Abstract:
A method, a network and an apparatus having the network in which a star network is formed by CAN type buses using a repeater where each arm can be isolated from the other arms. CAN buses are connected to one another by means of the repeater that duplicates the signals observable on one bus on all the other buses connected to it. Communications circuits and/or controllers are connected to the repeater Depending on the reception signal received, the repeater organizes operations of sending transmission signals to the communications circuits and the controllers.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of a priority under 35 USC 119(a)-(d) to French Patent Application No. 04 52752 filed Nov. 24, 2004, the entire contents of which is hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     An embodiment of the present invention relates to a method for making a network formed by CAN (Control Area Network) type buses, and in particular, setting up a star network formed by CAN type buses using a repeater. An embodiment of the invention is a network and an apparatus having the network. The invention can be applied but not exclusively in the field of medical systems such as in a radiology apparatus, and in particular, an X-ray apparatus.  
         [0003]     CAN type communications buses or CAN buses correspond to one of the standards used for electronic communications buses. Controllers associated with devices such as motors or actuators are connected to these buses to communicate with each other. These controllers manage signals that the device sends or receives on a bus. These controllers may either play a sender role and send a signal addressed to another controller of the bus, or play a receiver role and receive a signal sent by another controller. In one example, a controller is a microcontroller or a microprocessor, provided with memories, coupled to a CAN controller circuit.  
         [0004]     Under the CAN bus standard, when a controller sends or receives signals on a bus, all the other controllers connected to this bus receive these signals. Furthermore, the signals observable on the bus encode priority numbers in an address zone. Thus, when a first controller sends a first signal associated with a first address and when, at the same time, a second controller sends a second signal associated with a second address, the sending operations are organized. Indeed, if the second address has a higher priority level than the first address, the second controller is authorized to send while the first controller will be authorized to send only after the second controller has finished sending.  
         [0005]     There are known medical systems comprising CAN buses intended for obtaining mutual communication between different controllers connected to these buses and associated with devices.  
         [0006]      FIG. 1 a  shows a known X-ray apparatus  1  with a pedestal  2  to which an intermediate arm  3  is hooked by means of a first motor-driven link  4 . A C-shaped arm  5  is hooked to the arm  3  by means of a second motor-driven link  6 . Arm  5  has an X-ray emitter  7  as well as an X-ray detector  8  located on either side of a means for object support, such as medical table  9 . An object, such as a patient (not shown) reclines on table  9  for the duration of an examination.  
         [0007]     Table  9  has devices by which the patient can be spatially oriented during the examination. A device may, for example, be an operating handle or lever that controls the motor, or it may be the motor itself. Controllers  20 - 22  respectively associated with a lever and with the motors may communicate with each other by means of a CAN type bus  14 . Although the bus  14  is represented here by only a line, bus  14  generally has two connections to provide for the transmission of differential signals.  
         [0008]     Controllers  20 - 22  are located inside hollow metal frames  16 - 18 . In the prior art, the standard for CAN buses lays down the definition of a main bus segment, in this case the bus  14 , to whose ends two resistors are connected. This standard stipulates the connection of the controllers  20 - 22  to this main segment by means of segments  24 - 26  of connections. Segments  24 - 26  have a distance that is smaller, in a given ratio, than the length of the main bus segment  14 . This means that the bus  14  must wind about within these armatures, in order to limit the length of the connection segments  24 - 26 . Such a bus configuration therefore gives rise especially to a waste of connection.  
         [0009]     Furthermore, in such a configuration, a connection problem may impair communications between all the controllers. If there is a break of a bus inside the frames  16 - 18 , on one of the connection segments  24 - 26 , the communication on the entire bus is cut off and the medical system becomes unusable.  
         [0010]     If one of the controllers  20 - 22  is at a distance from the bus  14  greater than a limit distance, this controller can be connected to the bus  14  by means of a connection segment having one end connected to a termination resistor. The resistor prevents the segment from playing an antenna role relative to the bus  14 . However, the resistor is perceived by the bus  14  as a resistor connected to it in parallel. Hence, the greater the number of segments included in the bus  14 , the lower is the total impedance of the bus. Consequently, the controllers have their outputs connected almost in short-circuit and cannot let through enough current to send a signal on the bus  14 .  
         [0011]     There is commercially available CAN bus switches to duplicate signals on different buses. These switches are also known as gateways. However, in these switches or gateways, the signals undergo processing through software used by a microcontroller and they are reproduced after filtering by software on another bus. This software filtering causes a loss of time in the transmission of signals on a bus. Furthermore, the systems necessitate a programming of parameters to define buses on which signals will be duplicated. As a consequence, the signals observable on different buses connected to a switch or gateway may be different from one another.  
