Abstract:
A system, apparatus and method for efficient utilization of available band-width on the system&#39;s bus connection. The system includes a scheduler configured to receive a virtual schedule that provides at least one slot for sending a message over the communication bus. A module is configured to send a message over the communication bus.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 13/728,590, filed Dec. 27, 2012, now U.S. Pat. No. 8,700,836, which is a continuation of U.S. application Ser. No. 13/238,225, filed Sep. 21, 2011, now U.S. Pat. No. 8,364,875, which is a continuation of U.S. application Ser. No. 12/717,822, filed Mar. 4, 2010, now U.S. Pat. No. 8,037,226, which is a Continuation of U.S. application Ser. No. 10/539,005, filed Mar. 3, 2006, now U.S. Pat. No. 7,711,880, which is a U.S. national phase application filed under 35 U.S.C. 371 of International Application PCT/SE03/01736 filed Nov. 11, 2003, designating the United States, which claims priority from Sweden Application 0203756, filed on Dec. 17, 2002. 
    
    
     FIELD 
     The present invention relates to a distributed control or monitoring system with an arrangement in a control, monitoring and/or vehicle system that comprises locally distributed module units (nodes) carrying out one or more functions and links connecting one or more units, with the respective module unit being connected to the relevant link via a connection arrangement or a connection point and requisite communication device, preferably for the protocol CAN. 
     BACKGROUND 
     The CAN-system (Control Area Network) is currently well-known in various connections for controlling, processes, monitoring, etc. Among other things, it is used in various vehicle applications in order to control various processes in the vehicle. In addition, it is used for function control in machinery, for example within the construction industry, in weaving sheds, etc. In connection with this, reference can be made to the patent literature and the patent applications and patents that have been submitted and obtained by the Applicant of the present patent application. 
     In connection with control and monitoring systems of this type, there is an ongoing need to make the system as such more efficient, to improve it and to make it cheaper. A bottleneck for system performance is the bandwidth of the bus. To date, most systems based on CAN have been constructed on the basis that messages are sent event-driven. This means that the bus is poorly utilized and that the time for message transfer varies greatly. This fact has resulted in CAN generally being considered to be unsuitable for safety-critical systems. Such systems should be based on time-controlled scheduling of message transfers between the nodes in the system. Special protocols for this have been developed, for example TTP and Flexray. A new standard for CAN with support for time scheduling is in preparation by ISO (“Controller area network (CAN) B Part 4: Time-triggered communication” ISO/CD 11898-4) which proposes how CAN messages can be time scheduled and how clocks for this can be arranged. An overview and discussion of the subject is published in the publication IEEE Micro July-August 2002. TTP can be considered best to correspond to the current technology concerning distributed systems with high real-time and security requirements. This is a modern, advanced and well-documented protocol for time-controlled systems that are used in vessels. For detailed information, refer for example to H. Kopetz, “TTP/C Protocol”, TTTech 1999. A study of TTP/C shows that it makes high demands to the effect that the clocks in the respective modules must operate within a high given tolerance and that special circuits must be used, so-called Bus Guardians, in order to ensure that no module transmits messages that collide with other messages on the bus. In general, serial protocols are divided into three classifications: Collision Avoidance (CA), Collision Detection (CD) and Collision Resolution (CR). Definitions of the classifications and the boundaries between them can vary. TTP is a typical CA protocol, that is all measures and steps are taken in order to avoid collisions. This is a generally accepted starting point for traditional time-controlled systems. The Ethernet is a good example of a CD protocol. Collisions normally arise on the bus and are detected. Colliding messages are discarded and the transmitters send the messages again after a delay. The protocol has rules concerning this delay so that at least the modules that collided do not immediately collide again. The collisions thus give rise to reduced bandwidth and mean that the maximum delay for a given message can not be calculated and guaranteed. CAN is an example of a CR protocol. Collisions are resolved in a predictable way. The price of this characteristic is a low maximum bandwidth and this is one of the reasons that CAN has been considered not to be a suitable protocol for time-controlled systems. In addition, the collision-resolving characteristic has been considered by most experts not to be of value for time-controlled systems, as collisions are not supposed to occur. 
