Abstract:
A bus driver circuit which permits input of data, which can be identified reliably by modules on the bus line, onto the bus in the event of a short circuit on a conductor in a two-conductor bus system without requiring complicated circuitry is composed of several electrically controllable switches which form an H-bridge configuration, at least one switch being present in each of the five H-bridge branches. The two bus conductors are each connected to one end of the shunt arm of the H-bridge. The individual switches in the longitudinal branches of the H-bridge can be switched to at least two positions which are at different voltage levels.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a bus driver circuit for a two-conductor bus system connected to a control unit containing the bus driver circuit and multiple actuator modules and/or sensor modules in a motor vehicle, with the bus driver circuit supplying signals in the form of a pulse train formed by different voltage levels into the healthy bus conductor in the event of a short circuit on one of the two bus conductors. 
     BACKGROUND INFORMATION 
     Actuators and sensors are being installed in increasing numbers in motor vehicles to regulate the power train, brake systems, driving performance and restraint systems for the protection of occupants of the vehicle. Heavy and bulky cable harnesses can be eliminated by introducing bus systems linking together the actuators, the sensors and the respective control units. 
     In the case of a bus for a safety-relevant system such as a restraint system in particular, measures must be taken so that a fault on the bus conductors will not result in a system failure resulting in the restraint systems not being deployed in the event of a crash situation. For such a bus, there is therefore the requirement that signal transmission must be possible between a central control unit and actuators and/or sensors connected to the bus even in the event of a short circuit to the battery voltage of the vehicle or to ground on one of the two bus conductors. For this reason, the German Published Patent Application No. 198 13 952 describes a bus driver circuit with which it is possible to transmit messages in the form of pulse trains formed by two different voltage levels over the intact bus conductor in the event of a short circuit on one of the two bus conductors. The bus driver circuit is a series connection of three switches at two different voltage potentials. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a bus driver circuit which requires the least possible circuitry complexity and permits data to be injected into the bus in such a form that it can be identified reliably by the actuator modules and/or sensor modules connected to the bus. 
     This object is achieved through the fact that the bus driver circuit is composed of multiple electrically controllable switches forming an H-bridge configuration, at least one switch being present in each of the five H-bridge branches, the two bus conductors are each connected to one end of the shunt arm of the H-bridge, and the individual switches in the longitudinal branches of the H-bridge can be switched into at least two positions which are at different voltage levels. 
     Accordingly, it is expedient that the switches in two longitudinal branches of the H-bridge, starting from different ends of the shunt arm, can be switched to positions which are at different high voltage levels, and the switches in the two other longitudinal branches can be switched to positions which are at different low voltage levels. In the event of a short circuit on one of the two bus conductors, the switches in the longitudinal branches of the H-bridge that are connected to the healthy bus conductor are switched to switch positions so that a desired pulse train occurs on the healthy bus conductor. In the event of a short circuit in a bus conductor to ground, a switch connected to this bus conductor and having a switch position at ground potential is preferably switched to this position. 
     In the case of messages having a high transmission rate, the at least one switch in the shunt arm is expediently closed briefly between switching through different voltage levels over switches in the longitudinal branches of the H-bridge. This increases steepness of the edges of the pulses transmitted, thus permitting a high pulse transmission rate. Messages requiring a high transmission rate include, for example, deployment commands for restraint systems. 
     The pulse trains transmitted are preferably Manchester encoded, thus permitting easy synchronization of actuator modules and/or sensor modules. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a circuit diagram of a bus driver and multiple modules connected to the bus. 
     FIG. 2 a  shows a first possible switching operation of the bus driver circuit in the case of healthy bus conductors. 
     FIG. 2 b  shows a second possible switching operation of the bus driver circuit in the case of healthy bus conductors. 
     FIG. 3 a  shows switching operations of the bus driver circuit in the event of a short circuit to ground in the first bus conductor. 
     FIG. 3 b  shows switching operations of the bus driver circuit in the event of a short circuit to ground in the second bus conductor. 
