A method is provided for use in a host module, for identifying at least one accessory module on a bus, wherein the bus is configured to allow multiple accessory modules to be connected to the host module. The method includes sending a query to any accessory module connected to the bus, the query concerning whether the or each accessory module meets a specified criterion; and receiving synchronized responses from any accessory module that meets the specified criterion connected to the bus where said responses are specific to the query but non-specific to an effectively uniquely distinguishing feature of the individual module. It is then possible to determine from redundant information contained in an aggregate of the synchronized responses whether there is (a) no accessory module meeting the specified criterion, or (b) at least one accessory module meeting the specified criterion.