     Traditionally, time-scheduled transmission of messages is considered to be more efficient than event-driven transmission. However, the time slot that is allocated to a message must be longer than the message in order to take account of the difference between the modules&#39; clocks.  FIG. 1  shows schematically a traditional schedule with time slots in which the modules A and B place their messages. As their clocks are not precisely synchronized with the system clock, the messages are placed in practice slightly earlier or later in the allocated time slot than what is specified by the scheduling. This means that in traditional time-controlled systems the allocated time slot must be large in order for inaccurate clocks to be able to be used in the modules, which results in loss of bandwidth, or means alternatively that very accurate clocks must be used, which results in high costs. The present invention aims to solve this problem, among others. In addition, immediate retransmission of discarded messages is not permitted in traditional time-scheduled systems as this would require longer time slots. Extract from ISO 11898-4: “Arbitrating time window conflicts are resolved by the identifier arbitration of CAN and a CAN node may not start transmission if the bus is not idle. Several CAN nodes in the network may start a transmission within the Tx_Enable window of an arbitrating time window. The immediate automatic retransmission is disabled”. The quotation shows that it is a widespread belief that automatic retransmission should not occur at all in CAN-based time-controlled systems and that a node is not to request start of message transmission of the bus if the bus is not expected to be free. The present invention aims, among other things, to allow immediate retransmission of discarded messages. A characteristic of the invention is that a CAN node starts transmission when the bus is engaged (not idle). 
     Time-scheduled communication requires each node to keep time within a given accuracy in relation to the system clock and requires each message to be transmitted/received in a given time slot for the respective message. This leads to the communication being sensitive to interference and to additional bandwidth having to be kept in reserve in order that important messages will be certain to get through in time. A further development of the basic idea of the invention avoids this need and makes possible the construction of a very robust communication system with predictable characteristics. A communication system can be constructed by means of the invention, that has the respective advantages of event-driven and time-controlled communication while at the same time eliminating their respective disadvantages. A robust communication system with great adaptability to the requirements of the total system and with “graceful degradation” characteristics can be constructed by means of the invention. 
     In accordance with the concept of the invention, the problems described above are solved by utilizing the known characteristics of a CAN message and by the introduction of new concepts such as virtual and actual schedules and virtual system clock. The invention is applicable to other protocols than CAN that have the utilized CAN characteristics. 
     SUMMARY 
     The principal characteristics of an arrangement according to the invention are apparent from the characterizing part of the following claim  1 . 
     In the simplest form of the invention, the protocol utilizes CAN&#39;S characteristic that, in the event of a collision, the message that has commenced transmission is not terminated, but the colliding message waits until the bus is again free and then immediately commences the transmission. A practically 100% utilization of the bus bandwidth is obtained with the following method: 
     1. A virtual time schedule is set up where each message, at least those that are to be found under normal operating conditions, is allocated a time in accordance with a virtual clock at which the transmission of the message is to commence in ideal conditions. The time slot for the start of transmission for the next message can be set to a value that must be larger than the average length of the message, measured in time. 
     2. Each module is allocated an actual schedule, related to an actual clock in the module, for the transmission of messages. The time for transmission is set earlier than the time allocated in the virtual schedule. 
     3. The actual clocks in the modules are set in relation to the virtual clock. 
     4. The transmissions commence. 
     The method above shows a basic idea of the invention. First, there is a scheduling in relation to an ideal, virtual clock, where each message is given a time slot in which it can be transmitted without colliding with another message. By this means, it is ensured that the bandwidth is sufficient for transmitting the required information. The next problem is to make this scheduling work in practice. There are many ways to set up a global time in a system. In traditional time-synchronized systems, it is necessary that all modules in the system have a common time base related to a common clock, at least at each given moment in time. An unorthodox solution is described in Chapter 6.2 in CanKingdom version 3.01, where a message triggers transmission of another message and it is thereby possible to achieve a form of time-scheduled transmission without a common time base. Variants of these methods are discussed in the article “CAN for Critical Embedded Automotive Networks” in IEEE Micro July-August 2002 on page 33 under Time schedules. In a system constructed according to the invention, the different nodes can base their time on different references and can be synchronized in various ways. The only thing that a system designer must ensure is that in one way or another each node is set in relation to the time of transmission and reception of messages within a given tolerance in relation to the virtual clock and that part of the virtual schedule that relates to the respective node. A common factor relating to solutions in accordance with traditional time-scheduled communication is that transmission attempts are commenced when the bus is free. This means that there are gaps in the communication. By means of instead deliberately starting the transmission in the previous time slot, the collision-detecting mechanism will ensure that the message is sent as soon as this is possible, and in this way the highest possible bandwidth utilization is obtained. This idea deviates radically from current thinking, and requires clarification. In current thinking, it is taken as fundamental that absolute time constitutes the basis for the length and position of the time slots, not the time a message takes up on the bus. The schedule is based on the common, ideal system time and all practical deviations from this must be taken into account in the schedule by means of the messages being allocated larger time slots than the time they take up for transmission. Drifting of the respective local clocks means that the deviation between them increases with time. By instead looking at the actual time that a message takes up and letting the schedule vary in relation to the common, ideal system time, the messages can be packed closer together in the scheduling. The time that the respective messages take up on the bus varies very little between the different modules. This new way of thinking also means that the actual schedule will oscillate depending upon how long “pre-ignition” is set, the length of the messages, the deviation of the local clocks from the virtual ideal time and the presence of empty time slots. Maximum tolerance is achieved if the nominal time for transmission is set to half the previous time slot according to the virtual schedule and if the maximum deviation between the virtual clock and the clocks of the transmitting and receiving modules is less than half the time slot for the message. A receiving module then interprets a message that is started within the previous time slot according to the virtual schedule and is ended within the current time slot as belonging to the current time slot. In this way, by means of setting the tolerance for the actual schedule instead of the tolerance for the appearance of the message within the schedule, a more efficient utilization of the available bandwidth is achieved, while retaining the advantage of time-scheduled systems that the identity of the message can be determined by where it appears in the schedule. If the CAN protocol is used, the “pre-ignition” must be reduced by at least one bit time in order to ensure that the arbitration function does not come into effect in the event of a collision. 
