Abstract:
A device for connecting to a two-wire communications bus, a bus station that, while utilizing the device, is able to send messages, represented on the bus lines as dominant and recessive bus levels, to additional connected bus stations and receive same from them. The device includes (a) an arrangement setting a dominant bus level in the form of a first voltage difference between the two bus lines by driving a first electric current, and the device is suitable for the recessive bus level to set in as the second voltage difference between the two bus lines, at least partially by the flowing of a discharge current via terminating resistors connected to the bus lines; and (b) an arrangement to speed up the setting of at least one of the bus levels by driving at least one additional electric current, at least in response to the presence of a switching condition.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a device and a method for rapid transmission of data between at least two users of a bus system. 
     BACKGROUND 
     From ISO Standard 11898, for example, a Controller Area Network (CAN) as well as a “Time-Triggered CAN” (TTCAN) version of the CAN is known. The media access control method used in the CAN is based on a bit-wise arbitration. In a bit-wise arbitration, a plurality of user stations is simultaneously able to transmit data via the channel of the bus system, without thereby interfering with the data transmission. Furthermore, the user stations are able to ascertain the logical state (0 or 1) of the channel while transmitting a bit over the channel. If a value of the bit sent does not correspond to the ascertained logical state of the channel, the user station terminates the access to the channel. In CAN, the bit-wise arbitration is usually carried out in an arbitration field within a message that is to be transmitted via the channel. 
     Because of the bit-wise arbitration, a non-destructive transmission of the message over the channel is achieved. Because of this, good real time properties of the CAN come about, whereas media access control methods, in which the message sent by a user station is able to be destroyed during transmission via the channel, based on a collision with a further message sent by another station, have a clearly more unfavorable real time behavior since, based on the collision and the new transmission of the message required thereby, a delay in the data transmission comes about. 
     The protocols of the CAN are particularly suitable for transmitting short messages under real time conditions. If larger data blocks are to be transmitted via a CAN domain, the relatively low bit rate of the channel becomes a limiting factor. In order to assure the correct functioning of the bit-wise arbitration, during the arbitration, a minimum duration is must be maintained for the transmission of a bit, so that all the bus users have a uniform picture of the state of the bus (0 or 1) and equal priority access to the state of the bus, the minimum duration being a function of an extension of the bus system, the signal propagation speed on the channel, and intrinsic processing times in the interface modules of the bus users. 
     Therefore, increase of the bit rate would be problematic since this would reduce the duration of the individual bits. Since the signal propagation speed on the channel is essentially fixed, then, in case shorter bit lengths are to be achieved, a smaller extension of the bus system or lower intrinsic processing times are required. With respect to the intrinsic processing times, those times essentially correspond to that which is required until the respective bus level sets in. In this connection, one should note that, to set the dominant bus level, the sending station in each case, using a corresponding output stage, drives a current onto the bus which leads to the buildup of a corresponding voltage difference. In contrast, a recessive bus level sets in by the voltage difference between the two bus lines becoming reduced via terminating resistors or changing to a value deviating from the dominant bus level. This deviating value may be zero but may also be a voltage difference different from zero. 
     G. Cena and A. Valenzano, in “Overclocking of controller area networks” (Electronics Letters, vol. 35, No. 22 (1999), p. 1924) treat, from a theoretical point of view, the effects of overclocking the bus frequency in subsections of the message on the effectively achieved data rate, without, however, going into details of the methodology and the problem of the factors limiting the bus frequency, such as the signal propagation speed and intrinsic processing times in the interface modules of the bus users. 
     It may be seen in the cited documents that the related art does not supply satisfactory results from every point of view. 
     SUMMARY 
     An object of the present invention is to provide a device and/or method by the use of which user stations, such as control units, are able to transmit messages in a short time in a two-wire bus system including dominant and recessive bus levels. 
     The present invention is explained with the aid of a further development of a CAN bus system, in which, because of the device proposed, messages are able to be transmitted in a shorter time than is possible in a CAN network from the related art. For this purpose, a transceiver is proposed that is modified compared to CAN Standard 11896. The present invention is also able to be used in the same way in other bus systems in which one of the bus levels sets in, that is, a voltage difference between lines, by the flowing of a current via terminating resistors. The present invention described may, of course, also be used in TTCAN networks. 
     The object described is attained in the device according to the present invention in that a sending station speeds up the setting of the recessive bus level by driving a suitable, if necessary, additional electric current. 
