Abstract:
A bus system includes at least two data buses, the first data bus having a first number of users and the second data bus having a second number of users. At least two TTCAN buses, which are interlinked by users connected simultaneously to both TTCAN buses, are used as the data buses. Fewer users than the sum of the number of users of the first TTCAN bus and the number of users of the second TTCAN bus are connected to the two TTCAN buses at the same time. A generating arrangement is included to generate a scalable fault tolerance in the bus system as a function of the number of users connected to the at least two TTCAN buses at the same time.

Description:
FILED OF INVENTION 
     The present invention is directed to a bus system. 
     BACKGROUND INFORMATION 
     In recent years there has been an increase in networking of control units, sensors, and actuators with the help of a communication system, i.e., a bus system, in today&#39;s automobile industry and machine construction, in particular in the machine tool sector and in automation. Synergistic effects may be achieved here due to the distribution of functions among a plurality of control units. These are referred to as distributed systems. Communication between different stations is increasingly being accomplished via at least one bus, i.e., at least one bus system. Communication traffic on the bus system, access and reception mechanisms, and error handling are regulated by a protocol. 
     The CAN (controller area network) protocol is used in the automotive sector. This is an event-driven protocol, i.e., protocol activities such as sending a message are initiated by events having their origin outside of the communication system. Unique access to the communication system, i.e., bus system, is triggered via a priority-based bit arbitration. A prerequisite for this is that a priority is assigned to each message. The CAN protocol is very flexible, allowing for adding additional nodes and messages as long as there are still free priorities (message identifiers). The collection of all messages to be sent in the network together with their priorities and sender nodes plus possible reception nodes are stored in a list referred to as the communication matrix. 
     An alternative approach to event-driven spontaneous communication is the purely time-triggered approach. All communication activities on the bus are strictly periodic. Protocol activities such as sending a message are triggered only by the passing of time, which is valid for the entire bus system. Access to the medium is based on the allocation of time ranges in which one sender has the exclusive sending right. The protocol is comparatively inflexible; new nodes may be added only if the corresponding time ranges have been freed up in advance. This circumstance requires that the order of messages be determined even before starting operation. Therefore, a schedule is drawn up which must meet the requirements of the messages with regard to repeat rate, redundancy, deadlines, etc. This is referred to as a bus schedule. The positioning of messages within the transmission periods must be coordinated with the applications which produce the message content to minimize the latency between the application and the transmission time. If this coordination does not take place, the advantage of the time-triggered transmission (minimum latent jitter in sending the message on the bus) would be lost. High demands are made of the planning tools. TTP/C is such a bus system. 
     The requirements outlined above for a time-triggered communication and the requirements for a certain measure of flexibility are met by the method of time-triggered CAN known as TTCAN (time-triggered controller area network), which is described in German Published Patent Application No. 100 00 302, German Published Patent Application No. 100 00 303, German Published Patent Application No. 100 00 304 and German Published Patent Application No. 100 00 305 as well as ISO Standard 11898-4 (currently in the form of a draft). TTCAN meets these requirements by establishing the communication cycle (basic cycle) in exclusive time windows for periodic messages of certain communication users and in arbitrating time windows for spontaneous messages of a plurality of communication users. TTCAN is based essentially on a time-triggered periodic communication, which is cycled by a user or node which gives the utilization time and is known as the time master, with the help of a time reference message, or, for short, reference message. The period until the next reference message is referred to as the basic cycle and is divided into a predefinable number of time windows. A distinction is made between the local times, i.e., local timers of the individual users, i.e., nodes, and the time of the time master as the global time of its timer. Additional principles and definitions based on TTCAN are given in the ISO Draft 11898-4 or the related art cited above and are thus assumed to be available and will not be described further explicitly here. 
