Abstract:
An arrangement for generating digital signals on first and second bus lines is tolerant of most line faults, and includes modules connected to both lines, an upper switch between a first potential and the first line, a middle switch between the two lines, and a lower switch between the second line and a second potential lower than the first potential. The switches are controlled to generate digital signals as the potential difference between the two lines. If a fault potential (e.g. a short to ground or other potential) arises on one line, then the switches drive the non-faulty line to provide the required signals relative to the fault potential on the faulty line. Closing the middle switch ensures that the two lines are at the same potential for a low signal. This arrangement is for is signaling in various bus systems, e.g. in vehicle occupant protection systems in motor vehicles.

Description:
FIELD OF THE INVENTION 
     The invention concerns a signalling output stage for generating digital voltage signals on a bus system connecting a central processor unit via two signal lines to a plurality of modules. 
     BACKGROUND INFORMATION 
     According to the state of the art, so-called push-pull-signalling output stages with two sets of transistor-switching equipment are usually used as signalling output stages, connecting a line either with the supply voltage or to ground. In general, short circuits to an interference potential (e.g. to ground), the supply voltage, or another third potential may lead to an attenuation of the necessary :signal amplitude and thus to faults in the signal recognition up to and including the failure of the signalling output stage. 
     In order to prevent this, an integrated transmit and receive circuit is to be taken from DE 196 11 944 A1 for coupling a control unit to a two-wire bus, where a test device for fault recognition on the bus system lines is provided and where, in addition to a standard operating mode, further different operating modes are provided which, in the event of a fault, allow communication adjusted to the actual fault type. This is known as a fault-related change-over of the circuit termination elements. Employing the switching equipments in FIG. 3 a  makes it possible, if signal reception is no longer possible with VCC, to initially apply CAN_L to Vbatt, instead of using termination resistors  16  and  17  which in normal operation mode apply CAN_H to GND and CAN_L to VCC. Moreover, it is also possible to deploy very low current sources  26  and  27  permitting a current-limited retention of the signal reception. Column 7, starting at line  56 , indicates that in the event of a defect, signalling can be abandoned as the transmission signal TXD can no longer have any effect on the bus. The change-over thus mainly affects the reception of signals in a faulty bus, but not their transmission. 
     DE 39 01 589 A1 describes the coupling-up of a bus participant, in which a resistor network assures that even in the presence of faults on the bus lines the data existing on the data bus can always be recognized. Here again, only those faults which occur during reception, and not at the time of transmission, are being considered, even though transmission outputs are also connected by means of this resistor network, but being connected in a rigid way and consequentially not adaptable to the fault type. 
     DE 195 09 133 A1 also describes a reception device compensating bus faults, that changes between one- and two-wire reception. 
     Therefore, these publications may suggest the various fault types, particularly short circuits to supply voltages or ground potential, but they do not consider a signal generation adapted thereto. 
     DE 44 03 899 A1 teaches such a generic signalling output stage in a device for serial data transmission between at least two stations. Thus, FIG. 2 shows a signalling output stage where an upper switching equipment or device (T 2 ) is connected between a first voltage potential (V 2 ) and a first line (S+), and a lower switching equipment or device (T 3 ) is connected between a second voltage potential (ground) and the second line (S−). In addition to this, various operating modes are provided for the case of a fault on one of the lines, which enable a signal generation adapted to the type of the fault. Checking the lines for short circuits, and signalling control, are implemented by direct technical switching or circuit means, that is, shifting voltage potentials on the lines will directly lead to different electrical conditions and thus to a different signalling. Thus, an independent signalling is provided for on both lines S+ and S−, that is, line S+ is not only connectable to V 2  via R 5 ,T 2 , but it can also be permanently grounded, via the high ohmic resistance R 7 , while S−, via R 4 , highly ohmically quiescently rests at V 2  and can be pulled to ground by switching T 3 . 
