Abstract: An assembly including a host device and an add-on module, in which the host device and the add-on module include features to provide a rugged electro-mechanical interface or connection between the host device and the add-on module. The host device includes a body that has an open or non-enclosed docking bay on a side, and the module is placed into the docking bay such that a top side of its body mates with the inner surfaces and features of the docking bay. The interface is rugged so that the assembly is useful in the field, and the interface provides a secure attachment that can withstand dropping. A hook is provided on the top side of the body of the module, and the hook feature's shape, when combined with the location of fasteners on the module body that mate with threaded holes in the docking bay, is designed to withstand dropping.