     By providing each message with a unique identity, a redundancy in the communication is obtained that can be utilized in various ways. If the communication is designed in such a way that messages are normally to be found in given time slots, then it is easily detected if such is not the case. This is described in the article “CAN for Critical Embedded Automotive Networks” in IEEE Micro July-August 2002. Another way of utilizing this solution is to deliberately let messages try to be transmitted in time slots on each side of the allocated time slot. This can be carried out by allowing there to be a greater deviation from the virtual clock than a half time slot. Messages will then change places, but as they have an identity, this does not matter. It is true that the receiver has to be prepared to receive several messages and sort out the correct one, but the number of possibilities is limited in advance and the requisite bandwidth is ensured in advance. 
     Many protocols, including CAN, utilize so-called “bit stuffing” in order to synchronize the bit flow. In the case of CAN, the number of stuffing bits can vary from zero up to 24% of the number of bits in the original message. In a traditional time-controlled system, a time slot would be required that was 24% longer than the nominal length of the message. With the solution proposed in the invention, where the time slots are allowed to vary in length, the margin can be reduced to 12%. Alternatively, the time slots can be determined by the nominal message length and empty recovery slots can be inserted into the schedule in order to compensate for the stuffing bits. 
     There is a prevalent belief among experts in the field concerning time-scheduled systems that retransmission of discarded messages cannot take place. In the original CAN protocol, it is laid down that a discarded message is to be retransmitted automatically. In later versions of the standard, it is stated that this function need not be implemented and the reason for this is that there is a desire also to use CAN in time-controlled systems. The traditional solution to the problem is to increase the updating frequency by two or three times in order to compensate for lost messages. This leads to a very poor utilization of the bandwidth. In connection with CAN, it has been proposed that a time slot that is twice or three times as long should be allocated to the message and that one or two retransmissions should be permitted. The second and third time slot are allocated to messages with lower priority. This provides a considerably better utilization of the bandwidth, but has the disadvantage that one or two messages can be lost. By letting the actual time schedule vary, by utilizing non-discarding collision detection with immediate transmission after the cessation of the collision and error signalling and by utilizing a unique identity for each message as described above, now also utilizing the CAN characteristic that each identity is associated with a unique priority and that a discarded message is immediately retransmitted depending on its priority, it is possible elegantly and efficiently to solve the problem of achieving more efficient utilization of the bandwidth. By allowing the automatic retransmission and co-ordinating the allocated time slot with the message&#39;s priority, the characteristics of the communication can be tailored according to the requirements of the total system. If the previous message has a higher priority than the next one, the previous message will go out onto the bus immediately in the event of retransmission. Lost messages will conflict with subsequent messages in the same way. If all the following messages have lower priority, a retransmission will result in the following messages being displaced by one time slot. If the following message has higher priority than the discarded message, then the discarded message will not go out until there is a message with lower priority or until the bus becomes free. In normal circumstances, discarded messages seldom occur. It is considered in general that if there is more than one discarded message in 1000, then that is an indication of a system problem. With the proposed invention, it is therefore possible in practice to obtain practically 100% bus utilization with a short and easily predictable waiting time for each message. 
     By designing the system so that it allows messages to change position in the virtual schedule within given limits, it can be permitted that alarm messages are transmitted not according to schedule, even if practically all the available bandwidth is utilized for the normal communication. 