     In any emergency, signalling can thus be performed both just via S+ on its own as well as via S− on its own. In normal operation mode, both lines feature straight inverse signals to one other (compare FIG. 4 a ). In addition, both lines feature a quiescent potential. 
     In correspondence to this, DE 195 03 460 C1 also describes a failure tolerant output stage of the generic type which features a test device (condition detection module) designed for the detection of faults and their particular type, as well as a transmission module with different operating modes, where again, however, the first and second line of the bus system can signal independently of one another, because both lines each have their own connection to a high and a low voltage potential, as can be seen from FIG.  2 . 
     SUMMARY OF THE INVENTION 
     Based on this state of the art, this invention has the task to describe a further bus failure-tolerant signalling output stage, that does not have a fixed quiescent current load and yet provides a simple way, at least for the majority of possible faults, to continue signal generation in a different operating mode. Additionally, a particularly preferred application within a bus system is to be stated, where the certainty of data transmission being maintained, even in the event of any faults occurring on the bus system, will be further improved. 
     The above stated task is achieved according to the invention in a signalling output stage for the generation of digital voltage signals on a bus system with a central processor unit and a number of modules, connected to it by means of two lines, where the digital voltage signals assume one high voltage level and a corresponding lower voltage level. An upper switching equipment between a first voltage potential and the first line, as well as a lower switching equipment between a second, comparatively lower voltage potential and the second line are provided for. The central processor unit features a test device, by means of which faults on the lines, particularly short-circuits to a voltage potential can be detected. A standard operating mode is provided, in which signal generation is effected by the high voltage level being generated by closing the upper and lower switching equipments, and the low voltage level by opening at least the upper switching equipment. In case of a fault, additional different operating modes are provided for, which in spite of the fault enable digital voltage signals to be generated in such a way that they are adjusted to the type of fault. Further, a middle switching equipment is provided between the first line and the second line, by means of which signal generation will be maintained at least in the case of a fault on one line. 
     The above stated task is further achieved according to the invention in a circuit arrangement including the inventive signalling output stage, within a bus system, and further comprising the following features. All of the modules are further furnished with at least one short-circuit testing device for the two lines. The testing device is adapted to check the output of the respective line for an effective short-circuit, namely an effective resistance which is too low. For each of the two lines, there is further provided one switching device located between an input and an output of the respective line. The switching devices are respectively adapted to switch a connection between the input and the output of each respective one of the two lines, only after a test has been executed at the respective output by means of the respective short-circuit testing device and if the test has proven the absence of a short-circuit. 
     Advantageous further developments and embodiments of the invention are as set forth in the claims following this written description. 
     By using the two lines and three sets of switching equipment, it will become possible to drive the modules jointly via both lines, or individually, via only one of the lines, if a fixed interference potential is being applied to the other line. Both lines can assume either of the two voltage potentials. 
     If a fixed interference potential, e.g. to ground or supply voltage, is applied to a line, signalling or signal transmission will be implemented as a potential difference between the fixed interference potential of the short-circuited line and the line not subject to interference, the potential of which will be controlled accordingly. 
     Although the expenditure will initially be greater, due to the three sets of switching equipment and the two lines, this really becomes negligible if the new and varied options for signal generation, as well as the transmission certainty gained, are taken into account. 
     The different processes for generating the digital voltage signal are each optimized for one particular operating mode. A ground fault as well as a short circuit to a third voltage potential in one of the two lines do not necessarily lead to a failure of the modules or the signalling output stage. The relevant particular operating mode will be detected by means of a testing device, and the signalling output stage will be driven according to the test result, that is, the processes used for generating the voltage signals will be changed over., if necessary. 
