Disclosed is an interface between a drug delivery cassette and a medical effector system. The cassette may be mounted to the medical effector system in such a way that a fluid tube located on the cassette is positioned adjacent to a pump located on the medical effector system. The medical effector system may purge the fluid line of air by activating the pump and forcing fluid through the fluid line until a sensor positioned to monitor the fluid line indicates that fluid and not air is present in the tube. To prevent air purging of the fluid tube when connected to the patient, the medical effector system prohibits air purging unless the drug delivery end portion of the fluid tube is in a designated storage site located on the cassette. This is accomplished with a position sensor at the storage site that monitors the position of the fluid tube.