       BRIEF DESCRPTION OF THE INVENTION  
       [0012]     An embodiment of the invention is directed to overcoming a constraint dictated by the use of CAN type buses. In an embodiment of the invention, the CAN buses are connected to one another by means of a repeater. The repeater duplicates signals observable on one bus on all the other buses that are connected to it. There is then no longer any question of connection segments and main bus segments, since all the buses connected to the repeater are independent and behave as if they form one and the same bus.  
         [0013]     Thus, a signal sent on one of the buses will be observable on all the other buses connected to the repeater. As a consequence, even if the buses are physically isolated from one another, they are virtually connected to a common bus and exchange signals by means of the repeater.  
         [0014]     Furthermore, in an embodiment of the invention, the resistors connected to the ends of each bus are not seen as resistors parallel-connected by a controller. A large number of buses may therefore be connected to the repeater without any disturbance being caused, by the addition of a new bus, to communications between the other buses.  
         [0015]     Furthermore, in an embodiment of the invention, the signals are processed in real time because the time taken by the repeater to process a signal is short and known. Indeed, the signals are duplicated in the different buses by means of logic elements made in an ASIC or an FPGA, whose switching time is known with precision. The latency time of the system formed by the repeater and all the buses connected to it are therefore always short and known, whereas the latency time of the system formed by a switch or gateway and all the buses connected to it is a changing latency.  
         [0016]     An embodiment of the present invention relates to a method for making a network formed by CAN (Control Area Network) type buses. An embodiment of the invention relates to a method to make a star network formed by CAN buses using a repeater. The method comprising: first controllers connected to ends of the buses are linked to the repeater connected to all the buses, and a repeater reproduces the signals observable on each bus on all the other buses. An embodiment of the invention is a network and an apparatus having the network. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     An embodiment of the invention will be understood more clearly from the following description and the accompanying figures. These figures are given by way of an example that in no way restricts the scope and extent of the invention. In the figures:  
         [0018]      FIG. 1   a  is a schematic view, of a known network, already described, of an X-ray apparatus having a CAN bus;  
         [0019]      FIG. 1   b  is a schematic view of a medical table comprising CAN buses connected to one another by means of a repeater according to an embodiment of the invention;  
         [0020]      FIG. 2   a  is a schematic view of controllers connected to a repeater according to an embodiment of the invention either directly or by means of a CAN type bus;  
         [0021]      FIG. 2   b  is a schematic view of a communications circuit;  
         [0022]      FIG. 2   c  is a schematic view of a signal observable on a CAN bus; and  
         [0023]      FIG. 3  is a state diagram of operation of the repeater made according to an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]      FIG. 1   b  is an embodiment of the invention in which three CAN type buses  141 - 143  respectively connect controllers  20 - 22  to a repeater  19 . Repeater  19  reproduces the signals sent on one bus in the other buses. For example, when the controller  20  sends a signal on the bus  141 , this signal is reproduced in the buses  142  and  143 . The repeater  19  therefore enables the bus  14  to be replaced by three distinct buses  141 - 143 . The different buses  141 - 143  behave as if they form only one and the same bus. The repeater organizes the sending of signals on these buses  141 - 143 .  
         [0025]     In this configuration, it is no longer necessary for the buses  141 - 143  to describe loops within each frame  16 - 18  in order to connect all the controllers  20 - 22  to one another. This bus configuration thus provides for savings in bus connection length.  
         [0026]     Furthermore, when a break in connection occurs on a bus, an embodiment of the invention enables the other buses to still communicate with one another by means of the repeater  19 . In this configuration, the buses  141 - 143  can be physically isolated from one another and the signals observable on these buses  141 - 143  are independent of one another. The architecture of the buses around the repeater  19  is also called a star architecture, by analogy with the shape that they may have around the repeater  19 .  
         [0027]      FIG. 2   a  shows a schematic view of first two controllers  231  and  232  connected to the repeater  19  by means of two CAN type buses  261  and  262  and a second controller  233  directly connected to the repeater  19 .  
         [0028]     In  FIG. 2   b,  the buses  261  and  262  are two-way buses on which signals  351  and  352  are observable. Each of these buses  261  or  262  has a first communications circuit  241  or  242  and a second communications circuit  251  or  252 . Each of these buses  261  or  262  furthermore has two resistors  341 ,  342  or  361 ,  362  that are situated at its ends and are parallel-connected electrically with connections  271 ,  272  or  281 ,  282  of the bus  261  or  262 . These connections  271 ,  272  or  281 ,  282  link the first communications circuit to the second communications circuits or transceivers. In  FIG. 2   c,  the communications circuits or transceivers generally carry out the conversion of a digital all-or-nothing signal into a physical transportation signal.  
