A central control unit (ZS) and several data processing units (DV1, DV2) for controlling restraint devices (RH1, ..., RHk) are coupled to a bus line (BL). The central control unit (ZS) sends interrogation or control signals in the form of data telegrams to an individual or to all data processing units (DV1, DV2). The data processing units (DV1, DV2) send back response signals to the central control unit (ZS) to answer given interrogation or control signals. In order to enable sure allocation of interrogation and control signals to the data processing units (DV1, DV2) and reliable detection of monitoring data of the data processing units (DV1, DV2) and their restraint devices (RH1, ..., RHk), the interrogation or control signals are data telegrams, all having the same frame structure (I, II, III) consisting of a first area (I) with n control bits, a second area (II) with m information bits and a third area (III) with p check bits.