     In this connection, it may be of advantage to design the device according to the present invention to be switchable, so that the additional driven current strength is used for setting the bus level only in a certain state. In this way, for example, the method according to the present invention is able to be used only in certain sections of a message that is to be transmitted. Furthermore, it is possible thereby to switch over, in running operation, between bus communication according to the current standard and one that is sped up. 
     In the case of a change from dominant to recessive bus level, the device according to the present invention advantageously drives a specified current between the two bus lines, so that the target voltage difference is achieved more rapidly in the recessive case. In this connection, the driven current may be limited by a temporal limitation or by a power limitation or by measuring the voltage difference and switching off upon reaching a certain threshold value. This is particularly important because other stations have to have the possibility of overwriting the recessive bus level by a dominant bit. This case is able to occur both during the arbitration and upon detection of an error, to name just two examples. 
     It is also of advantage for the device according to the present invention, in response to a change from a recessive to a dominant bus level, to modify the driven current compared to the devices known from the related art. It is possible that the respective current is increased, or an additional current is supplied in parallel. A reduction in the current strength, and accordingly a slowing down of this edge change may, however, also be meaningful, for instance, in order to hold the electromagnetic radiation, radiated on the average per time unit, within specified limits. This response, too, may be made a function of a switching condition. 
     In one particularly advantageous form, the device is designed so that the switching condition of the device is ascertained by itself by the analysis of the data signals received. It is also advantageous if the device reports back its current switching state to the actuating controller, through a suitable output. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic block diagram of a transceiver according to the related art, including inputs and outputs to the controller and the bus connections, voltage supply and ground. 
         FIG. 2  shows a schematic block diagram of a device according to an example embodiment of the present invention, including an additional transmitting subcircuit and an additional input to its change-over switching. 
         FIG. 3  shows a schematic block diagram of a device according to an example embodiment of the present invention, including an additional sending subcircuit as well as a device for autonomous switching as a function of the analysis of the signals at input TxD. 
         FIG. 4  shows an exemplary cutout from the circuit diagram of a device according to the related art including the essential elements of the sending subcircuit. 
         FIG. 5  shows an exemplary cutout from the circuit diagram of a device according to an example embodiment of the present invention including elements of the modified sending subcircuit, made up of the original sending subcircuit and additional elements of the sped-up subcircuit. 
         FIG. 6  shows an exemplary cutout from the circuit diagram of an additional example embodiment of a device according to the present invention, including elements of the modified sending subcircuit, made up of the original sending subcircuit and additional elements of the sped-up subcircuit. 
     
    
    
     DETAILED DESCRIPTION 
     Example embodiments are described for the method and device according to the present invention. These specific examples are used to explain, but do not limit the scope of, the invention. The bus system under discussion is assumed to be a two-wire communications bus, with two conducting connections whose potential difference represents the bus signal. One of the two lines may also be represented, for example, by conductive body parts of an automobile, which conductive body parts are connected to one another in a conductive manner. 
       FIG. 1  shows a prior art block diagram of a device  100 . The device includes a circuit including circuit elements which may be subdivided into a sending subcircuit  110  and a receiving subcircuit  120 . The sending subcircuit  110  and the receiving subcircuit  120  may be connected to each other or have common circuit elements. The device also includes the connecting possibilities that are usual for transceivers from the related art, such as terminals CANH, CANL for connecting to communications bus  130 , terminals RxD and TxD for receiving and sending logical data from or to the communications controller, terminal Vcc for providing a supply voltage, and GND for supplying a ground. Further possible terminals according to the related art may include, for example: enable inputs, prompting inputs, standby inputs, etc. These have been omitted here for the sake of simplicity. Sending subcircuit  110  generates output signals for bus terminals CANH and CANL, based on at least the sending signal TxD of the communications controller. Receiving subcircuit  120  generates a receiving signal RxD for the communications controller, based on at least the difference of the input signals of bus terminals CANH and CANL. 
     In contrast, device  200  according to the present invention of the form shown in  FIG. 2  includes an additional terminal ACCL which is used for switching between the response known from the related art and the sped-up response according to the present invention. A signal is read in via terminal ACCL from which the presence of a valid switchover condition is read out by the device. The terminal is connected to an additional speeding-up subcircuit  210 , which is additionally in connection to sending subcircuit  110 . 
