Abstract:
In an electronic system for the transmission of data between a number of stations, sub-system operation can be achieved by a suitable choice of signal levels and wake-up levels, so that some of the stations can communicate with one another while other stations are in a sleep mode in which power is saved.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a system for the transmission of information between a plurality of stations which are interconnected via a bus, consisting of at least one conductor, and transmit, via said bus, information encoded in the form of different signal levels. 
     A system of this kind is already known, for example from DE 195 23 031 A1. 
     Therein, a station is essentially in one of two different modes. In the normal or active mode the communication is possible between all stations, i.e. all stations can correctly receive a message transmitted by any other station, a transmitting station imposing the appropriate signal level on the bus by means of a bus driver. In the sleep mode a sleep level occurs on the bus and communication is not possible; however, a message transmitted by an arbitrary station can be interpreted by the other stations as a request for changing over the system to the normal mode, so that via the bus the system can be woken up from the sleep mode and communication can be resumed. 
     Typical applications of such systems involve a large number of stations so that there is a need to save electric power, for example in vehicles in the parked state, so that the sleep mode should be entered as often and for as long as possible. However, at the same time there is usually a need that some of the stations would occasionally like to communicate with one another also in the sleep mode. The temporary normal operation of the overall system with all stations would not be acceptable for this purpose, considering the power consumption. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to enable sub-system operation in which some of the stations communicate with one another, whereas the remaining part of the stations remains in the sleep mode without interpreting the ongoing communication by some of the stations as a wake-up request. 
     This object is achieved according to the invention in that in addition to the signal levels there is defined an additional wake-up level which is situated outside the signal level range and can hence be clearly distinguished from the signal levels for all stations. If this wake-up level does not occur for a communication, the stations already in the sleep mode will remain in the sleep mode. 
     In order to achieve a sub-system operation starting from a sleep mode, first all stations are briefly woken up by the transmission of a wake-up level. This wake-up level normally succeeds a data message. If desired, the message can define the subscribers for the sub-system operation, so that the stations which are not addressed can immediately enter the sleep mode again. The further communication in the sub-system operation takes place on the basis of signal levels. 
     The invention also relates to a station which includes at least one receiver for receiving signals received via a bus, the station being switchable to the normal or active mode by way of a wake-up signal having a voltage level which lies outside the signal level range. 
     In order to avoid mix-ups and enhance the system consistency, in the stations the wake-up level can be interpreted logically identically to a signal level, so that during normal operation a message transmitted by a station by means of a bus driver which unduly operates with wake-up levels is correctly received by all stations in the same way as if the bus driver had used the normal signal levels. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention will be described in detail hereinafter with reference to the drawing. Therein: 
     FIG. 1 shows a system with a plurality of stations which are interconnected via a bus, 
     FIG. 2 shows diagrammatically the construction of a station, 
     FIG. 3 shows various signal levels and threshold values, and 
     FIG. 4 shows diagrammatically the construction of a bus driver in a 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a system in which a number of stations  14 ,  15 ,  16  are connected to a bus  11  via which signals representing information are transmitted. These signals are binary signals which are transmitted serially via a single conductor or in phase opposition via two conductors. The bus may also include a conductor for power supply of the stations in as far as the stations are not provided with an own power supply source. Each station includes a receiver for the signals transmitted via the bus and normally also a bus driver whereby signals can be transmitted, via the bus, from the relevant station to all other stations. 
     As is shown in FIG. 2, each station, for example the station  14 , includes at least one bus driver  21  which is connected to the bus  11 , a circuit  22  for executing a protocol for the reliable transmission of the individual messages, and at least one control circuit  23  for the execution of application-specific tasks. The bus driver converts the serial data, arriving from the protocol circuit  22  via the line  24 , into signal levels or wake-up levels on the bus  11 . Via lines  26  and  27 , the bus driver  21  can be set to the sleep mode in which the power consumption is very low, to the normal mode or to the wake-up mode. The receiver in the bus driver  21  transmits signals received from the bus  11  as data, via the line  25 , to the protocol circuit  22  for further processing, said protocol circuit applying the data to the control circuit  23 . The protocol circuit  22  also receives data from the control circuit  23  for application to the bus driver  21 . The bus driver can signal a detected wake-up request via the line  28 . The station is then set to an active mode; for example, parts of the station, such as the bus driver or also parts of the control circuit, are then connected to the supply voltage. 
     As is shown in FIG. 3, the bus driver  21  can adjust the following levels on the bus  11 : 
     a sleep level V 0  which can at the same time be interpreted as a logic “0”, 
     a signal level V 1  which is interpreted as a logic “1”, 
     a wake-up level V 2  which is also interpreted as a logic “1”; upon a change-over of the level from V 0  to V 1 , or vice versa, only the receiving level V 3  is exceeded whereas upon a change-over of the level from V 0  to V 2 , or vice versa, both receiving levels V 3  and V 4  are exceeded, V 4  representing the wake-up receiving level. 
     FIG. 4 shows the construction principle of the bus driver  21 . From an applied voltage V 7  a voltage converter  31  forms the voltages V 1  as the signal level and V 2  as the wake-up level for the control circuit  32  which itself, in the event of a logic “1” on the line  24 , originating from the control circuit  23  in FIG. 2, switches the transistor  37  as a voltage follower in such a manner that the level V 1  or V 2  prevails on the bus  11 . Via the line  27 , also originating from the control circuit  23 , the switch  35  is switched over from V 1  to V 2  and hence from the normal mode to the transmission of a wake-up level. In the case of a logic “0” on the line  24 , the transistor  37  remains switched off and the sleep level imposed by the resistor  38  occurs on the bus. 
     The receiver in the bus driver  21  is formed essentially by the comparators  33  and  34 . The voltage converter  31  also provides the reference voltage V 3  for the reception of data by means of the comparator  33  and the supply voltage V 5  for feeding the comparator  33 . Irrespective of the fact whether a station transmits by way of signal levels or wake-up levels, the data is always correctly received because of the receiving threshold V 3  which is effective for both levels, said data being applied to the protocol circuit  22  in FIG. 2 via the line  25 . The circuit  39  supplies the reference voltage V 4  and the supply voltage V 6  for the reception of the wake-up signals by means of the comparator  34 , said voltages being applied to the control circuit  23  in FIG. 2 via the line  28 . 
     Via the line  26  from the control circuit and the switch  36 , the bus driver can be set to the sleep mode in that the voltage converter  31 , and hence the voltages V 1 , V 2 , V 3 , V 5  and V 8  are switched off and only the circuit  39  remains active (and hence also the comparator  34 ) in order to evaluate a wake-up level arriving and to output a signal via the line  28 . Via the line  26 , further circuits of the stations can also be switched off, for example the protocol circuit.