This invention relates to a communication system of the multiplex type, that is with an information channel which is accessible to a plurality of users, provided between a master processing unit and slave units with the possibility that information exchange can be initiated from one of the slave units. In particular the communication system according to this invention can be applied to a plurality of units associated with the electrical devices in a vehicle, and especially for activation and control of the electrical devices fitted to a vehicle door, for example, the window winding device, the door locking device, the device actuating various electrical components in an outside rear-view mirror and the manual control switch device for operating the components of the various abovementioned devices as described in Italian patent application No. 67158-A/89 lodged on the Mar. 7, 1989 entitled: "Activation and control system for a plurality of electrical devices in a vehicle".
As known, multimaster systems in which each unit has the characteristics of a master unit and is therefore capable of communicating directly with every other unit are used in communication systems between various units. This has disadvantages in that:
all the units must be approximately equal in complexity PA1 because all the units are connected by means of a single data transmission line. A fault in the latter may cause failure of the entire system if redundancy is not provided, but this entails costly duplication and operational difficulties.
The structure of the messages is complex, either because they have to include double addressing, or because of the need to interpret all messages by a single unit.
Master-slave systems have therefore been constructed in which there is a central master unit which controls all communication sending commands to, interrogating or synchronizing the slave units, which in turn can only transmit after these communications have been received. A result of this however is that the master unit must always be active whenever communication is required with any slave unit. In order to overcome this disadvantage while maintaining the structure of the master-slave system additional transmission lines dedicated to communication requests are required.