In a medical data transfer system the patient wears a sensor assembly (1) collecting data for wireless transmission to a medical data collection system (2). The sensor assembly (1) also incorporates a wireless communications unit (7), an electronic circuit (3, 5) generating medically relevant data, and a data transfer storage unit (6). The circuit (3, 5) is linked to the storage unit (6) by a first data cable (10). The communications unit (7) is linked to the storage unit (6) by a second data cable (12). The sensor assembly (1) has a switch (11) that in a first position makes the connection in the first data cable and in a second position makes the connection in the second data cable. The switch (11) alternates between the two conditions without communication to the electronic circuit (3, 5).