     In traditional time-controlled systems, great importance is placed upon there being a physical reference clock (master clock) in the system and upon all clocks being set in relation to this. See, for example, the standard ISO 11898-4, referred to above. With the proposed invention, this problem can be simplified considerably. The virtual clock does not exist physically, but the local physical clock in the respective node can be set in relation to the virtual clock in many ways. In its simplest form, this is carried out simply by utilizing the ever-present internal clock in the CAN Controller which is used for bit coding according to the Non-Return-to-Zero concept. If, for example, it is determined that the virtual clock is zero when message A 1  in module A is transmitted, then this can be used as reference for module B by instructing this to transmit its message B 1  when the message A 1  has been received. B 1  can be used as a reference for the virtual clock for module C, and so on. In this way, modules without special clock-function can be integrated into the system. In this way, a module D with a primitive clock can also be integrated into the system by instructing it to transmit message D 1  50 ms after it has received A 1  and to transmit D 2  10 ms after B 2 , etc. It can be seen that, in this way, module schedules can be created simply in modules that are not designed for time-controlled systems and that considerable redundancy can be incorporated. The module A can be related to an actual clock with high accuracy, for example GPS, and in this way the whole system can be kept within small tolerances. It can also be seen that systems can be constructed with a hierarchy of virtual clocks and that complex systems can be constructed which are in principle time-controlled, but in which many, and in the extreme case all, of the modules in the system in reality do not have physical module clocks. The virtual clocks are tools to help the system designer to develop communication according to principles for time-controlled systems and thereby make his system fulfill stringent real-time requirements. The actual schedule is constructed by programming the respective modules to transmit their messages in suitable relation to the virtual schedule. As is shown by the examples, there are many ways of achieving this by utilizing the characteristics of CAN which were originally developed to solve other problems. 
     A detailed description of the invention in accordance with what follows shows how the bus connection is utilized more efficiently and how lower requirements are imposed on the clock function in the respective module, or alternatively how it is completely replaced by utilizing one or more previous messages as triggers for transmission attempts. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A currently proposed embodiment of an arrangement that has the significant characteristics of the invention will be described below, with reference to the attached drawings in which: 
         FIG. 1  shows schematically a CAN system with the nodes A and B and additional nodes C, D and Z and a traditional scheduling of some messages from A and B, 
         FIG. 2  shows time slots in a virtual schedule with associated virtual clock and how the modules insert their messages in accordance with their actual clocks, 
         FIG. 3  shows a traditional system operating with TTP and a corresponding system operating with the invention, 
         FIG. 4  shows a virtual schedule, applicable for all the following examples, an actual schedule in a respective module and resulting actual schedule on the bus for all modules for scheduling according to traditional principles. In addition, it shows resulting actual schedules from the use of the invention without pre-ignition and a comparison of the two methods, 
         FIG. 5  shows corresponding actual schedules with pre-ignition and a discard bus with retransmission of messages in comparison with a non-discard bus according to the traditional solution, 
         FIG. 6  shows corresponding actual schedules for a high-discard bus with retransmission of messages in comparison with a non-discard bus according to the traditional solution, 
         FIG. 7  shows corresponding actual schedules where a message is set in relation to the virtual clock and where other messages are indirectly set in relation to the virtual clock by the respective messages being the trigger for one or more other messages, and 
         FIG. 8  shows the construction of the system in the form of a schedule. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows schematically a network with the modules A and B connected to a few-core serial connection  1 . The network can contain additional connected modules which have been indicated by C, D and Z. Each module can transmit and receive messages. The transmission of messages is time-scheduled. This scheduling is symbolized by  2 , where  3  shows how messages appear on the few-core connection, consecutively in time. The time is divided into time slots and the starting points for the respective slots are indicated by  4 ,  5 ,  6 ,  7  and  8 . According to traditional scheduling, each module is now allocated smaller slots. Module A receives  5 A to  6 A for the message A 1  and  7 A to  8 A for the message A 2 . In the same way, module B receives  4 B to  5 B for B 1  and  6 B to  7 B for B 2 . Now the clocks in A and B are not synchronized, for which reason the messages A 1 , B 1 , A 2  and B 2  in reality come within the nominal slots  4 - 5 - 6 - 7 - 8  but not according to the allocated slot. In the figure, the clock in A runs slower than the clock in B, for which reason the gap between A 1  and B 1  becomes greater than the gap between A 1  and B 2 . During the scheduling, buffer zones  5 B- 5 A,  6 A- 6 B, etc, must be inserted between the messages in order that these will not collide as a result of the relative deviations between the module clocks. In the example described above according to traditional thinking, the actual correct time is taken as the starting point and buffer zones are scheduled in order to compensate for local deviations from the correct time. 