     FIG. 4 a  shows switching operations of the bus driver circuit in the event of a short circuit to the battery voltage of the vehicle in the first bus conductor. 
     FIG. 4 b  shows switching operations of the bus driver circuit in the event of a short circuit to the battery voltage of the vehicle in the second bus conductor. 
     FIG. 5 shows switching operations of the bus driver circuit in the event of healthy bus conductors with a very high-speed data transmission. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a bus system composed of a two-conductor bus L 1  and L 2  connected to a control unit SE and n actuator modules and/or sensor modules, this figure showing modules M 1  and Mn. The two-conductor bus system illustrated here has a ring structure, for example, i.e., both ends of bus conductors L 1  and L 2  are connected to control unit SE. 
     In control unit SE there is a bus driver circuit composed of multiple electrically controllable switches S 1 , S 2 , S 3 , S 4  and S 5  forming an H-bridge configuration. In practice, switches S 1 , . . . , S 5 , which are identified with a simple switch symbol, are preferably MOS field-effect transistors. Instead of switch S 1 , . . . , S 5  shown here, multiple switches may also be provided in the individual branches of the H-bridge, their switching function optionally depending on the direction of the current flowing through the switch. As indicated by broken lines, individual switches S 1 , . . . , S 5  are controlled by a processor PZ in control unit SE. Two bus conductors L 1  and L 2  are connected to the shunt arm of the H-bridge in which switch S 5  is located. First bus conductor L 1  is connected to a tie point between the shunt arm having switch S 5  and the two longitudinal branches having switches S 3  and S 4  of the H-bridge. Second bus conductor L 2  is connected to tie point  2  between the other end of the shunt arm having switch S 5  and the two longitudinal branches having switches S 1  and S 2 . Switches S 1 , S 2 , S 3  and S 4  in the longitudinal branches of the H-bridge have three different switch positions  0 , I and II. Switch positions I of two switches S 1  and S 3  in the two upper parallel longitudinal branches of the H-bridge are at a voltage level U 2 , and switch positions II of two switches S 1  and S 3  are at a voltage level U 1 . Voltage level U 1  is between approximately 20 and 30 V, for example, and voltage level U 2  is a few volts (e.g., 2 V) lower than voltage level U 1 . In their switch positions I, two switches S 2  and S 4  in the two lower parallel longitudinal branches of the H-bridge establish for two bus conductors L 1  and L 2  a connection of connection points  1  and  2  for the two bus conductors L 1  and L 2  to a voltage level U 4  having a much lower value than voltage level U 1  and preferably being the ground potential of 0 V. In switch position II, switches S 2  and S 4  put connection points  1  and  2  of the two bus conductors L 1  and L 2  at a voltage level U 3  a few volts (e.g., 2 V) above lowest voltage level U 4 . 
     FIGS. 2 a  and  2   b  show how processor PZ controls individual switches S 1 , . . . , S 5  of the H-bridge to transmit a message to modules M 1 , Mn when both bus conductors L 1  and L 2  are healthy, i.e., there is no short circuit. Messages sent out over the bus by control unit SE to modules M 1  through Mn are in the form of a pulse train made up of two different voltage levels, a high differential voltage ΔU between two bus conductors L 1  and L 2  corresponding to a logical 1, and a low differential voltage ΔU corresponding to a logical 0. The bottom portion of each of FIGS. 2 a  and  2   b  shows a curve for differential voltage ΔU between two bus conductors L 1  and L 2  when a bit string 10101, for example, is to be transmitted. The high level of differential voltage ΔU, which corresponds to a logical 1, arises from the difference between the two voltage levels U 1  and U 4 , and low differential voltage level ΔU corresponds to the difference between the two voltage levels U 2  and U 3 . The two differential voltage levels ΔU=U 1 −U 4  and ΔU=U 2 −U 3  thus occur either due to the fact that two switches S 2  and S 3 , which are connected to bus conductors L 1  and L 2  as shown in FIG. 2 a  , or switches S 1  and S 4 , which are also connected to the two bus conductors L 1  and L 2 , are controlled in common mode. To form higher differential voltage levels ΔU=U 1 −U 4 , either switch S 2  is switched to position I and switch S 3  is switched to position II (FIG. 2 a  ) or switch S 1  is switched to switch position II and switch S 4  is switched to switch position I (FIG. 2 b  ). The lower differential voltage level ΔU=U 2 −U 3  occurs due to the fact that either switch S 2  is switched to switch position II and switch S 3  is switched to switch position I (FIG. 2 a  ) or switch S 1  is switched to switch position I and switch S 4  is switched to switch position II (FIG. 2 b  ). FIGS. 2 a  and  2   b  illustrate how a symmetrical voltage modulation in phase opposition is produced on bus conductors L 1  and L 2 , thus minimizing any possible emission of interference signals. 
