In a fieldbus system, sub-subscribers include a functional switch-on unit connected to a sub-switch-on unit. Status information indicating whether the functional switch-on unit is active is recorded in each sub-subscriber. A telegram is output by a main subscriber, including at least one datagram. The status information of the sub-subscriber addressed in the datagram is recorded with the telegram. A counter field is changed by the sub-switch-on unit of sub-user, having processed the user data field in a predefined manner. The datagram is assigned an expected value for the counter field, which results when the user data field has been processed in accordance with the command field. The main subscriber determines the expected value for the counter field on the basis of the status information, and compares it with the value of the counter field processed in order to detect a processing error, in the event of a deviation.