       FIG. 2  now shows the new thinking. The scheduling is carried out against a virtual clock. In the virtual schedule  22 , the messages A 1 , B 1 , C 1 , A 2 , B 2 , C 2 , etc, are now inserted at the required times. In  FIG. 2 , the messages have been tightly packed for the sake of clarity. The times of transmission of the respective messages are transmitted to the respective modules. Locally each module thus receives an actual schedule for its messages: A receives an actual schedule for messages A 1  and A 2 , B for B 1  and B 2  and C for C 1  and C 2 . Each module has an actual clock which is set is relation to the virtual clock,  21 A,  21 B, and  21 C respectively, to which their schedules are related. Now the clock  21 A happens to coincide with the virtual clock and the message A 1  is sent out at the correct time. The clock in B runs fast and attempts to transmit before the transmission of A 1  is completed. The collision detection in CAN means that B 1  is retained automatically until A 1  is finished and is then sent out immediately. In spite of the fact that the clock  21 B is wrong, the message will be sent out correctly according to the virtual schedule. In the same way, the messages A 2  and B 2  will be sent out correctly. The clock  21 C is slow, so that a gap is created between B 2  and C 1 . This would have been able to be prevented if the actual local schedules had been put forward in relation to the virtual schedule, that is if there was deliberate scheduling for collision. In traditional thinking, the schedule is stationary in time while the messages oscillate (jump forward and back) in time. With the new thinking, the whole of the actual schedule will oscillate in time. 
       FIG. 3  shows schematically a system  31  with the protocol TTP/C and an identical schematically drawn-up system  32  according to the invention with the protocol CAN. The TTP system has the modules  31 ′,  31 ″ and  31 ′″ which are connected to the bus. Each module has a usual application with requisite hardware, symbolized by HA, and the communication between these is carried out via the communication units  33 ′,  33 ″ and  33 ′″. These communication units are specially developed for TTP and contain a protocol processor with associated ROM data, symbolized by PP and two Bus Guardians designated BG (only one is illustrated). Each PP and PG has a clock, symbolized by C. The description of TTP is greatly simplified, but the key feature for the comparison between TTP and a system according to the invention is the existence and placing of clocks. TTP is based on the correct time being maintained. Therefore PP and BG each have their own clocks. PP handles the transmission and reception of messages according to a time schedule. BG has a smaller part of this schedule, namely at which times the module is entitled to transmit. Only at these times can PP physically send signals on the bus. At other times BG prevents this. The time schedule is constructed by a system designer outside the system and this is downloaded to the system using a tool, symbolized by the unit V. TTP requires the special communication units which provide a common time in the system and check that messages are transmitted at the correct times and that message collision is avoided. In contrast to this is shown the system  32  according to the invention. Each module has also an application with hardware, H′, H″ and H m . The communication is carried out according to the CAN protocol and standard circuits for this are used in the modules  32 ″ and  32   m . The module  32 ′ is assumed to have a CAN controller specially developed for the system according to the invention, with an integrated clock. The module  32 ″ has a clock that is handled by the application, while  32 ′″ does not have a clock at all. As for TTP, a system designer constructs scheduling for the system, but this is broken down and modified for each module, which receives a local schedule. These local schedules can be completely different in essence, but taken as a whole they form a well co-ordinated schedule for the system. The local schedules are downloaded to the respective modules from a common unit symbolized by VV. This can be temporarily or permanently connected to the system, or alternatively can be incorporated as a module in the system which has other tasks in addition to the scheduling of messages. 
       FIG. 4  shows schematically a virtual schedule represented by  41 . This is created with a virtual clock  42  which generates a continuous timing whereby the time slots T 1 , T 2 , T 3 , T 4 , T 5 , etc, are defined. The required transmission of the messages A 1 , B 1 , C 1 , A 2  and B 2  which are to be sent from the respective modules A, B and C in the system have been inserted in these time slots. The messages do not occupy the whole of the time slots as the virtual time is interpreted by an actual clock in the respective modules. These all deviate from the virtual time. Each module will have an actual schedule for transmission of its messages, which is illustrated by  43 , and these schedules will deviate from each other as a result of the clocks&#39; deviations from the virtual clock.  43  shows that the clock in the module A is fast by the amount dA but runs at the same speed as the virtual clock. The clock in the module B is slow by the amount dB and runs at the same speed as the virtual clock. The clock in the module C is correct at the start but runs slightly too quickly, for which reason the deviation gradually increases which is indicated by dC and dC′. Each module transmits in accordance with its actual schedule and the resulting actual schedule for the system as a whole which appears on the bus is shown by  44 . Here it can be seen that there are slots of varying sizes between messages and thereby a poor utilization of the bandwidth. The messages B 1  and C 1  almost collide and will do so at the next occasion unless their clocks are adjusted relative to the virtual clock before then. If the module clocks keep poor time, they must be synchronized frequently via synchronization messages, which steals bandwidth. This bandwidth loss can be reduced if the modules are provided with more accurate clocks, which makes the module more expensive. 