     In particular, for a bus system within a vehicle occupant protection system, e.g. as used in a motor vehicle, in which—via the bus system—control modules for triggering vehicle occupant protection devices are connected to one another, and to the central processor unit, in a way that allows them to communicate, protection of this kind against simple line short circuit faults is essential. These short circuits can occur particularly against the ground potential, which is generally routed along the metal vehicle body or against the operating supply voltage, which is conducted in a vehicle&#39;s cable harnesses as a supply voltage network on an immediately adjacent basis. Due to the exposure to extreme mechanical stresses, a possible destruction of the cable insulation found in motor vehicles can never be entirely excluded. In particular, during the course of an accident, the cables can be subjected to destruction. In order to be able to still transmit the signals, that are relevant for the safety of vehicle occupants, to the control modules in order to trigger the vehicle occupant protection devices, the option to transmit signals even in the event of faults on one of the lines, using the respective other line represents a significant progress. In order to still be able to transmit signals even in the event of a short circuit to the supply voltage, e.g. battery power supply voltage, it is advantageous to select the two voltage potentials applied to the switching equipment such that a potential difference arises which is still detectable as a high voltage level. The higher of the two voltage potentials should thus be increased by a corresponding amount compared to the supply system voltage. Naturally, applications with regard to other signalling tasks are not excluded. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be explained below by means of an embodiment example and the following figures. Short description of the figures: 
     FIG. 1 Block diagram of a bus system featuring a signalling output stage with three sets of switching equipment and two lines in accordance with the invention. 
     FIG. 2 Overview over the possible signalling processes in accordance with the invention. 
     FIG. 3 Selection decision table with regard to the signalling processes used, based on the measured operating conditions or modes. 
     FIG. 4 Details of two modules featuring longitudinal switches for limiting the effect of a short circuit. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In FIG. 1, the bus system complete with a signalling output stage  1  is initially shown in the form of a block diagram. In this embodiment example, this signalling output stage forms a component part of the central processor unit  2 , but in principle it can also be used within the various individual modules, if these are to transmit actively, and thus require a signalling output stage. Via the two lines L 1  and L 2 , the modules M 1  . . . Mx are connected to the central processing unit  2 , which is effected such that the modules are arranged respectively in parallel one behind another between the two lines L 1  and L 2 . The individual modules M 1  . . . Mx respectively receive the arising particular voltage potential difference U between the first and second lines L 1 , L 2 . A high potential difference U between L 1  and L 2  is considered to be a high voltage level, in analogy to this a low potential difference, particularly a zero voltage, is considered to be a low voltage level. The assignment of a digital logical value (logical 0 or 1) is independent of this. Thus, particularly, a high voltage level can be used as the quiescent condition to ensure that the supply voltage is supplied to the individual modules M 1  . . . Mx, even if no signals are being transmitted. 
     The signalling output stage  1  comprises the three sets of switching equipment or switching devices S 1 , S 2  and S 3 , that are respectively located between a first voltage potential φ 2 , the line L 1 , the line L 2 , and a second voltage potential φ 2 , which is defined as the lower one of the two. This determination is to be understood to be merely a precondition for the unambiguousness of the processes to be described in the following. The switching equipments or devices S 1  to S 3  are schematically shown as switches in FIG. 1, in reality, however they are of course embodied in the form of the usual known transistor switches. 
     In addition, and by way of example, FIG. 1 shows a short circuit of the first line L 1  to a third voltage potential φ 3 . Such short circuits to ground or to a third voltage potential occur (e.g. due to insulation faults) on lines L 1  and L 2 , as well as possibly on spatially adjacent components and conductors. Since the case or housing is used as the electrical ground potential, particularly in the field of motor vehicle electronics, even a slight defect in the line insulation may lead to a permanent short circuit to ground. Even a short circuit e.g. to the operation supply voltage potential may occur in a different line that is similarly insufficiently insulated. 
     With open switching equipment S 1 , S 2 , S 3 , the lines L 1  and L 2  are free of potential and floating in normal operation mode, so that both lines—with an appropriate combination of switching equipment states—can assume both the first as well as the second voltage potentials. Only when a short circuit occurs on a line, this will then be subject to a fixed potential (second or third voltage potential), such that the signalling needs to take this into account. 