     Thus there are various real time bus systems for networking control units in automation, in motor vehicles or elsewhere, including the aforementioned CAN, TTP/C or Byteflight as well as TTCAN, also mentioned above. CAN, TTCAN and Byteflight are single-channel bus systems, which means that redundancy may be achieved by duplication of the corresponding system. TTP/C is an intrinsically two-channel system, i.e., redundancy is always built in. As a service, many bus systems provide a time base synchronized to the bus. In the bus systems originally designed as two-channel or multichannel systems, synchronization may be forced by design, such as by the fact that one node, i.e., user, must transmit on both buses simultaneously. This may have some advantages, (e.g., synchronization is always ensured) but it may also have a number of disadvantages, e.g., the fact that each bus may not be operated independently, the time patterns on the two buses may differ only to a very limited extent and the modularity of the two or more bus systems is lost due to the coupling which is provided by design. 
     As explained, the systems described alone may not yield optimum results in all regards. 
     SUMMARY OF THE INVENTION 
     In the case of buses, i.e., bus systems, designed as single-channel systems, synchronization is performed explicitly if needed. In the following, a TTCAN network and/or a plurality of TTCAN buses, i.e., bus systems, and their coupling are assumed as the bus system, but this is to be understood as restrictive with regard to the object of the exemplary embodiment of the present invention to be explained later only inasmuch as the properties of the TTCAN are the prerequisite, i.e., are necessary, for representing the object according to the exemplary embodiment of the present invention. 
     The exemplary embodiment of the present invention relates to a bus system composed of at least two data buses, the first data bus having a first number of users and the second data bus having a second number of users, at least two TTCAN buses, which are interlinked by users connected simultaneously to both TTCAN buses, advantageously being used as the data buses, and fewer users than the sum of the number of users of the first TTCAN bus and the number of users of the second TTCAN bus are connected to the two TTCAN buses at the same time, an arrangement being included for generating a scalable fault tolerance in the bus system as a function of the number of users connected to the at least two TTCAN buses at the same time. 
     In addition, it may be advantageous that when there are more than two TTCAN buses, two TTCAN buses, as at least one coupled pair of data buses, are always interconnected by at least one user. 
     An arrangement, e.g., the synchronization layer (SL, SLZ), is expediently provided in the bus system by which the at least two TTCAN buses are synchronized. 
     The global time may be synchronized when there is at least one coupled pair of TTCAN buses and/or the cycle time may be synchronized when there is at least one coupled pair of TTCAN buses. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows the coupling of two TTCAN bus systems via a user which functions as a gateway user. 
         FIG. 2  shows the coupling of three TTCAN bus systems by coupled pairs. 
         FIG. 3  shows the coupling of four TTCAN bus systems by various users to illustrate scalable tolerance. 
         FIG. 4  shows a flow chart of frequency adjustment between two TTCAN buses, i.e., TTCAN bus systems. 
         FIG. 5  shows a flow chart to illustrate the phase adjustment of two TTCAN buses, i.e., TTCAN bus systems. 
     
    
    
     DETAILED DESCRIPTION 
     The exemplary embodiment of the present invention concerns how a fault-tolerant bus system, i.e., network, may be created from a combination of a plurality of TTCAN buses. This may be particularly advantageous in conjunction with the mechanisms with regard to synchronization of the global time of a plurality of TTCAN buses and/or synchronization of the cycle time of a plurality of TTCAN buses so that mutual synchronization of all these buses in the total bus system or network may be accomplished. 
       FIG. 1  shows a bus system, i.e., network composed of a plurality of TTCAN buses, i.e., TTCAN bus systems, namely two in the present case. A first bus  103 B 1  and a second bus  103 B 2  are shown. Two users  101  and  102  are connected to the first bus  103 B 1 . A user  105  is connected to second bus  103 B 2 . User  100  is connected to both buses  103 B 1  and  103 B 2  and functions as a connection user or gateway computer, i.e., gateway user or gateway controller which has access to both buses. A coupled pair of TTCAN buses ( 103 B 1  and  103 B 2  here) is thus defined as a combination of two TTCAN buses such that there is at least one gateway user having access to both buses. The connection of the individual users to the particular bus is accomplished via a corresponding interface element, e.g., interface element  110 B 1  in the case of user  101 . Likewise, user  100  is connected as a gateway user to bus  103 B 1  via an interface element  104 B 1  and to bus  103 B 2  via an interface element  104 B 2 . As an alternative, one interface element having two terminals may also be provided for connection to bus  103 B 1  and to bus  103 B 2  in contrast with two interface elements  104 B 1  and  104 B 2 . 
