An input/output connector system with a cable assembly that routes signals over a circuit board to which a receptacle connector is attached. A cage housing the receptacle has a channel with an opening at one end for mounting in a panel opening. The cage has an opening in a top surface through which a plug may be inserted into the channel to mate with the receptacle. The cage may be formed with one or more keys that engage channels on the plug to constrain motion of a plug inserted through the top surface to a direction perpendicular with the circuit board until the plug is inserted sufficiently into the cage that it can slide towards the receptacle without damaging the plug or receptacle. The plug may have one or more latch components on the sides, which engage with corresponding latch components on the cage when the plug is slid into engagement with the receptacle connector.