     The signalling output stage  1  or the respective sets of switching equipment S 1 , S 2 , and S 3  thereof are driven by a control device  3  of the central processor unit  2 . Moreover, in this embodiment example, all sets of switching equipment S 1 -S 3  comprise a respectively assigned current testing device I 1 -I 3 . This will determine the current which respectively flows through the switching equipment, and compares the determined current to a permissible value range. If this value range is exceeded, the control device  3  connected with these testing devices I 1  to I 3  will determine whether the signalling process is to be changed. The selection decision used in this case will be explained in more detail in connection with FIG.  3 . The testing devices I 1  to I 3  are to be used to monitor the operating mode or condition of the switching equipments S 1  to S 3 , as well as of the lines L 1  and L 2 ; in particular, the occurrence of a short circuit on one of the lines to a third voltage potential or ground is to be detected. Naturally, it is also possible—instead of detecting the current flow through the switching equipments—to detect the voltage potentials of the two lines L 1  and L 2  by means of voltage testing devices not shown here, and to compare these in accordance with adapted value ranges. 
     The special advantages provided by this signalling output stage can also be seen with regard to the signalling processes which have now become possible by this means, and which will be explained as follows by means of the following overview in FIG.  2 . 
     Thus, in the event of a short circuit in one line, signalling will fundamentally still be possible in almost all cases, with one exception, in that the now short-circuited line is in principle used as a fixed reference potential and the respective other line is switched as required for the signalling. Using the switching equipment S 2  located between the lines L 1  and L 2 , it will now be possible to generate the low voltage level even if one of the lines has a fixed potential. This is done such that the switching equipment S 2  will then be closed, respectively, and, via this equipment S 2 , the line which is still floating will be able to assume the voltage potential of the defective line, such that a potential difference U arises across the modules M 1  . . . Mx which corresponds to the low voltage level. On a line with a fixed potential, the high voltage level is respectively generated as a potential difference U of the first or second voltage potential with reference to this fixed potential (U=φ 1 -φ 3  or U=φ 2 -φ 3 ). The sign of the potential difference can fundamentally be accounted for by means of a rectifying circuit at the inputs of the modules M 1  . . .Mx. 
     The sets of switching equipment S 1  to S 3  are again arranged analogous to FIG.  1 . The switching states of the same are marked “closed” for the conductive state of the respective switching equipment, and “open” for the non-conductive state. “High” marks the high voltage level, and, correspondingly, “low” stands for the low voltage level. In operating mode B 1 , the change from “high” to “low” is additionally discussed by means of the arrow “→”. 
     In operating mode B 1 , the lines L 1  and L 2  are floating, that is, free of potential, if all three sets of switching equipment S 1  . . . S 3  are open. In this mode, both lines can thus be actively used for signalling with regard to the switching equipment position shown, “high” is provided by the potential difference from φ 1  and φ 2 . 
     When changing from High→Low, while S 1  is in its opening movement, S 2  will be initially closed for at least a short time, but for the maintenance of a low voltage level it should preferably be kept closed. Initially, the capacitive parts of the switching equipment, lines, and modules will be more quickly discharged due to the short circuit via S 2 , and the switching speed will be increased. In the subsequent operating modes B 2  . . . B 4 , this changeover will no longer be shown, since it can ultimately be applied independently from the individual signalling processes. The switching equipment S 3  is not absolutely necessary for the changeover and for achieving the low is voltage level, but it can advantageously be closed and L 2  set to φ 2 , which is usually the ground potential. The only exception will be the operating mode, in which L 2  is shorted to φ 3 , since closing S 3  would lead to a short-circuit. 
     In the overview in FIG. 2, the operating mode B 2  is initially shown as the signalling for a short-circuit from L 1  to the third voltage potential φ 3 , which e.g. could be the operating supply voltage. In this case, the signalling is effected through S 2  and S 3 . The high voltage level can then be derived as the potential difference from φ 3  and φ 2 , “low” —as usual—by means of the short-circuit of L 1  and L 2  via S 2 . The only reason why the state of S 1  is preferably permanently open is provided by power dissipation considerations, as otherwise a short-circuit would arise from φ 3  to φ 1 . The signalling shown for this case can obviously also be applied to a short-circuit from L 1  to φ 1 . 