         [0029]     In this embodiment, each of the first communications circuits  241  or  242  is connected to the repeater  19  by means of two wire links  301 ,  311  or  302 ,  312 . Each of the second communications circuits  251  or  252  is connected to one of the first controllers  231  or  232  by means of two links  371 ,  381  or  372 ,  382 . The second controller  233  is directly connected to the repeater  19  by means of two connections  41  and  42 .  
         [0030]     First transmission signals  321 - 322  are sent by the repeater  19  to first communications circuits, and first reception signals  331 - 332  sent by these first communications circuits are received by the repeater  19 . A second transmission signal  44  is sent by the repeater  19  to the second controller  233  and a second reception signal  43  is sent by the second controller  233  to the repeater  19 .  
         [0031]     In  FIG. 3 , the repeater  19  organizes the sending of the transmission and reception signals to first communications circuits  241  and  242  and the second controller  233 . This organization of the sending of signals is achieved so as to simulate the interconnection of the first controllers  231  and  232 , and of the second controller  233  to a same bus.  
         [0032]     As a variant embodiment, other controllers may be connected to the buses  261  or  262 . For example, the controller  46  is connected to the bus  261  by means of a communications circuit  47 .  
         [0033]     In practice, each of the second controllers is physically assembled with the second communications circuit  251 ,  252  and the resistor  361 ,  362  corresponding to it on the electronic circuits  27  and  28 . Furthermore, the repeater  19 , the resistors  341 - 362 , the first communications circuits  372 ,  373 , and the second controller  233  may also be physically grouped together on one and the same electronic circuit  29 .  
         [0034]      FIG. 2   b  shows a detailed schematic view of the communications circuit  241 , whose structure is substantially identical to that of the circuits  242 ,  251  and  252 . The communications circuit  241  provides for two-way communications on the bus  261 . The circuit  241  is capable of both sending the signal  321  on the bus  261 , and receiving the signal  331  sent on the bus  261 . Thus, the circuit  241  converts the all-or-nothing type transmission signal  321  into a transportation signal  351  and the transportation signal  351  into an all-or-nothing type reception signal  331 . More specifically, when a transmission signal  321  is sent by the repeater on the bus  261 , a first conversion element  50  converts this signal into a differential type of signal  351 . Connections  52  and  53  pick up this signal and apply it to the terminals of a second conversion element  51 . This second conversion element  51  then converts the differential voltage signal observable on the bus into a reception signal  331 . Such a signal pick-up operation enables the repeater  19  that is connected to the circuit  241  to receive all the observable signals on the bus  261  including those that its sends itself. The repeater  19  can thus synchronies operations of sending transmission signals as a function of the other signals sent on the bus  261 . The transmission and reception signals possess either a recessive level or a dominant level. A dominant level signal cannot be modified by a recessive level signal, while a recessive level signal can be modified by a dominant level signal. In general, in an idle state, a controller sends recessive level signals.  
         [0035]     In one example, the communications circuit is an 82C250 type circuit. As a variant embodiment, the communications circuits carry out a conversion of all-or-nothing signals into optical or RF transportation signals.  
         [0036]      FIG. 2   c  shows a shape of the differential signal  351  observable on the bus  261 . This signal  351  is more specifically observable between the connections  271  and  281  of the bus  261 . This signal  351  is of a differential type because the potentials of the two connections  271  and  281  measurable relative to a ground possess a same difference relative to a mean value A. At an initial instant for example, a signal  351  is observed with a voltage level of A volts at the terminals of the resistor  341 . This voltage level A corresponds to a recessive level. At an instant t 1 , a dominant type of signal is sent on the bus  261 . The voltage  351  then starts rising and, at an instant t 2 , it reaches a level 2*A corresponding to a dominant level. At the instant t 2 , one of the connections then has a potential of 2*A volts while the other has a potential of 0 Volts. At an instant t 3 , a recessive level signal is sent on the bus  261 . The voltage  351  then falls and, at an instant t 4 , reaches a level corresponding to the recessive level.  
         [0037]     There is a certain delay between the instant when a change in level is imposed on the signal and the instant when the signal reaches the requested level. This delay corresponds in fact to the charging or discharging of the capacitors used in the communications circuits or to parasitic capacitive effects introduced especially by cables or lugs of components. The duration needed for the signal to pass from a dominant level to a recessive level is called an overlapping period  49 . During this overlapping period  49 , the level of the signal observed on the bus cannot be detected with certainty.  