     Timers  111  and  106  having an internal clock source or time source  107  and  112  in particular a quartz crystal or an oscillator, in particular a VCO (voltage controlled oscillator) are also shown in users  100  and  101 , respectively. In addition, time detection modules, in particular counters  108  and  113  are also contained in particular timers  106  and  111 , respectively. 
     Control functions in the particular user in particular for input/output of data to the bus system, for transfer of time information from the timer and for synchronization of the buses, i.e., bus users and other methods and method steps, etc., in particular those according to the present invention may be performed by modules  109  and/or  114  as processing modules, in particular a microcomputer or microprocessor or controller. Parts of this functionality or the entire functionality may also be provided directly in the particular interface module. 
     A user may also be preselected as a time reference generator as defined by TTCAN, in particular for one bus system. This user, the time reference generator as the time master, thus specifies the basic cycle as described in the related art. It is also possible for the gateway user to function as a time reference generator, i.e., as a time master for both bus systems. The time generator of the corresponding reference user, i.e., the time master of the particular TTCAN system by which the local time of this time master is determined is thus considered to be the reference time generator, i.e., it specifies the reference time for the corresponding bus system, i.e.,  103 B 1  and/or  103 B 2 . In other words, the local time generator, e.g.,  106  and/or  111  of the specified reference user as the time master is thus considered a global time generator of the corresponding bus, i.e., bus system  103 B 1  and/or  103 B 2  and it specifies the global time of the corresponding bus. 
       FIG. 1  thus shows a coupled pair of TTCAN buses having a gateway user or gateway node. For more precise representation of this coupling according to the exemplary embodiment of the present invention, the following description according to the exemplary embodiment of the present invention is used. 
     At least two TTCAN buses B 1 , B 2  are coupled when there is a series Pi=(BXi, BYi), where i=1 to n, and there are n elements N having the following properties:
         BXi, BYi are TTCAN buses for all i,   For an i, BXi and BYi form a coupled pair of TTCAN buses,   BX(i+1) is BYi (for i=1 to n−1),   BX 1  is bus B 1  and BYn is bus B 2 .       

     In other words, two TTCAN buses B 1  and B 2  are coupled when they are connected by any path of coupled pairs, regardless of its complexity. A system of at least two TTCAN buses is referred to here as a fault-tolerant TTCAN bus system if two of the buses are coupled (in the sense described above). All system architectures using a fault-tolerant TTCAN bus system or network are thus detectable. 
     Additional examples are shown in  FIGS. 2 and 3 .  FIG. 2  shows three TTCAN buses  203 B 1 ,  203 B 2 , and  203 B 3  as well as bus users  200 ,  201 ,  204 , and  205 . Buses  203 B 1  and  203 B 2  are linked together by user  200 , and buses  203 B 2  and  203 B 3  are also linked together by user  201 . Thus, in the sense according to the present invention, bus systems  203 B 1  and  203 B 3  are connected in a fault-tolerant system according to the definition given above, in particular with respect to synchronization, by the coupling of bus systems  203 B 1  and  203 B 2  as well as  203 B 2  and  203 B 3  as coupled pairs by users  200  and  201 . In contrast with conventional redundant systems, i.e., two buses in the entire bus system, in which each node or user is connected in a redundant manner to each other bus, i.e., each user has a connection to each bus, the system architectures proposed here allow a scalable fault tolerance and a mixture of fault-tolerant and non-fault-tolerant systems due to the use of coupled pairs. 