     In the simplest case, the evaluation of the ACCL input takes place in the following manner. If a logical TRUE condition is present on the ACCL terminal, speeding-up subcircuit  210  generates output signals based on the signals received by sending subcircuit  110 . If a logical FALSE condition is present on the ACCL terminal, speeding-up subcircuit  210  is inactive. One may, however, also specify another optional signal or a sequence of signals, which have to be present at the ACCL terminal so that the device of the present invention derives a valid switchover condition. It is possible to design this signal to be configurable, for instance, by storing a value in a register provided for this, which the device is intended to interpret as the presence of a valid switching condition. 
     Speeding-up subcircuit  210  is provided access to bus terminals CANH and CANL, so that when a valid switching condition is present, the output signals generated by sending subcircuit  110  are modified by the output signals generated by speeding-up subcircuit  210 . 
     The combination of sending subcircuit  110  and speeding-up subcircuit  210  may be regarded as a modified sending subcircuit  220 , which is indicated in  FIG. 2  by a dashed line. Example embodiments of this sending subcircuit  220  are given in  FIGS. 5 to 6 . 
     A further example embodiment of a device  300  according to the present invention is shown in  FIG. 3 . In this case, the device is designed without switching input ACCL of  FIG. 2 . Instead, the device includes an analysis arrangement  310  which, in the case shown, is connected to terminal TxD and speeding-up subcircuit  210 , and is equipped to decide autonomously on switching the device into the sped-up mode according to the present invention. 
     For this purpose, an analysis arrangement  310  evaluates the input signals which reach the device from the communications controller, that is, preferably, as shown in  FIG. 3 , the TxD signal, and generates from this a control signal for speeding-up subcircuit  210 . The evaluation of the data signals received from the communications controller may include, for example, the ascertainment of a bit length in time within the signals received, the evaluation of an identification within the message, or even the ascertainment of a switching point of a bit length in time within the message. For the control signal generated by analysis arrangement  310  there applies analogously what was described for the input signal at the ACCL terminal in connection with  FIG. 2 . In the simplest case, the control signal includes a logical TRUE or FALSE condition. If analysis arrangement  310  generates a logical TRUE as the control signal, speeding-up subcircuit  210  generates output signals based on the signals received by sending subcircuit  110 . If a logical FALSE condition is present, speeding-up subcircuit  210  is inactive. Thus, as a function of the control signal which analysis arrangement  310  generates, the output signals generated by sending subcircuit  110  are modified using the output signals generated by speeding-up subcircuit  210 , or not, as the case may be. As described in  FIG. 2 , for the control signal an evaluation may of course also be required, in order to derive the presence of a switching condition. It may be characterized to be configurable. 
     In addition to or alternatively to basis on the evaluation of sending signal TxD, it is also possible that analysis arrangement  310  obtains its output signal, which activates speeding-up subcircuit  210 , for instance, by evaluating receiving signal RxD and/or the signals present at bus terminals CANH, CANL. For this, it is, of course, necessary that the analysis arrangement  310  be provided access to the signals to be evaluated, by corresponding connections. Here too, the evaluation of the data signals received may include, for example, the ascertainment of a bit length in time within the signals received, the evaluation of an identification within the message, or even the ascertainment of a switching point of a bit length in time within the message. In this example embodiment, the device is able to react, for example, not only at a sending attempt but also upon receipt of sped-up messages with reduced bit length, by switching to the sped-up mode according to the present invention. Transceivers of the related art frequently include a prompting logic which analyzes the signals at bus terminals CANH, CANL, and derives from this a prompting request. Analysis arrangement  310  is advantageously able to be integrated into such a prompting logic, if this is also provided in the device according to the present invention. The promptability of devices according to the invention will be discussed in more detail below. 
     Alternatively, one might do completely without the switching of the device according to the present invention, and the sped-up method according to the present invention could permanently be used. This specific embodiment, which is also not shown in greater detail, comes about from device  200  in  FIG. 2  by leaving out the ACCL terminal or by applying a durable TRUE condition. 
     Device  200  or  300  according to the present invention may be designed so that speeding-up subcircuit  210 , in the case of a change from a dominant to a recessive bus level speeds up the setting of the correspondingly provided voltage difference, which may be equal to, or different from zero. This is particularly advantageous, since this edge change in the devices known from the related art takes place relatively slowly, because the voltage difference provided for the recessive level is set, without the speeding-up subcircuit, only by a discharge current which flows via terminating resistors to the signal line. 
