A portable expansion unit controls safety-relevant functions of a medical device. The expansion unit has a retaining device to be adjusted to the size of a flat, portable operating unit of the medical device. The retaining device mechanically connects the expansion unit detachably with the portable operating unit. A communication module establishes a safety-related data connection with the medical device. This combines a mobile, portable consumer operating device, such as a tablet computer or smartphone, which does not fulfill any particular safety-related requirements, with an expansion unit, which permits the control of safety-relevant functions with the aid of a safety-related radio protocol.