     This is explained again using the example of  FIG. 3 , which shows four buses  303 B 1 ,  303 B 2 ,  303 B 3 , and  303 B 4 . In addition, bus users  301 ,  302 ,  300 ,  304 , and  305  are also shown. Buses  303 B 1  and  303 B 2  are linked together by user  300 , and buses  303 B 3  and  303 B 4  are linked by user  301 . At the same time, three buses  303 B 1 ,  303 B 2 , and  303 B 3  are linked by a user  302 , so that a mixture of fault-tolerant and non-fault-tolerant systems is made possible, while the desired fault tolerance in the system is scalable, i.e., it may be represented in various degrees of redundancy (single, dual, multiple redundancy). It is thus possible to introduce higher degrees of redundancy into a system without thereby coupling systems which should remain separated, which also permits a reduction of common mode errors, i.e., common-mode faults of the bus system(s). 
     In combination with the synchronization mechanisms in TTCAN, a uniform synchronized communication system may be created that permits all conceivable degrees of fault tolerance in the simplest manner. Synchronization is described in greater detail below with respect to the global time as well as the cycle times of individual users or bus systems. 
     A method of how two or more TTCAN buses, in particular level  2  buses (see ISO Draft) are able to synchronize their global times with one another will be described first. This method may be used by dedicated hardware, by the application running on the corresponding host, or by a special software layer. 
     The method of synchronization including possible variations in the sequence is described below. A pair of buses is referred to here as being directly synchronizable if there is at least one gateway computer having access to both buses, as described above. A general prerequisite is also that there is a chain of gateways between two buses to be synchronized, connecting the two buses via directly synchronizable pairs, i.e., the coupled pairs mentioned above. The synchronization layer (hardware or software) which performs the synchronization is referred to below as synchronization layer SL. In doing so, the SL need not necessarily be present on each node, as also indicated by the following description. 
     In one exemplary embodiment, the gateway user or gateway computer is the time master in at least one of the two buses to be synchronized for adjusting the time in the bus in which it is time master. Optionally, as another exemplary embodiment, a message may be transmitted from the gateway user to the time master to perform the corresponding time adjustment for the synchronization. Then it is not necessary for the gateway user to also be time master of at least one of the bus systems to be synchronized. 
     For initialization, the initialization procedure specified for this bus according to ISO 11898-4 and the state of the art for the TTCAN runs on each TTCAN bus. As a result this yields two or more TTCAN buses running independently of one another and having different global times and different time masters at a given time. In an optional variant, the system design may ensure that the same node, i.e., user, becomes the time master on both buses or on a plurality of buses. 
     Then on the basis of  FIGS. 4 and 5 , frequency adjustment and phase adjustment are described, whereby the frequency and the phase of the buses are adjustable independently of one another; in an exemplary embodiment, first the frequency is adjusted because as long as the frequency is wrong, i.e., deviates, the phase will be changed continuously. 
     A) Frequency Adjustment Between Two Buses 
     In the TTCAN the rate of the global time, i.e., the length of time unit NTU, is determined by the timer frequency of the time master, i.e., in particular the oscillator frequency or quartz crystal frequency and its TUR (time unit ratio) value. NTU (network time unit) refers to the time units of the global time of the particular bus, and TUR refers to the ratio between the length of an NTU and the length of a specific basic time unit, e.g., the local timer period, i.e., in particular the local oscillator period as described in ISO Draft 11898-4. Synchronization layer SL must ensure that the NTUs on the various buses have the intended ratios to one another. The SL may be implemented in hardware or software by processing units  109  and/or  114 , etc., but the SL need not necessarily be inherent in all users, as mentioned already. 