     In one alternative example embodiment, a device according to the present invention may, however, also be designed so that additionally or exclusively during a change from the recessive to the dominant bus level, the setting of the voltage difference is sped up. For this, either an additional speeding-up subcircuit (not shown) may be provided or the behavior of at least one of subcircuits  110  and  120  may be modified to the extent that it drives the current required for setting the dominant bus level more strongly, as a function of the switching over. In this case, an additional switching input ACCL 2  (not shown), compared to device  200 , may be provided for modified sending subcircuit  220  or, compared to device  300 , a separate connection between analysis arrangement  310  and modified sending subcircuit  220 , in order to be able to activate separately the speeding up for the two directions of the edge change. 
       FIG. 4  shows an example of a schematic design of sending subcircuit  110  of a device according to the related art. It is made up essentially of a driver component  400 , whose two outputs each controls the levels of the two bus lines CANH, CANL via a transistor T 1  or T 2 . Sending signal TxD is modified in a suitable manner by driver component  400 , so that when a logical zero is present, the post-connected transistor circuit pulls bus terminal CANH to potential V_CANH and bus terminal CANL to potential V_CANL (dominant bus level), while, when a logical one is present, the post-connected transistor circuit T 1 , T 2  blocks in connection to diodes D 1  and D 2 , and the bus lines, via the terminating resistors (not shown) at the line ends equalize again their potential to a recessive bus level including a voltage difference of approximately zero. In an alternative development from the related art (“low-speed CAN”), in the recessive case, the bus lines are at a voltage difference that significantly deviates from zero. 
     In contrast,  FIG. 5  shows schematically an exemplary embodiment of a modified sending subcircuit  220 , in which original sending subcircuit  110  is supplemented by the additional circuit elements of speeding-up subcircuit  210 . The further circuit elements include at each output of the driver component an inverting circuit element  13  and  14  that is switchable by switching signal ACCL, as well as post-connected, for instance edge triggered pulse generators P 3  and P 4 . Instead of edge triggered pulse generators, other pulse generators known to one skilled in the art may naturally also be used, such as threshold value triggered pulse generators. The output pulse is triggered after an edge change of logical zero to logical one (dominant to recessive) at the input of the pulse generator. The outputs of the pulse generators supply the gate voltage for transistors T 3 , T 4  which connect bus lines CANH, CANL via diodes D 3  and D 4  to potential sources V_CANH and V_CANL. 
     If there is an activation present by switching signal ACCL (or the switching signal coming from analysis arrangement  310 ) at switching elements  13  and  14 , the latter supply an output signal, inverted with respect to the input signal, to pulse generators P 3  and P 4 , which are edge triggered in the present example, and which, by the application of a suitable gate voltage, briefly switch transistors T 3 , T 4  at low resistance. If there is no activation present, the entire circuit is inactive and does not affect the bus level. 
     Thus, the circuit design leads to the fact that, when transistor T 1  is switched at high resistance by the signal of driver component  400 , transistor T 3  briefly becomes low-resisting, and analogously, when transistor T 2  is switched at high resistance, transistor T 4  briefly becomes low-resisting. In this way, speeding-up subcircuit  210  influences the level of bus lines CANH, CANL. In particular, the combination of transistors T 3 , T 4  and diodes D 3 , D 4  is selected so that the line levels are driven exactly in reverse, as by sending subcircuit  110 . 
     By the inverting of the output signal of the driver component in combination with the pulse width limitation, the desired behavior is achieved that at a change from a dominant to a recessive bus state, while the circuit of T 1  and D 1 , and T 2  and D 2 , respectively, blocks, for a short time bus line CANH is connected at low resistance via transistor T 4  and diode D 4  at potential V_CANL and bus line CANL is connected at low resistance at potential V_CANH via transistor T 3  and diode D 3 . At a suitable selection of the pulse duration, the setting of the recessive bus level is sped up by the circuit, without properties, such as, for instance, the possibility of overwriting the recessive bus level by other control units within the scope of arbitration, or even upon detection of an error, being lost. 
     In the reverse case, that is, at the transition from a recessive to a dominant bus level, in the exemplary embodiment shown in  FIG. 5 , the behavior of sending subcircuit  110  essentially does not change by the addition of speeding-up subcircuit  210 . While transistors T 1 , T 2  in this case are switched at low resistance by a suitable gate voltage, the gate voltage for T 3  and T 4  is held by inverters  13 ,  14  and pulse generators P 3 , P 4  at a value which leads to a high-resistance behavior, since the pulse generators are only triggered by an edge change of logical zero to logical one. During a transition from a recessive to a dominant bus level, if a speeding up is also to be achieved, the driver component, for example, may be modified, as will be explained in connection with  FIG. 6 . 