     All the other switches except for switches S 2  and S 3  or S 1  and S 4  remain in their switch position  0 , so that no voltage potential at all flows over them to bus conductors L 1  and L 2 . 
     Because of severe mechanical loads, in particular in the course of an accident, one of two bus conductors L 1  or L 2  may become short circuited to ground or to battery voltage. It is very important for a safety-relevant device such as a restraint system to have communication between the control unit and the actuator modules and/or sensor modules M 1 , Mn even in the event of such a short circuit on one of the two bus conductors L 1 , L 2 . In the case of a restraint system, communication between control unit SE and modules M 1 , Mn connected to bus conductors L 1 , L 2  is composed of a diagnostic inquiry directed by control unit SE to individual modules M 1 , Mn and—in the event of a crash—commands for deployment of the restraint systems (airbags, seat-belt tightening systems, etc.) controlled by modules M 1 , Mn. 
     If the bus driver circuit is to be controllable by processor PZ in a manner suitable for signal transmission over the bus in the event of a short circuit, it is to have an arrangement for detecting whether and on which of the two bus conductors L 1 , L 2  there is a short circuit. Such short-circuit detection is known from German Published Patent Application No. 195 09 133, for example. Details of this short-circuit detection, which is essentially known per se, will not be presented here because they are not included in the object of the present invention. 
     FIG. 3 a  shows how switches S 1 , . . . , S 5  of the H-bridge are to be controlled in the event of a short circuit to ground in bus conductor L 1  if a bit string 10101, for example, is to be transmitted to modules M 1 , Mn. A logical 1 appears due to the fact that switch S 1 , connected to bus conductor L 2 , which is not short-circuited, is switched to switch position II, and switch S 2 , which is also connected to bus conductor L 2 , is switched to switch position  0 . Then bus conductor L 2  is at voltage U 1 , and thus a voltage difference ΔU=U 1  exists between conductor L 2  and conductor L 1 , which is short-circuited to ground. If a logical 0 is to be transmitted, switch S 1  is switched into switch position  0  and switch S 2  is switched into switch position II. Then conductor L 2  is at voltage level U 3 , and there is a voltage difference ΔU=U 3  between conductor L 2  and conductor L 1  which is short-circuited to ground. It is expedient to keep switch S 4  constantly in switch position I in order to thus keep short-circuited conductor L 1  fixedly at potential U 4 =0 V corresponding to the ground potential. However, switch S 4  may also be kept in switch position  0 . Other switches S 3  and S 4  also assume switch position  0  permanently. 
     FIG. 3 b  shows the switch positions for the case when conductor L 2  is short-circuited to ground. To transmit a logical 1 over the bus here, switch S 3  is brought into switch position II, and switch S 4  is brought into switch position  0 . Then conductor L 1  is at voltage level U 1 . A differential voltage ΔU=U 1  is now applied between conductor L 1  and conductor L 2 , which is short-circuited to ground. A logical 0 occurs due to the fact that switch S 3  is switched into switch position  0 , and switch S 4  is switched into switch position II. A voltage difference of ΔU=U 3  then exists between two conductors L 2  and L 1 . Switch S 2  can be switched permanently to switch position I, so that conductor L 2  which is short-circuited to ground is kept at potential U 4 , which amounts to 0 V and therefore corresponds to the ground potential. Other switches S 1  and S 5  are kept permanently in switch position  0 . FIG. 4 a  shows the switch positions of switches S 1 , . . . , S 5  for the case when first bus conductor L 1  is short-circuited to battery voltage UB of the vehicle. Switch S 1  is switched to switch position II, and switch S 2  is switched to switch position  0  for transmission of a logical 1. Then a differential voltage ΔU=UB−U 1  exists between conductor L 2  and conductor L 1 , which is short-circuited to battery voltage UB. A logical 0 occurs because switch S 1  is switched to switch position  0  and switch S 2  is switched to switch position II. The differential voltage then existing between conductors L 1  and L 2  amounts to ΔU=UB−U 3 . The other switches S 3 , S 4  and S 5  remain in switch position  0 . 