     Essentially different procedures may be used. One bus may be coordinated with the others by selecting the frequency of one bus, i.e., rate of the global time of the one bus as a setpoint and determining the difference in rate, i.e., frequency, of the other bus, the at least one additional bus being coordinated with the others or the at least two buses being close to one another with respect to the rate of the global time, i.e., in particular the frequency of the timer of the particular time masters. In addition, the adjustment may be performed in one step or gradually. The particular strategy depends on the SL, i.e., the synchronization layer, and the requirements of the particular applications. All methods have in common the following system:
         The SL determines the correction to be performed for a bus and notifies the time master at that time. The correction may be, for example, the ratio of the instantaneous TUR value and the new TUR value to be set. It may be particularly advantageous if the SL determines the correction on the time master so that the correction value may be the new TUR value directly, for example.   The SL determines the new TUR value in the time master and uses it after the following cycle.   All the other nodes of this bus then follow the time master via TTCAN synchronization.       

       FIG. 4  shows in block  400  the global time of the first bus, e.g., B 1  being determined, e.g.,  103 B 1 . This is determined at a point in time T 1  in block  401 , i.e., captured, so that a first capture value of the global time of bus B 1  is obtained in block  401 . Likewise at time T 1  a value of the global time of bus B 2  determined in block  404 , e.g.,  103 B 2  is detected, i.e., captured in block  405 . At the next point in time T 2  the particular first capture value is advanced from block  401  or block  405  to block  402  or block  406  and a new second capture value is detected with respect to the global time of the particular bus in block  401  or  405 . From these two capture values in block  401  and block  402  and/or block  405  and block  406 , the particular rate of the global time or the clock rate of particular bus B 1  and/or B 2  is determined by forming the difference in block  403  and/or block  407 . Then by forming the difference in block  409  a correction value is obtained from these values for the rate of the global time of the particular bus, this correction value representing the difference in the rate of the global times of the buses in question, i.e., the difference in the particular clock rate, i.e., the timer rate. As mentioned above, this may also be accomplished by way of the particular TUR value, for example. The rest of the procedure according to the present invention then takes place with the help of the value for the frequency adjustment in block  401  determined in block  409 . 
     The frequency adjustment is to be illustrated again using an example. There are two buses B 1  and B 2 . The synchronization strategy is that B 2  must adjust the length of the NTU to the length valid on B 1 . In a simplest case it is assumed that both are nominally the same length. The application (SL) of the present time master of B 2  is assumed to have direct access to the bus B 1  and the same timer, i.e., the same clock source or time source, i.e., the same oscillator or quartz crystal is to be used for B 1  and B 2 . Then the NTUs of the two buses B 1  and B 2  are exactly the same length when the TUR value of this node or user with respect to bus B 1  is equal to the TUR value of the node and/or user with respect to bus B 2 . Thus, the SL must use the TUR value with respect to B 1  as the TUR value with respect to B 2 . 
     In principle, the situation may be handled in the same way if the NTUs of the two buses have any desired ratio to one another. This is then implementable in hardware to advantage if the ratio or its inverse is an integer, namely in an exemplary embodiment a power of two. 
     If the same timer, i.e., time source, is not used for both buses, then the difference in the global times of the two buses may be measured twice or more often, i.e., periodically in succession, and calculating a correction factor from a comparison between the observed change in difference and the nominal change in difference. This may be done if the present time master does not itself have access to both buses. As an alternative, the SL may measure the length of a basic cycle on a bus in units of the other bus and then determine the correction value therefrom. 
     B) Phase Adjustment 
     For the phase adjustment the SL measures the phase difference between two global times on two buses and determines the jump, i.e., correction value to be set for each of the two buses. This may be performed with the help of the stopwatch register of the TTCAN. 
     The SL then notifies the two time masters present on the two buses regarding the particular jump, i.e., correction value to be set. A time master that has set a jump presupposes a predefined bit, specifically in the case of TTCAN the discontinuity bit in the next reference message and shifts its global time by the corresponding amount. Then the time reference message, i.e., the reference message of the corresponding time master is sent at the adjusted point in time. After successfully sending this reference message(s) on at least one of the two buses, if necessary, the at least two buses are synchronized with one another. 
