A system includes a housing having multiple slots. The system also includes multiple adaptor modules each configured to be inserted into one of the slots and perform at least one function related to an associated input or output signal. For each slot-adaptor module pair, the slot includes one of a key and a receptacle configured to receive the key, and the adaptor module includes the other of the key and the receptacle. Each key fits into the receptacle associated with one type of adaptor module and cannot fit into the receptacles associated with other types of adaptor modules. Each receptacle accepts the key associated with one type of adaptor module and cannot accept the keys associated with other types of adaptor modules. Each of at least one of the keys and the receptacles is rotatable to achieve one of multiple different orientations. Different ones of the keys have different cross-sectional shapes.