     In order to avoid the requirement for accurate clocks, the CAN characteristic is utilized to resolve collisions of messages without discarding and to deliberately put forward the transmission in the module schedules. This is shown in  45 . Here the respective messages have been scheduled to be transmitted almost 50% in the previous time slot according to the virtual schedule. The module A starts its transmission even earlier, as its clock is fast. The transmission of message Z is carried out in this time slot and the message A 1  will not go onto the bus until this is completed. In the example shown, this is carried out at the same time as A 1  goes out onto the bus in the example  44 . The module B attempts to send its message almost 50% in the time slot T 1  but, as its clock is slow, the message A 1  has already been sent and a small time slot arises between A 1  and B 1 . In the same way, C sends out C 1  and, on account of the relative effects of the clock deviations, C 1  goes out onto the bus immediately upon the attempt at transmission. A 2  goes out immediately thereafter, while there is a larger slot to B 2 . 
     In both examples, we have used the same virtual schedule as the starting point. With the traditional method, the highest possible bandwidth has been utilized. With the new method, large margins have been created and the messages are sent more quickly. 
     In  FIG. 5 , the same virtual schedule has been utilized as above and is illustrated by  41 ′. The example  51  shows that automatic retransmission can be utilized. It is assumed that the message A 1  in the example  45  has been discarded just before it was terminated and that it is sent again. The correct message goes out immediately after the corrupt message A 1 ′. (This ignores components not directly affected by the invention in the CAN protocol&#39;s error handling such as error frames, etc, for which is only required a time analysis during the scheduling, which is simple for experts in the field and which does not affect the fundamental discussion and the result in the main.) The module B starts its transmission according to schedule with pre-ignition, but as A 1  has already then started, B 1  will go out onto the bus immediately after A 1  irrespective of their relative priorities. In the same way, C 1  goes out immediately after B 1  and thereafter A 2 . A comparison with the traditional, maximally utilized schedule shows that, in spite of the retransmission of A 1 , B 2  goes out earlier by means of the invention. Both better utilization of the bandwidth and shorter maximum delay of messages have been achieved. 
       FIG. 6  shows the same case as in  FIG. 4 , but in which A 1  is retransmitted twice and this results in a temporary 100% bus utilization, but in spite of this B 2  goes out slightly delayed. In this example, it has been assumed that B 1  cannot be delayed, for which reason it has been given a higher priority than A 1 . After two failures of almost complete A 1  messages, B 1  goes out with a small delay, after which the correct A 1  goes out, followed by C 1  and A 2 . 
       FIG. 7  shows how a time-scheduled communication can be set up with modules without clocks. Here the module A is co-ordinated with the virtual clock and with the modules B and C via the messages A 1  and A 2 . In addition, it shows how redundancy is achieved by means of one message triggering several messages. 
     The example  71  shows in principle the same virtual scheduling as above. In this case, the fact has been taken into account that there is a certain delay after a message has been received correctly by a module before this triggers a transmission of a message by the module in question. Here the message A 1  is set in relation to the virtual clock. In the module B, the message B 1  is set to be sent when A 1  is received and in the module C, the message C 1  is set to be sent when B 1  is received. In the module A, A 2  is set to be sent when C 1  is received. As is shown by the figure, the messages go out onto the bus better synchronized than with the use of the traditional method represented by  44 ″. It can easily be seen that the method has a disadvantage in that one omitted message can stop all further transmission. In order to overcome this problem, each message is allocated two or more messages that trigger transmission. This is shown in the example  72 . Here A 1  has been set to trigger both B 1  and C 1 , B 1  to trigger C 1  and A 2 , C 1  to trigger A 2  and B 2 , A 2  to trigger B 2  and C 2 , and B 2  to trigger A 3  and C 3 . Now A 1  fails to trigger B 1  but as it also triggers C 1 , this goes out instead of B 1 . The whole schedule is thereby displaced one step to the left. This does not matter, as each message has a unique identity. 