       FIG. 5  shows this phase adjustment. The global times of two buses are determined in block  500  and  504  and detected, i.e., captured, in block  501  and block  505 , respectively. Capturing may occur at the same point in time T 1 . Then the two capture values are sent directly for forming a difference in block  509  where the phase difference, i.e., the difference in the clock phase, the clock source phase or the time source phase may be determined based on the single capture values of the two buses. The remaining method described according to the present invention is then executed in block  510 . 
     In particular when there are more than two buses it is advantageous if in the case of such a paired synchronization of two buses, the jump, i.e., correction takes place on only one of the two buses, i.e., one of the two buses assumes a master role for the global time. In other words, the global time on the first bus remains unchanged while the global time on the second bus jumps, i.e., is changed. In this case more than two buses may be synchronized successively via the paired synchronization with no problem without this synchronization of a pair having a particularly complex influence on that of another pair. 
     This is to be illustrated on the basis of one example. There are five buses B 1 , B 2 , B 3 , B 4 , B 5  in the system. The synchronizable, i.e., coupled pairs are (B 1 , B 2 ), (B 1 , B 3 ), (B 2 , B 4 ), (B 3 , B 5 ). If B 1  is the master for B 2  and B 3 , B 2  is the master for B 4 , and B 3  is the master for B 5 , then the paired synchronization (B 2  and B 3  are synchronized first to B 1 , then B 4  is synchronized to B 2 , and B 5  is synchronized to B 3 ) results in system-wide synchronization within two cycles. 
     In the same system, synchronization could also be performed without the master principle. Then, however, the SL must ensure that once buses are synchronized they will remain synchronized, i.e., a jump, i.e., a change on one bus will also take place on all buses synchronized with it. 
     In addition, it may be advantageous (but not necessary) for the time master of a bus to be synchronized to also have direct access to the global time of the corresponding partner bus. In this case, the SL may be set up only on the potential time masters of one bus, i.e., the message of the SL regarding the size of the jump to be set is omitted or becomes very simple. 
     It may not be necessary for the NTUs, i.e., the time units of the global time on the buses to be synchronized to be the same. However, it may be particularly simple and useful if the (nominal) NTUs between two coupled buses differ only by an integral factor (powers of two in a particularly advantageous manner). 
     C) Phase Adjustment by Frequency Shift 
     As an alternative to the mechanism described above with respect to the phase adjustment in point B), a phase adjustment may be achieved through a long-term change in rate (see point A) in this regard). The procedure in principle is exactly the same as that described in point A) for the frequency adjustment between two buses. In this case, however, the goal is not to adjust the NTU of the bus to be adjusted exactly to the target value there, but instead to lengthen or shorten this NTU slightly so that the clock, i.e., timer, to be adjusted is incremented somewhat more slowly or more rapidly, and thus a phase adjustment is achieved over a longer period of time. 
     D) Preserving the Synchronized State 
     After a phase adjustment as in B) or C) there are various ways for maintaining the synchronized state—first by repeating the phase adjustment as soon as the observed phase exceeds a certain value and second by adjusting the frequency as described in point A). In addition, there is a combination of these two possibilities. In the method described in point A), the frequency adjustment may take place one cycle later than a corresponding change in frequency in the master bus, so even in the case of a very precise matching of the frequencies of the buses involved there may result an accumulation of a difference. In this case a minor phase adjustment or compensation by a frequency which does not correspond exactly in a targeted manner, in particular a predefined manner must be performed occasionally. 
     Use of the corresponding TTCAN interface may be demonstrated with a debugging instrument. The performance on the bus may also be analyzed by bus monitoring. 
     A method of synchronization is described below, showing how two or more TTCAN buses may synchronize their cycle times mutually. Here again, this method may be performed by dedicated hardware, by the application running on the corresponding hosts or by a special software layer. 
