Abstract: The invention relates to a bus participant device (30) for receiving and transmitting data telegrams (18) via a serial data bus (12) according to a master/slave method, wherein the bus participant device (30) is configured to function, in a slave mode, as a slave (16) on the data bus (12) by means of a slave unit (36) that is part of the bus participant device (30), wherein the bus participant device (30) comprises a master unit (38) that can be activated. In particular, by means of the slave unit (36) and/or the master unit (38), a transmission of data telegrams (18) via the data bus (12) can be monitored for a malfunction of an active bus master (14). Preferably, subject to a result of the monitoring process, an activation of the master unit (38) by means of the slave unit (36) or the master unit (38) can be effected. The invention further relates to a method for operating a serial data bus (12), and to various safety-critical devices.