The invention relates to a method for transferring data between a secure computer (1), e.g. an error protected stored program control (1), and a number of input/output units (2-4) via a bus control unit (6) connected to a secure computer (1) and a serial bus system (5). The bus control unit (6) responds in a cyclical manner to the input/output units (2-4) that are connected to the bus system (5) and transmits a multi-bit message (8) to the respective input/output unit (e.g. 4) that is addressed. In order to create a data transfer method that enables security-relevant signals to be transmitted via a bus system (5) which is not error protected, the invention provides that at least one of the input/output units (4) is designed as a security unit (4), the multi-bit message (8) transmitted to the security unit (4) contains at least one control bit and the security unit (4) interprets the multi-bit message (8) as being correct only if the control bit alternates within a given monitoring period.