     This method including various variants in the sequence is described below; the same prerequisites and definitions apply as was the case in synchronization of the global time, i.e., direct synchronizability when there is at least one gateway computer having access to both buses, and there is a chain of gateways between two buses to be synchronized, connecting the two buses via directly synchronizable coupled pairs. Here again the synchronization is performed by a synchronization layer in hardware or software which performs the synchronization and is referred to below as synchronization layer SLZ with respect to the cycle times. Here again, the SLZ need not necessarily be present at each node. First, the frequency adjustment between two buses will again be discussed here in point AZ). 
     AZ) Frequency Adjustment Between Two Buses 
     A frequency adjustment is for use in TTCAN level  2  via protocol mechanisms. However, it is sufficient if the time master is operated in level  2  operation. For other nodes this is not necessary. The adjustment proceeds there like the corresponding adjustment for the global time as described above and thus cannot be made independent of this adjustment if the global time is also synchronized in the same network. 
     In TTCAN level  2 , the rate of the cycle time, i.e., the length of unit of time NTUZ is determined by the frequency of the timer, in particular the oscillator of the time master and its TUR value, as already described above for the global time. The SLZ must ensure that the NTUZs have the specified ratios to one another on the different buses. The NTUZs may be the same as or different from the NTUs mentioned above. 
     In principle, different procedures may be used. Either one bus may be adjusted to the others or two or more are mutually approximated. In addition, the adjustment may in turn take place in one step or gradually. The particular strategy depends on the SLZ and the requirements of the application. In particular the synchronization of the global time and synchronization of the cycle time may be performed by the same software layer, i.e., the same synchronization step SL, which may be expressed as follows in an advantageous embodiment:
 
SL=SLZ.
 
     All the methods with regard to the frequency adjustment within the scope of the cycle time have in common the following scheme:
         The SLZ determines the correction to be performed for a bus and informs the instantaneous time master thereof. The correction may be, for example, the ratio of the TUR value currently in effect and the new TUR value to be set. It is in turn particularly advantageous if the SLZ determines the correction on the time master. Then the correction value may be, for example, the new TUR value.   The SLZ determines the new TUR value in the time master and uses it starting with the following cycle.   All the other nodes of this bus then follow the time master via the TTCAN synchronization.       

     This method may be explained again on the basis of  FIG. 4 , where the sequence is essentially the same except that the cycle time is used instead of the global time. Therefore, reference is made here to the preceding discussion with regard to  FIG. 4  and details will not be described explicitly again. 
     Another Example 
     There are two buses B 1 , B 2 . The synchronization strategy is that B 2  must adjust the length of the NTUZ to the valid length on B 1 . In the simplest case, both are nominally the same length. The application (SLZ) of the current time master of B 2  is assumed to have direct access to bus B 1  and the same oscillator is to be used for B 1  and B 2 . Then the NTUZs of the two buses B 1  and B 2  are exactly equal in length when this TUR value of this node with respect to B 1  is equal to the TUR value of this node with respect to B 2 . The SLZ must thus use the TUR value with respect to B 1  as the TUR value with respect to B 2 . 
     The situation may be handled in principle the same if the NTUZs of the two buses have any ratio to one another. It may then be advantageous in the hardware if the ratio or its inverse is an integer, in particular a power to two. If the same timer, in particular oscillator, is not used for both buses, there is again the aforementioned possibility of measuring the difference in the global times of the two buses at least twice and/or periodically in succession and calculating a correction factor from the comparison between the observed change in the difference and the nominal change in the difference. This is also given again if the current time master does not have access to both buses. As an alternative, the SLZ may measure the length of a basic cycle from one bus in units of the other bus and then determine the correction value from this. 
     BZ) Phase Adjustment 
     
         
         
           
             The SLZ again measures the phase difference between two times on two buses and determines for each of the two buses the jump, i.e., change, to be set. The SLZ then notifies the two instantaneous time masters of the two buses regarding the particular jump to be set. 
             A time master that must set a jump in turn sets a predefined bit, here in particular the Next_is_Gap bit of the TTCAN (see ISO Draft) in the reference message. From the local SLZ it receives the starting point in time of the next basic cycle. 
             After successfully sending this reference message(s) on one bus or both buses, the cycle time phases of the two buses are synchronized. 