       FIG. 6  shows how the behavior according to the present invention may also be achieved by using a suitably modified driver component  600 . The driver component is activated by the switching signal coming from analysis arrangement  310  to the extent that it converts its behavior to the sped-up behavior according to the present invention. For instance, it provides the inverted outputs of speeding-up subcircuit  210 , which applies the brief gate voltages to transistors T 3 , T 4 , using, for example, the edge triggered or threshold value triggered pulse generators P 3 , P 4 . In the variant shown here, the pulse generators are still shown separately. In one specific embodiment that is not shown, they could also be integrated into the driver component, so that the driver component, directly by providing an appropriate gate voltage, briefly makes transistors T 3  and T 4  low-resistant. Furthermore, of course, deviating from  FIG. 6 , the switchover could also take place because of external switching signal ACCL. In addition, modified driver component  600 , as a function of the switching signal, is able to adjust additionally or exclusively the output voltages which refer to transistors T 1  and T 2 , so that the change from the recessive to the dominant bus level is sped up. As was touched upon before, the latter behavior may also be activated independently of the behavior first described, by a separate switching signal. 
     In the specific embodiments described, it was assumed that the gate voltages present at transistors T 3 , T 4 , which are supplied by pulse generators P 3 , P 4  are suitably brief. This may be achieved by a corresponding presetting or precontrol. It is also possible, however, that a recording arrangement is provided in the device which, for example, record the duration of the presence of the gate voltage, the charge quantity transmitted, the current strength, the electrical power or the energy applied of the current that has flowed from potential sources V_CANH, V_CANL to bus lines CANH, CANL, and that, as a function of the recorded variable(s), the duration of applying the gate voltages to transistors T 3  and T 4  is set, or when a threshold is reached, the voltage is interrupted. Additional specific embodiments may provide that the voltage potentials lying on bus lines CANH, CANL are ascertained, for example, at diodes D 3  and D 4 , and that the pulse width is set depending on these values, or, upon reaching a threshold, the voltage is interrupted. 
     Beyond that, the device according to the present invention, in one example embodiment, may include an additional output, via which the currently present mode (sped up/not sped up) is fed back, for instance, to the communications controller or other switching elements. This feedback may advantageously be used to ensure that the communications controller transmits sped-up data to the device via terminal TxD only when the device has been successfully switched over into the sped-up mode. 
     Furthermore, the device according to the present invention, in one example embodiment, is able to be equipped to be itself activated or prompted by evaluation of the signals presently on bus lines CANH, CANL, as well as to be able to initiate a prompting process at connected switching elements, for example, the communications controller or a microprocessor. Devices such as those described in the present invention, besides the active state and the switched-off state, usually have at least one additional state, the sleep-operating state, in which the current consumption is minimized to the greatest extent. To prompt from the sleep mode, a separate input may be provided on the device, which is connected to a separate prompting line. However, a prompting process may also be initiated via bus lines CANH, CANL. 
     In the sleep mode, the device according to the present invention, in the simplest case, observes the bus state, and a prompting process is carried out if the occurrence of a dominant bus level is determined. Then, on the one hand, the device itself becomes active, and, on the other hand, it is able to activate the connected switching elements, such as the communications controller or the microprocessor, by suitable signals. The device may also be defined in such a way that, in sleep mode, it still maintains the capability of analyzing messages present on the bus, and that it only leaves the sleep mode upon receipt of a specified message or a message with a specified address, or a message with a certain identification, for instance, within the data field. A device designed in two stages is also possible, in which the device first changes to an intermediate state, when a dominant bus level is established, in which it develops the capability of analyzing messages present on the bus, for instance by the start of an oscillator provided for this. Upon detection of a specified message or a message with a specified address, or a message with a certain identification, the device is then fully prompted or activated. Moreover, there is the possibility that additionally or exclusively the activation of the connected switching elements, such as communications controllers or microprocessors, is made dependent on established identifiers, the address or the content of the message received on the bus. In this way, a selective prompting of bus stations is able to be implemented via suitable messages. 
     In the devices according to the present invention, one should note for the selective prompting, if it refers to parts of the message present on the bus, that, even in the sleep mode, the device is to be able to detect the sped-up message sections and to be able to decode them, if necessary.