     The example above shows that, using the invention, not only do we achieve a better utilization of the bus&#39;s bandwidth and the use of simpler clocks, but also that the communication can be constructed to be fault-tolerant with completely predictable maximum delays and that these can be optimized according to the system&#39;s real-time requirements by means of suitable choice of identifiers and priorities, maximum number of permitted retransmissions, choice of accuracy of the clocks in the respective modules, choice of setting in relation to the virtual clock for the respective module and message, etc, and that modules can be given special rules for how the communication is to be carried out. Taken as a whole, these rules give a predictable appearance on the bus for each message. If any module should infringe any given rule, this is easily discovered by one or more system-monitoring modules and the communication does not break down. System-monitoring nodes can always issue correcting command messages, for example according to CanKingdom, by these messages being given higher priority than other messages. Traditional methods such as “bus guardians” used in traditional time-controlled systems can be used without problems, if considered necessary. 
     In certain cases, it cannot be tolerated that messages are sent one or more time slots earlier than scheduled. The case described above where a displacement arises due to a node not sending its message, can be avoided by one or more modules being scheduled to send a dummy message with lower priority. If these find that they do not go out onto the bus during the first half of the time slot as a result of this being occupied, the transmission attempt is discontinued. 
     ISO 11898-4 is a standard for scheduling of messages in a CAN bus. The standard describes two levels for the clock function, Level 1 and Level 2. Level 1 is characterized by the total time schedule being made up of shorter partial schedules which are introduced by time synchronization messages being sent from a time master. In order to ensure that the system does not fail as a result of the time master failing, there is a complex set of rules that enable one or more reserve time masters to take over in a secure way. In addition, there is a complex set of rules in the standard for how the local clocks are to be synchronized with the time master and how the respective module is to be able to determine whether it is synchronized or not. All time measurement in Level 1 is related to the bit rate on the bus. Using the invention described, a system can be constructed with much simpler means for obtaining an identical appearance of messages on the bus, without the modules having special clock functions that are synchronized with special time masters. This can be carried out by the virtual clock being set in relation to one or more messages via a selected event, for example indication of a position of a motor&#39;s flywheel. When one or more modules indicate this position, this triggers the transmission of a special message. If this message is identical for all the modules that send it, it will be understood as being owned by all the modules that send it out within the first bit, Start Of Frame. If the deviation is larger than one bit length, the message will be put in a queue in the losing module. A local time-out terminates the transmission attempt after a time that is shorter than the length of message after the indication. An alternative to this is to put the first message in the schedule after the start message at a higher priority than this. The start messages that are in a queue will then lose out in the arbitration which makes it possible to let the application in the respective module terminate the transmission attempt after the receipt of the start message. Subsequent messages can then be scheduled with the start message as a reference to a local clock which is started upon the receipt of the start message or by using some later message in the schedule as a clock reference or by using one or more previous messages in the schedule which directly trigger the transmission attempt in the way described above. 
     By means of the invention, the problem with the scheduling is changed from following rules laid down that require all the modules to be synchronized with a given time master in the system or with a given set of rules applicable for all the modules for obtaining timing that is common to the system for a planned virtual scheduling which is converted in practice by local rules in each module which are co-ordinated in a design process which is unique for the system. By this means, the characteristics of the system can be optimized both with regard to cost and features, by means of, among other things, eliminating the earlier basic rule that messages must not collide on the bus in time-scheduled systems. In addition, modules without clocks can be integrated into systems based on time-scheduled communication. 
     Many time-controlled communication systems rely on a selected edge in the bit flow on the bus for the synchronization of the local clocks within the system. CAN utilizes falling edges for hard and soft synchronization of the clocks that are used for the synchronization of the time for interpreting the individual bits in the bit flow on the bus. The first edge, SOF, is often proposed to be used for clocks for other purposes, for example for scheduling of messages. However, in order to distinguish this falling edge from a falling edge caused by an interference, SOF is not indicated until at the sampling point. ISO 11898-4 (TTCAN) utilizes this delayed indication of SOF in order to distinguish easily false edges from real ones. A disadvantage of this method is that the distance between the edge and the sampling point can be different in different modules within the system, which leads to an inexact synchronization of the clocks. This can, however, be corrected easily at local level if, for the clocks&#39; synchronization, reference is made to the edge and not to the sampling point, as this local deviation is known in the respective nodes. 
     For the invention, it is, on the other hand, an advantage to utilize the sampling point as a reference. In this way, it is possible easily to integrate modules that are developed for event-driven CAN systems in time-controlled systems. When the message two time slots before is correctly received, the transmission trigger is set to be activated at the next SOF. When SOF in the message in the preceding time slot is detected, then the CAN controller is activated for transmission. As the message has then already passed the arbitration stage, it will be sent immediately after the preceding message has been terminated, irrespective of the message&#39;s priority. In this way, a module without a clock can be set in relation to a virtual clock and a virtual schedule with a high degree of precision. 
