Abstract: A bus subscriber includes: a first logic unit; a second logic unit; a monitoring circuit; a transmission unit; and a first test line. The bus subscriber is adapted such that the first logic unit communicates with a bus via the transmission unit in a communication phase and the monitoring circuit emits a monitoring signal to the transmission unit upon a fault so that the transmission unit blocks the communication. The first and/or the second logic unit is coupled to the monitoring circuit via the first test line so as to emit a first test signal to the monitoring circuit.

Abstract: The invention relates to a data transmission system for the serial asynchronous data transmission between two devices (1; 3). The aim of the invention is to provide a secure data transmission system for the communication between two devices that can be produced with as little technical effort as possible. To this end, two circuit parts that are interconnected by means of a two-wire circuit (8) are used which are provided with respective receiver circuit elements (2b; 4b) and respective emitter circuit elements (2a; 4a). The inventive system also comprises a source of current (6) via which a current (1q) can be fed to the data transmission line (8a) so that signal states can be modified depending on the input signals of the circuit parts while at the same time parts of the circuits are supplied with current.