding the PCI bus. Cold spare capability is thus provided to systems with PCI and similar communication buses.