A method, system and computer program product are provided for detecting and configuring an external input/output (IO) enclosure in a computer system. A PCIE Host Bridge (PHB) in a system unit is connected to a plurality of PCIE add-in card slots. One or more cables are connected between the PHB and the external enclosure. System firmware including detecting and configuring functions uses sideband structure for detecting a PCIE cable card and configuring the external input/output (IO) enclosure.