     The signalling for a short-circuit from L 2  to the third voltage potential φ 3  is shown as operating mode B 3 . This signalling is performed via S 1  and S 2 , with “high” being the potential difference between φ 1  (S 1  closed) and φ 3 , and with “low” being effected by a short-circuit of L 1  and L 2  via S 2 . In order to avoid excessive power dissipation caused by a short-circuit at S 3 , S 3  will preferably be in a permanent open state. 
     The signalling for a short-circuit from L 2  to p 2  is shown as operating mode B 4 . Signalling is again effected via S 1  and S 2 , but in this case “high” —as for the standard operating mode—is derived as the potential difference between φ 1  (S 1  closed) and φ 2 , but L 2  remains fixed on the second voltage potential φ 2 . “Low” is again generated by means of a short-circuit of L 1  and L 2  via S 2 . The state of S 3  is not crucial in this operating mode B 4 , e.g. set to “open” in this embodiment example. 
     FIG. 3 shows a selection decision table for the applied signalling process based on the measured operating modes or conditions. The example shown here was based on a measurement of the voltage potential of lines L 1  and L 2 . Those conditions that cannot be rectified or corrected by means of the signalling output stage alone are crossed out; i.e. the double faults and the following two operating modes, namely the short-circuit of 
     L 1  to φ 2  as well as that of L 2  to φ 1 , that is, the respective opposite voltage level with reference to the applied external switching equipment (S 1 , S 3 ). The currents in the individual current testing devices I 1  to I 3  for switching equipments S 1  to S 3  can be clearly assigned to the respective operating modes, e.g. in the case of an excessive current in I 3 , L 2  is set to φ 1  or φ 3 ; if, in addition, the current in I 2  is also excessive, L 1  will be set to one of these voltage potentials, and if, in spite of the fact that S 3  and S 1  are closed, there is no current flow in I 3 , but rather in I 1 , there is a short-circuit to φ 2  in one of the lines L 1  or L 2 . This can also be distinguished by closing only S 1 . If even now the current in I 1  is excessive, L 1  is set to φ 2 . The floating condition will always be detected by the potential difference between L 1  and L 2 , which in this case always equals zero for the closed switching equipment S 2 . 
     FIG. 4 now shows a further development of the invention by using modules with longitudinal switches for isolating a short-circuit on the line. In this way, as will be explained below, it will nonetheless be possible to effect a signalling even for those operating modes which so far could not be handled by the signalling output stage alone, if a bus system is used wherein at least one short circuit testing device  4  is provided for the two lines L 1 , L 2  in respect of all modules M, and wherein this testing device tests the output of the respective line for an effective short-circuit, namely a resistance value effective there which is too low. For each of the two lines L 1 , L 2  a switching device  5  is respectively provided between the input and the output of a line in a module (Mx, Mx- 1 ), that can be conductive ( 5   a ) or non-conductive ( 5   b ). 
     Using these so-called longitudinal switches  5  with short-circuit testing device  4 , however, it is possible to limit a short-circuit to a line section between two modules (Mx, Mx- 1 ), and the signalling outside this line section can continue unchanged in that the switching devices  5   a  (closed),  5   b  (open) only then make a respective connection ( 5   a ) between the input and the output of each of the two lines, once a test has been performed at the respective output by means of the short-circuit testing device(s)  4 , and if this test has proven the absence of a short-circuit. 
     The use of such longitudinal switches further increases the certainty of signal transmission and can be applied advantageously for the signalling output stage, as it permits the respective non-defective line to be used for signal transmission. Within the modules M, an internal ground potential is provided for, against which the signalling voltage potentials then build up a potential difference.