         [0038]      FIG. 3  shows a state diagram corresponding to the implementation of the method according to an embodiment of the invention. In the following description, the term “sender” is understood to mean an element that is directly connected to the repeater  19  and sends a signal to this repeater  19 , and the term “recipient” covers all the elements that are directly connected to the repeater  19 , except for the transmitter. When the repeater  19  receives a dominant level reception signal from the transmitter, the repeater  19  sends out transmission signals whose level depends on the recipient and/or the transmitter.  
         [0039]     More specifically, in a resting state  78 , the reception signals  331 ,  332  and  43  received by the repeater  19  have recessive levels. When the receiver  19  receives a first dominant level reception signal  331  sent by the first communications circuit  241  (which is then a sender), it goes into a first state  80 . In this state  80 , the receiver  19  sends dominant level transmission signals  322  and  44  to all the recipients  242  and  233 . This sending of dominant signals to the first communications circuit  242  and the second controller  233  makes it possible to simulate the fact that the controllers  231 - 233  are connected to the same bus. Furthermore, the receiver  19  sends a recessive level transmission signal  321  to the transmitter  241 . This sending of a recessive level transmission signal  321  is aimed at preventing blocking in this first state  80 . Indeed, if the signal  321  had a dominant level, the observable signal  351  on the bus  261  would constantly have a dominant level and no other controller  231 - 233  would then be authorized to send any dominant signal to the repeater  19 .  
         [0040]     The operations of sending signals to the sender  241  and recipients  242  and  233  by the repeater  19  take place as long as the sender  241  sends a dominant level signal  331 . Furthermore, so long as the sender  241  sends a dominant level signal, the repeater  19  does not process the reception signals  332  and  43  sent by the recipients  233  and  242 . This absence of processing also prevents a blocking of the system if the repeater  19  should receive only dominant level signals.  
         [0041]     The repeater  19  comes out of the first state  80  and goes into a timeout step  79  when the sender  241  sends a recessive level signal  331  to the repeater  19 . In the timeout step  79 , the repeater  19  sends recessive level transmission signals to the recipients  242 ,  233  and the sender  241  during a timeout period. This timeout step  79  makes it possible to overcome any problems that the system could encounter when the level of the observable signals on the buses  261  and  262  is indeterminate. The timeout duration is at least as long as the overlapping period  48 . This duration ranges from 0 ns to 700 ns, and is chosen as a function of a given application. When the timeout period has elapsed, the system returns to the idle step  78  when the repeater  19  is listening for dominant level signals that may be sent.  
         [0042]     When the first communications circuit  242  becomes a sender in turn, the repeater  19  behaves with the recipients in a way that corresponds to the way in which it behaves when the first communications circuit  241  is a sender.  
         [0043]     When the repeater  19  is in an idle state and receives a reception signal  43  sent by the second controller  233  (which then becomes the transmitter), it goes into a third state  82 . In this third state  82 , the repeater  19  sends dominant level transmission signals  321 ,  322  and  44  to all the recipients  241 ,  242  and the sender  233 . The controller  233  directly connected to the repeater  19  thus receives a reception signal of a same level as the transmission signal that it is sending, so that it can organize operations of sending signals and still receive a signal  44  corresponding to the signals sent on the bus.  
         [0044]     Here again, so long as the sender  233  sends a dominant level signal, the repeater  19  does not process the reception signals sent by the recipients  241 ,  242 . The repeater  19  comes out of the state  82  when the controller  233  sends a recessive level signal  43  to the repeater  19 . Then, as was done earlier, the repeater  19  goes into a timeout step  79 . At the end of this step  79 , the repeater  19  returns to the idle state  78 .  
         [0045]     In an exemplary embodiment of implementation, two first controllers (hence two first circuits  241  and  242 ), and only one second controller  233  are connected to the repeater  19 . However, in the general case, an unspecified number of first controllers and an unspecified number of second controllers may be connected to the repeater  19 .  
         [0046]     As a variant embodiment, it is of course possible to connect only first controllers or only second controllers to the repeater  19 .  
         [0047]     In addition, while an embodiment of the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made in the function and/or way and/or result and equivalents may be substituted for elements thereof without departing from the scope and extent of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. Moreover, the use of the terms first, second, etc. do not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element or feature from another. Furthermore, the use of the terms a, an, etc. do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced element or feature.