           
         
       
    
     This is again represented in  FIG. 5 , as described above; instead of the global time, the cycle time is used and the Next_is_Gap bit is used. This desired phase may be 0 here, i.e., the basic cycle begins at the same time on both buses. However this is not necessary. It is not necessary for the NTUZs, i.e., the units of time of the cycle time, to be the same on the buses to be synchronized. However it may be particularly simple and useful if the (nominal) NTUZs between two coupled buses differ only by an integral factor, in particular powers of two in an advantageous manner. 
     It may also not be necessary for the cycle lengths of the buses to be synchronized to be the same. However the method proposed here may be beneficial when the (nominal) cycle lengths of the two buses are in a rational ratio to one another which does not use excessively large natural numbers because then it is regularly possible to speak of a fixed phase in an easily reproducible manner. Example: Two (nominal) cycles are running on one bus B 1 , while three are running on another bus B 2 . Then every other cycle on B 1  (three cycles on B 2 ) there is a theoretically fixed phase relation between B 1  and B 2 . 
     In particular when there are more than two buses it is advantageous here if in the case of such a paired synchronization of two buses, the jump takes place only on one of the two buses, i.e., one of the two buses assumes a master role for the phase of the cycle time. In other words, the sequence of basic cycles on the first bus remains unchanged while a gap is inserted between two basic cycles on the second bus, and this gap is just large enough to adjust the desired phase. In this case, more than two buses may be synchronized successively by paired synchronization without any problem and without the synchronization of one pair having any influence, in particular, any complex influence on that of another. 
     Example 
     There are five buses B 1  through B 5  in the system. Synchronizable pairs include (B 1 , B 2 ), (B 1 , B 3 ), (B 2 , B 4 ), (B 3 , B 5 ). If B 1  is the master for B 2  and B 3 , B 2  is the master for B 4 , and B 3  is the master for B 5 , then the paired synchronization (B 2  and B 3  are first synchronized with B 1 , then B 4  is synchronized with B 2  and B 5  with B 3 ) will result in system-wide synchronization within two cycles. 
     In the same system, synchronization may also be performed without the master principle. Then, however, the SLZ would have to ensure that once buses have been synchronized they will remain synchronized, i.e., a jump, i.e., a correction on one bus must also take place on all the other buses synchronized with it. 
     In addition, it may be advantageous (but not necessary) if the time master of a bus to be synchronized also has direct access to the corresponding partner bus. In this case the SLZ may be set up only on the potential time masters of a bus. In other words, the message of the SLZ regarding the size of the jump to be set, i.e., the correction to be set is eliminated and/or greatly simplified. 
     CZ) Phase Adjustment by Frequency Shift 
     As an alternative to the mechanism described in section BZ) in this case, a phase adjustment may be achieved through more gradual changing of the rate than in point AZ). The basic procedure is exactly the same as that described in point AZ). However, in this case the goal is not to adjust the NTUZs of the bus to be adjusted precisely to the target value there, but instead to lengthen or shorten these NTUZs slightly, so that the clock to be adjusted is incremented slightly more slowly or more rapidly and thus a phase adjustment is achieved over a longer period of time. 
     DZ) Preserving the Synchronized State 
     After a phase adjustment as in BZ) or CZ), there are various ways to maintain the synchronized state as already described in point D) of synchronization of the global time:
         Repeating the phase adjustment as soon as the observed phase exceeds a certain value.   Adjusting the frequency as in AZ) or A) or a combination of the two possibilities.       

     In the method described in AZ) or A), the frequency adjustment may be performed one cycle later than a corresponding frequency change in the master bus, so even when there is a very precise matching of the frequencies of the buses involved, this may result in an accumulation of a difference. In this case a minor phase adjustment or compensation by a frequency which is known not to match exactly must be performed occasionally. As in synchronization of the global time, this use may be displayed via a debugging instrument, or performance on the bus may be analyzed with bus monitoring.