A receptacle block defines one or more sockets at which plugs may be received. Each socket contains a first set of contacts and a second set of contacts. Each socket also includes a sensing contact that interacts with the second set of contacts to close an electrical switch. For example, the sensing contact can interact with an arm extending from one of the contacts of the second set. Closure of the switch can be detected and interpreted to indicate that a plug has been received at the respective socket.