     FIG. 4 b  shows the switch positions for the case when second bus conductor L 2  is short-circuited to battery voltage UB. A logical 1 occurs here because switch S 3  is switched to switch position II and switch S 4  is switched to switch position  0 . A differential voltage ΔU=UB−U 1  then exists between two conductors L 1  and L 2 . A logical 0 occurs on the bus because switch S 3  is switched to switch position  0  and switch S 4  is switched to switch position II. In this case, a differential voltage ΔU=UB−U 3  exists between conductors L 1  and L 2 . Switches S 1 , S 2  and S 5  remain in switch position  0 . 
     If a message is to be transmitted at the highest possible rate over the bus to individual modules M 1 , Mn, e.g., an ignition command at a bit rate of 125 kbit/s, then switch S 5  in the shunt arm of the H-bridge is switched briefly into switch position I between switching through two different voltage levels as illustrated in FIG. 5, thus briefly connecting two bus conductors L 1  and L 2  and thus bringing them both to the same voltage potential. FIG. 5 shows as an example transmission of a message over the bus when both bus conductors L 1  and L 2  are healthy, i.e., there is no short circuit. FIG. 5 illustrates clearly that switch S 5  is closed briefly when both switches S 2  and S 3  are switched from switch position I to switch position II and vice versa. This measure achieves the result that the individual signal pulse edges have a greater steepness, and therefore a higher bit rate is possible. Especially in the case of signals having a high voltage range such as deployment commands, a greater edge steepness is used to achieve a high bit rate. 
     It is expedient to transmit the messages with the known Manchester II code because it permits transmission with the lowest possible susceptibility to fault and also guarantees easy synchronization of modules M 1 , Mn. 
     Modules M 1 , Mn each have electronic controls SE 1 , SEn. If modules M 1 , Mn are actuator modules, then electronic controls SE 1 , SEn have the function of controlling the deployment of the restraint devices connected to modules M 1 , Mn. Electronic controls SE 1 , SEn may also have a fault diagnostic function for the circuitry arrangement present in modules M 1 , Mn and the respective deployment devices of the restraint arrangement. Likewise, electronic controls SE 1 , SEn are also responsible for controlling a longitudinal switch SM 1 , SMn. If both bus conductors L 1 , L 2  are short-circuited together at one location or if they have simultaneous short circuits to ground and to battery voltage, then the longitudinal switches in the modules directly adjacent to this short-circuit site are opened. If the bus is a ring, as illustrated in FIG. 1, then data and power can be transmitted from control unit SE to all modules M 1 , Mn despite such a short circuit, because data and power can be transmitted in both directions (to the right and to the left) on a ring bus. 
     If the power supply for electronic controls SE 1 , SEn in modules M 1 , Mn travels over the bus from control unit SE, then a bridge rectifier composed of a diode ring GR 11  and GR 12 , GRn 1  and GRn 2  is connected between two bus conductors L 1  and L 2  on both sides of longitudinal switch SM 1 , SMn. It is thus possible to obtain a d.c. power supply voltage for electronic controls SE 1 , SEn from the data signal transmitted over the bus conductor, regardless of the direction in which this data signal is transmitted on the bus conductor. 
     A capacitor C 1 , Cn functioning as an energy buffer in the transmission of signals (deployment commands) having a high voltage range is connected upstream from each electronic control SE 1 , SEn.