     It can easily be seen that if the method above is combined with a suitable choice of priorities, then several messages can have one and the same trigger message and also each message can have several trigger messages and in normal circumstances the messages will go out at the correct time. If any message is not sent, for example due to a module fault, then all the other messages will still go out, displaced, however, by one time slot. 
     An alternative to using the sampling point as a reference is to use the falling edge in the RTR bit after the CAN identifier. This is indicated by all the CAN Controllers for soft synchronization and is thereby just as precise as SOF but has the advantage of being reliably identified. 
       FIG. 8  shows schematically how the construction of a system according to the invention can be carried out. A system designer  80  designs a control system  81  to be used in a product, for example a boat  82 . In the example, the system consists of a boat control unit  83  which communicates, via a bus  84 , with three motors with associated auxiliary devices for course setting, raising and lowering of rigging, logs, etc,  85 ,  85 ′ and  85 ″. The control electronics for the boat control unit  83  are represented by  86  and for the respective motors by  87 ,  87 ′ and  87 ″. The system designer builds up the system in a computer-based tool  88  which can be of a type according to U.S. Pat. No. 7,188,162, issued Mar. 6, 2007, and U.S. Patent Application Publication No. 2005/0086382 A1, published Apr. 21, 2005. In addition to a selection of characteristics that are described in the documents referred to, the tool has also a part 89 for scheduling according to the invention. In a first step, the system designer develops one or more virtual schedules  90  based on one or more virtual clocks  91  for the system as described above. The tool processes the virtual schedule into one or more local actual schedules for the respective modules which are indicated by  92 ,  93 ,  94  and  95 . What is important for the invention is that the schedule is associated with the correct choice of identifier and their associations with each other and their priority in the event of collision on the bus, for which reason this has been marked specially by  92 ′,  93 ′,  94 ′ and  95 ′. The tool supplies additional information i 1  for settings, program codes, etc, for the respective module which is indicated by  96 . The information i 2  thus created and processed is transferred to a configuring tool  97  via the connection  98  which can be selected in a suitable way, for example via a diskette, file transfer in a digital network, etc. In addition to configuring the system, the information can be used for later analysis of the same and the configuring tool can advantageously be combined with an analysis tool which, among other things, can compare the virtual schedule with the actual schedule which appears on the bus during the system&#39;s operation. The configuring tool is connected to the system&#39;s bus via the connection  99  and the connector  99 ′ and requisite information  100  for local schedules, settings, etc, is exchanged between the modules and the tool. The virtual schedule function can also convey direct information i 3  to the configuration tool  97  for requisite processing. 
       101  shows schematically the functions comprised in the respective module. The module is connected to the bus via a connector  101  which can be a contact or a wireless connection, for transferring the bus signals i 4  to and from the module. The signals pass through signal-adapting means  102 , for example a CAN transceiver, which in turn is connected to a protocol circuit  103 , for example a CAN Controller. This is connected to a CPU  104  with requisite peripherals and application software  105  for the module function. In addition, the module has at least software  107  which can set received messages  108  on the bus in relation to messages  109  which are to be sent. The module has an actual clock  110  which is set in relation to at least one edge on message signals on the bus, for example the falling edge  111  at Start of Frame in a CAN message. In its simplest form, the clock  110  consists of the clock incorporated in a CAN Controller for bit synchronization and interpretation. The module can be arranged to handle time-controlled means symbolized by the unit  112  and the outgoing and incoming signal connections  113 . The controls i 5  can be related to the clock  110  and/or a clock  114  with a second time base, for example a time base that varies with the engine speed. The module can also be arranged to handle event-driven or event-controlling means symbolized by the unit  115  and the outgoing and incoming signal connections  116 . Signals i 6  from events or triggering of events are co-ordinated with the time-related signals i 5  and with the actual schedule (the actual schedules) in the module and with the messages&#39; identities according to the arrangement  117  and  118 . Activities in the module trigger transmission of a message  119  on the bus according to the actual schedule which collides with the message  108 . If the collision takes place in Start of Frame then, if it has a higher priority than  108 , the module&#39;s message will appear on the bus instead of the message  108 . The sender of message  108  will receive  119  and then again commence transmission of  108 . If  108  has the highest priority, the module will wait until  108  has been received and then send  119 . If SoF has already been received, then the module waits to send  119  until  108  has been received. The transfer or exchange of information or signals between the virtual schedule and the system tool is symbolized by i 7 . 
     The invention is not limited to the embodiments described above, but can be modified within the framework of the following patent claims and concept of the invention.